30. Aging and the Body Systems

How Every System Changes Across the Lifespan

Learning Objectives

30.1 What Aging Is — and the One Idea That Organizes the Chapter

Almost every textbook statement about aging is a statement about an average, and averages hide the thing that matters. Two 80-year-olds can differ more from one another than either differs from a 40-year-old. One runs a 5 km race; the other cannot rise from a chair without using her arms. They have the same chronological age and nothing else in common.

So the first job is to take the word apart.

Chronological versus biological age

Chronological age is time since birth. It is precise, useless as a predictor for any individual, and the only number most systems record.

Biological age is an estimate of how far a person's physiology has actually drifted from the young-adult state. It has no single measurement, but it can be approximated from composites — gait speed, grip strength, forced expiratory volume, epigenetic clocks (§30.10) — and it predicts mortality and disability substantially better than the calendar does. Adwoa's gait speed of 0.68 m/s and grip strength of 15 kg place her biologically older than her 78 years; Nia's numbers place her biologically younger than 24.

Primary versus secondary aging

This distinction is the working tool of the chapter and it answers Case File question 1.

Primary (intrinsic) aging is the change that happens to everyone who lives long enough, independent of disease. Four criteria are traditionally applied: it is universal (occurs in all members of the species), intrinsic (not caused by the environment), progressive (accumulates gradually), and deleterious (reduces functional capacity). Presbyopia is the cleanest example in the body: every human eye loses accommodation on a nearly identical schedule, nobody escapes it, no lifestyle prevents it, and it is not a disease.

Secondary aging is change driven by disease, injury, and behavior — atherosclerosis from decades of hypertension and dyslipidemia, emphysema from tobacco, osteoarthritis from a previous joint injury, type 2 diabetes from adiposity and inactivity. It looks like aging, arrives on the same schedule as aging, and is routinely mistaken for aging. It is, unlike primary aging, substantially preventable.

The practical test: is the change universal, and is it proportional to age alone? Presbyopia is universal. A femoral neck T-score of −3.1 is not — it is at the far tail of the distribution for a 78-year-old woman, whose median is nearer −1.8. Adwoa has a disease (osteoporosis) superimposed on a normal age-related process (bone loss), and the whole clinical question is how much of the gap is which.

Usual versus successful aging

Rowe and Kahn's distinction, introduced in the 1980s, was a reaction against studies that described the average older adult and called the result "aging." Usual aging is what happens to most people in a given population — a mixture of primary aging and accumulated, largely preventable secondary aging. Successful aging is the trajectory of those with little disease burden, preserved physical and cognitive function, and continued engagement. The gap between the two curves is roughly the size of the preventable component, and it is large.

Life expectancy, healthspan, maximum lifespan

The number that should bother you is the third one. Over the last century life expectancy rose enormously; healthspan rose less. The gap — a decade or more of life lived with disease and disability — is the actual problem, and it is why §30.9 and §30.10 are about healthspan rather than longevity.

Physiological reserve: the organizing idea

Here is the concept that makes the rest of the chapter cohere.

Every organ has a resting level of function and a maximum level of function. The difference between them is its physiological reserve — the spare capacity available when something is demanded. A young kidney filters about 120 mL/min at rest and can raise that substantially; a young heart pumps 5 L/min at rest and 25 L/min at maximum; a young lung ventilates 6 L/min at rest and can reach 150 L/min or more.

Aging does not, in the main, reduce resting function. It reduces maximum function. Most organ systems lose roughly 1% of maximal capacity per year after about age 30, some faster and some slower, and the resting requirement barely moves. So for decades nothing appears to be wrong, because nothing measured at rest is wrong. What is shrinking is the gap.

   FUNCTION
   (% of young-adult maximum)
   100 ┤●●●●
       │    ●●●●●
    90 ┤         ●●●●●                MAXIMUM CAPACITY
       │              ●●●●●           (falls ~1%/year after ~30)
    80 ┤                   ●●●●●
       │                        ●●●●●
    70 ┤                             ●●●●●
       │            ← RESERVE →           ●●●●●
    60 ┤         (the shrinking gap)           ●●●●
       │                                          ●●●●
    50 ┤                                              ●●●●
       │                                                  ●●●
    40 ┤                                                     ●●●
       │                                                        ●●●
    30 ┤─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ●─ ─
       │                          DISABILITY THRESHOLD          ▲  ●●
    20 ┤        (capacity needed for independent living)         │
       │                                                    CROSSING
    10 ┤▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪
       │  RESTING REQUIREMENT — essentially flat across the lifespan
     0 └────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────
           20   30   40   50   60   70   75   80   85   90   95
                              AGE (years)

   ┌──────────────────────────────────────────────────────────────────┐
   │ THREE TRAJECTORIES TO THE SAME THRESHOLD                          │
   │                                                                   │
   │  A · LOWER PEAK      never built much reserve → crosses earlier   │
   │  B · STEEPER SLOPE   disease/inactivity accelerates loss          │
   │  C · ACUTE DROP      one event (hip fracture, ICU stay, stroke)   │
   │                      removes years of reserve in days             │
   │                                                                   │
   │  Adwoa's fall is trajectory C acting on a person already          │
   │  well down trajectory B.                                          │
   └──────────────────────────────────────────────────────────────────┘

Homeostenosis

Homeostenosis is the term for the consequence: the narrowing of the range across which homeostasis can still be maintained. Return to Chapter 1's control-loop diagram — receptor, control center, effector. Aging degrades all three. Receptors report less faithfully (baroreceptors, osmoreceptors, thermoreceptors, chemoreceptors all lose sensitivity). Control centers integrate more slowly. Effectors respond less strongly to the same signal (the aged heart's blunted response to beta-adrenergic stimulation, §30.5, is the classic case).

The loops still work. They simply work over a smaller range, and they take longer to get there. A young person perturbed 20% from a set point returns in seconds; an older person perturbed the same amount returns in minutes, or does not fully return at all. Notice this is precisely Thread 2 from Chapter 1 rewritten for a different timescale.

30.2 Theories and Mechanisms: The Hallmarks of Aging

For most of the twentieth century, theories of aging were arguments about a single cause — wear and tear, rate of living, free radicals, programmed death. None survived contact with the evidence in its pure form. What replaced them is better: a set of interacting cellular and molecular processes, each of which can be manipulated experimentally, each of which accelerates or is accelerated by the others.

The framework below follows the "hallmarks of aging" formulation. Read it as a network with causal arrows, not as a list to memorize. The exam question worth answering is not name the nine hallmarks but if I block this one, what happens to the others?

The primary drivers — damage that accumulates

1 · Genomic instability. DNA is under continuous assault: reactive oxygen species from mitochondria, replication errors, ultraviolet and ionizing radiation, chemical adducts. Repair systems — base excision, nucleotide excision, mismatch repair, double-strand break repair — are extensive but imperfect, and they themselves decline. Somatic mutations accumulate in every tissue across life. The strongest evidence that this matters causally is the progeroid syndromes: Werner syndrome (a defective RecQ helicase) and Hutchinson-Gilford progeria (a mutant lamin A producing progerin, which destabilizes the nuclear envelope) both produce accelerated, though not complete, phenocopies of aging.

2 · Telomere attrition. Chromosome ends are capped by TTAGGG repeats — about 10–15 kb at birth — bound by the shelterin complex, which disguises the chromosome end so the cell does not read it as a double-strand break. Because DNA polymerase cannot replicate the extreme 3′ end, each division removes roughly 50–100 base pairs. When telomeres shorten to about 4–6 kb the cap fails, a DNA damage response fires, and the cell arrests permanently. That arrest is the Hayflick limit — the roughly 40–60 divisions a human fibroblast can complete in culture. Germ cells, stem cells, and most cancers express telomerase and escape it; ordinary somatic cells do not. Telomere shortening is therefore a mitotic clock, and it matters most in high-turnover tissues: marrow, gut epithelium, skin, immune cells.

3 · Epigenetic alteration. The genome does not change much; its annotation does. Global DNA methylation drifts downward while specific CpG islands become hypermethylated; histone modification patterns shift; heterochromatin loosens and previously silenced regions, including transposable elements, become transcribed. These changes are so regular that they can be read as a clock (§30.10). Crucially, epigenetic marks are reversible in principle, which is why this hallmark attracts so much interventional interest.

4 · Loss of proteostasis. A cell's protein quality-control network — chaperones that fold, the ubiquitin-proteasome system that degrades, autophagy and lysosomes that clear bulk damaged material — becomes less efficient. Misfolded proteins accumulate and aggregate. Every major neurodegenerative disease is, at bottom, a proteostasis failure with a specific culprit protein: amyloid-β and tau, α-synuclein, TDP-43, huntingtin. In non-neural tissue the same failure appears as lipofuscin — the undegradable brown-yellow residue of oxidized lipid and crosslinked protein that accumulates inside long-lived post-mitotic cells.

The antagonistic responses — protective, then harmful

5 · Deregulated nutrient sensing. Four coupled pathways report nutritional state: the insulin/IGF-1 axis, mTOR (senses amino acids and growth signals; drives anabolism and suppresses autophagy), AMPK (senses a high AMP:ATP ratio, i.e. energy scarcity; drives catabolism and autophagy), and the sirtuins (NAD⁺-dependent deacetylases). Aging shifts the balance toward the anabolic, growth-promoting, autophagy-suppressing configuration — persistently high mTOR signaling — which is the state that caloric restriction and exercise reverse. Reducing insulin/IGF-1 or mTOR signaling extends lifespan in yeast, worms, flies, and mice. The evolutionary logic is antagonistic pleiotropy: strong growth signaling is advantageous when young and reproducing, and costly later.

6 · Mitochondrial dysfunction. Mitochondria lose respiratory efficiency, produce more reactive oxygen species per unit ATP, and accumulate mitochondrial DNA mutations (mtDNA sits close to the electron transport chain, has minimal histone protection, and has less capable repair). Damaged mitochondria are cleared by mitophagy, which itself declines. Skeletal muscle oxidative capacity falls roughly 8% per decade in sedentary adults — and, importantly, much of that is reversible with training, which tells you a substantial fraction is disuse rather than time.

7 · Cellular senescence. A cell that reaches a damage threshold — critically short telomeres, oncogene activation, oxidative or genotoxic stress — can enter a permanent, non-dividing but metabolically active state. Senescence evolved as a tumor-suppressive mechanism (a damaged cell that cannot divide cannot become a cancer) and it is also essential for wound healing and embryonic patterning. Its problem is persistence. Senescent cells resist apoptosis, accumulate with age (from well under 1% of cells in young tissue to roughly 3–15% in some aged tissues), and secrete the senescence-associated secretory phenotype.

The integrative consequences — what the phenotype is made of

8 · Stem cell exhaustion. Every renewing tissue depends on a stem cell compartment. Hematopoietic stem cells persist in number but shift their output toward myeloid lineages and away from lymphoid, contributing to immunosenescence (§30.7). Satellite cells in skeletal muscle decline in number and in activation capacity, limiting repair and contributing to sarcopenia. Intestinal, epidermal, and neural stem cell pools all show reduced regenerative output. The niche degrades along with the cell.

9 · Altered intercellular communication. Endocrine, neuronal, and paracrine signaling all drift, and the dominant drift is toward a chronic, sterile, low-grade pro-inflammatory state — inflammaging. Circulating IL-6, TNF-α, and C-reactive protein rise modestly but persistently with age, driven by senescent cells, mitochondrial damage-associated molecular patterns, visceral adipose tissue, failing autophagy, and gut barrier changes. Of all circulating biomarkers, IL-6 is among the strongest single predictors of mortality and disability in older adults. Inflammaging is not an infection and does not produce fever; it produces the slow catabolic, anabolic-resistant, endothelium-damaging background against which sarcopenia, atherosclerosis, insulin resistance, and frailty all develop faster.

     ┌─────────────────────── PRIMARY DAMAGE ───────────────────────┐
     │                                                              │
     │  GENOMIC        TELOMERE       EPIGENETIC      LOSS OF        │
     │  INSTABILITY    ATTRITION      ALTERATION      PROTEOSTASIS   │
     │      │              │               │              │         │
     └──────┼──────────────┼───────────────┼──────────────┼─────────┘
            │              │               │              │
            └──────┬───────┴───────┬───────┴──────┬───────┘
                   ▼               ▼              ▼
     ┌───────── ANTAGONISTIC RESPONSES (protective → harmful) ───────┐
     │                                                              │
     │   DEREGULATED           MITOCHONDRIAL         CELLULAR       │
     │   NUTRIENT SENSING ◄───► DYSFUNCTION ◄──────► SENESCENCE     │
     │   (mTOR ↑, AMPK ↓,      (↓ ATP, ↑ ROS,       (p16, p21,      │
     │    sirtuins ↓)           mtDNA mutation)      SASP output)   │
     │        │                      │                    │        │
     └────────┼──────────────────────┼────────────────────┼────────┘
              │                      │                    │
              │      ┌───────────────┘                    │
              ▼      ▼                                    ▼
     ┌──────────────────── INTEGRATIVE OUTPUT ──────────────────────┐
     │                                                              │
     │      STEM CELL              ALTERED INTERCELLULAR            │
     │      EXHAUSTION  ◄─────────► COMMUNICATION                   │
     │      (↓ repair,             = INFLAMMAGING                   │
     │       ↓ regeneration)        (IL-6, TNF-α, CRP ↑)            │
     │            │                        │                       │
     └────────────┼────────────────────────┼───────────────────────┘
                  └──────────┬─────────────┘
                             ▼
              ╔══════════════════════════════════╗
              ║   LOSS OF PHYSIOLOGICAL RESERVE  ║
              ║   ACROSS ALL ORGAN SYSTEMS       ║
              ║        (Figure 30.1)             ║
              ╚══════════════════════════════════╝

     KEY FEEDBACK ARROWS (why this is a network, not a list):
       SASP ──────────────► induces senescence in neighbors, damages matrix
       ROS ───────────────► damages nuclear and mitochondrial DNA
       Inflammaging ──────► impairs stem cell function and muscle anabolism
       Failing autophagy ─► retains damaged mitochondria → more ROS
       Short telomeres ───► DNA damage response → senescence → SASP

30.3 Integumentary, Skeletal, and Muscular Aging

Skin

Skin is the only organ whose aging everyone can see, which makes it a useful place to practice separating primary from secondary change. Compare the skin of your inner upper arm with the skin of your face or forearm: same person, same age, wildly different appearance. The difference is ultraviolet exposure. Intrinsic aging produces thin, dry, finely wrinkled, unblemished skin. Extrinsic aging (photoaging) produces coarse wrinkling, leathery texture, irregular pigmentation, and telangiectasia. Most of what is called "aged skin" is photoaging.

Layer by layer (Chapter 5):

Three consequences deserve emphasis because they drive real clinical problems.

Thermoregulation fails from both directions. Fewer functioning sweat glands, less subcutaneous insulation, blunted cutaneous vasomotor responses, and a reduced shivering response together mean an older adult tolerates neither heat nor cold. Heat-wave mortality is concentrated overwhelmingly in the elderly, and the mechanism is integumentary and cardiovascular before it is anything else.

Wound healing slows by roughly 25–50%. Every phase is affected — a delayed and blunted inflammatory phase, reduced fibroblast proliferation and migration, less collagen deposited and of poorer quality, slower angiogenesis, delayed re-epithelialization. The healed wound also reaches a lower final tensile strength.

Senile purpura. The flat, well-demarcated, dark purple patches on the forearms and dorsal hands of older adults are not a bleeding disorder. Dermal collagen loss removes the mechanical support around superficial vessels; the vessels themselves are more fragile; minor shear tears them; and the extravasated blood spreads through a loosened dermis rather than being contained. Coagulation studies are normal. It is a connective tissue finding, not a hematological one — a good example of predicting a sign from a structural change.

Bone

Bone mass follows a stereotyped lifetime curve (Chapter 6). It rises through childhood and adolescence — roughly 25% of adult bone mass is laid down in the two years around peak height velocity — and reaches peak bone mass between about 25 and 30. Thereafter, remodeling balance turns slightly negative: each remodeling cycle resorbs a little more than it replaces, and total mass falls by about 0.5–1% per year.

In women, menopause interrupts this gradual decline with an abrupt acceleration. Estrogen restrains osteoclasts (by suppressing RANKL and promoting osteoclast apoptosis) and supports osteoblast survival. When estradiol falls by more than 90% within a couple of years, bone loss accelerates to 2–3% per year overall and up to 5% per year in trabecular bone, for roughly 5–10 years, before settling back to the age-related rate. Trabecular bone — vertebral bodies, femoral neck, distal radius — is affected first and worst because it has far more surface area per unit volume and therefore more remodeling. Once a trabecular strut is perforated it cannot be rebuilt; connectivity is lost permanently, and strength falls faster than mass.

By age 80, a typical woman has lost roughly 30–40% of peak bone mass and a typical man roughly 20–25%. Men start higher, lose more slowly, and have no menopausal step — which is why hip fracture rates in men lag those in women by about a decade rather than differing in kind.

T-score, the standard reporting metric, is the number of standard deviations from the mean of a young adult reference population:

The consequence is fracture, and the consequence of fracture is the reason this chapter exists. Hip fracture in an older adult carries roughly 20–30% mortality at one year, and only about half of survivors regain their prior level of function. The fracture is rarely the direct cause of death; the cascade is — immobility, pneumonia, delirium, venous thromboembolism, pressure injury, deconditioning, loss of independence. This is Figure 30.1's trajectory C.

Joints

Articular cartilage is avascular, aneural, and has essentially no capacity for repair (Chapter 7). With age, chondrocytes senesce and their synthetic output falls; proteoglycan content and its water-binding capacity change; the collagen network accumulates advanced glycation end-product crosslinks that make it stiffer and more brittle. Cartilage becomes less able to distribute load, and the subchondral bone stiffens, transmitting more force back to the cartilage above it.

Osteoarthritis is the endpoint: cartilage loss, subchondral sclerosis, osteophyte formation, and low-grade synovitis. Radiographic OA is present in more than half of adults over 65 and the great majority over 75 — but only a fraction have symptoms, and the correlation between radiographic severity and pain is famously weak. Osteoarthritis is best understood as primary aging of cartilage plus secondary factors: joint injury (Toby's ACL rupture in Chapter 7 roughly quadruples his lifetime knee OA risk), malalignment, obesity (both mechanical loading and adipokine-driven inflammation), and occupational loading. Ligaments and tendons stiffen and weaken in parallel, and range of motion falls in every major joint.

Muscle: sarcopenia

Sarcopenia is the age-related loss of skeletal muscle mass, strength, and function. Current consensus definitions (EWGSOP2 and similar) put low muscle strength first, because strength predicts outcomes better than mass does: probable sarcopenia is low grip strength or a slow chair-stand time; confirmed sarcopenia adds low appendicular lean mass; severe sarcopenia adds low physical performance such as gait speed below 0.8 m/s.

The numbers:

• Mass falls about 0.5–1% per year from the fourth decade, accelerating after 70 to perhaps 1–2% per year. Cumulatively, 30–50% of muscle mass is lost between 30 and 80.
• Strength falls two to three times faster than mass — roughly 1.5–3% per year — so muscle quality (force per unit cross-sectional area) declines as well as quantity. Fat and fibrous tissue infiltrate the muscle (myosteatosis), excitation-contraction coupling becomes less efficient, and neural drive falls.
• Power (force × velocity) falls faster still, perhaps 3–4% per year. This matters more than strength for the tasks that fail first: rising from a chair, climbing a step, and — above all — the rapid corrective step that prevents a stumble from becoming a fall.

The cellular pattern is specific and it explains the functional pattern. Loss is preferentially of type II (fast, glycolytic) fibers: type II fiber cross-sectional area falls by roughly 25–40% by age 80, while type I (slow, oxidative) fiber area is comparatively preserved. The muscle becomes proportionally slower and more fatigue-resistant — well suited to standing still and poorly suited to catching yourself.

Underneath that lies motor unit remodeling. Alpha motor neurons die progressively; by age 70–80, the number of functioning motor units in a limb muscle may be reduced by about half. Surviving motor neurons sprout collateral axons that reinnervate orphaned fibers — an impressive rescue that preserves mass at the cost of control. The surviving units are larger (more fibers per neuron), so force can no longer be graded finely, and because the reinnervating neurons are disproportionately slow-type, the rescued fibers convert toward type I. Fiber-type grouping on biopsy — clusters of same-type fibers replacing the young mosaic — is the histologic signature of denervation and reinnervation, and it is the single most characteristic finding in aged muscle.

Then the crucial point: sarcopenia is substantially preventable and partly reversible. Disuse produces the same fiber-type-specific atrophy, the same fat infiltration, and the same strength loss, on a timescale of weeks rather than decades. Ten days of bed rest in healthy older adults costs roughly 1 kg of leg lean mass and a substantial fraction of leg strength — which is exactly what is happening to Adwoa in her hospital bed. Anabolic resistance — the reduced muscle protein synthetic response to a given dose of amino acids — is real and means older adults need more protein per meal to trigger the same response, not that the response is absent. See §30.9.

30.4 Nervous System and Special Senses

The aging brain

The old teaching that adults lose "100,000 neurons a day" is wrong, and correcting it changes how you interpret cognitive aging. Stereological counting shows that neuronal number in the neocortex and hippocampus is relatively well preserved in healthy aging. What changes is connectivity and volume.

• Total brain volume peaks in the early twenties and declines roughly 0.2% per year through middle age, accelerating to about 0.5% per year after 60 and up to 0.7–1% per year after 70.
• The decline is regionally selective: prefrontal cortex and hippocampus lose most, primary sensory and motor cortices and the occipital lobe least. This regional pattern predicts the cognitive pattern.
• White matter volume peaks later than gray — around 40 to 50 — then declines more steeply. Myelin sheaths degenerate and are incompletely repaired; diffusion imaging shows falling fractional anisotropy, especially in the frontal white matter and the corpus callosum. Reduced white matter integrity slows conduction between regions, and this is the leading structural explanation for the generalized slowing of processing speed.
• Synapse and dendrite loss: dendritic arbors of pyramidal neurons simplify and spine density falls, particularly in prefrontal cortex. This is the substrate of change, not cell death.
• Neurotransmitters: nigrostriatal dopamine neurons decline about 5–10% per decade and striatal D2 receptor density falls similarly, which contributes to slowing, reduced gait automaticity, and a stooped, shorter-stepped walk that is not Parkinson disease. Cholinergic projections from the basal forebrain decline, relevant to attention and memory encoding — and this is why anticholinergic drugs are so poorly tolerated (see Adwoa's diphenhydramine).

What cognition actually does with age

Popular belief and student expectation both hold that everything gets worse. The data are much more interesting: cognition dissociates.

The single largest mediator is processing speed. Statistically, much of the age-related decline in memory and reasoning tasks disappears once speed is controlled for, which suggests that a lot of what looks like a memory problem is a timing problem: information is encoded and retrieved more slowly, so under time pressure less gets through. Adwoa's four-choice reaction time of 610 ms versus Nia's 340 ms is that finding in a single number — and note that the choice reaction time gap (270 ms) is far larger than the simple reaction time gap (125 ms), because choice tasks add a central decision step, which is precisely the step that slows.

Compensation is real and visible on functional imaging: older adults performing the same task frequently recruit bilateral prefrontal activation where young adults are unilateral. Whether this is successful compensation or loss of specificity is debated; both probably occur.

Normal aging, MCI, dementia, and delirium

These four are constantly confused, and distinguishing them is one of the most consequential skills in this chapter.

Sleep

Sleep architecture changes markedly and predictably (Chapter 12). Total sleep time falls modestly. Slow-wave (N3) sleep falls by roughly 2% per decade from young adulthood and may be nearly absent by the eighties, with the largest losses in men. REM declines slightly. Sleep becomes fragmented: the number of arousals rises, wake-after-sleep-onset increases, and sleep efficiency falls from over 90% in young adults to 70–80%. The circadian phase advances — earlier sleep onset and earlier waking — driven partly by reduced melatonin amplitude and by reduced retinal light transmission through an aged, yellowing lens, which weakens the entraining signal reaching the suprachiasmatic nucleus. Sleep-disordered breathing becomes far more common, and periodic limb movements more frequent.

The clinical trap follows immediately: fragmented sleep is common, it is distressing, and it invites a sedative. The two drug classes most often reached for — sedating antihistamines and benzodiazepines or "z-drugs" — are among the most reliable precipitants of delirium and falls in this population. Adwoa's diphenhydramine is a textbook example (§30.6).

The special senses

Presbyopia. Accommodative amplitude falls from about 14 diopters in childhood to under 2 diopters at 50 and effectively zero by 60–65 — a near-linear decline that is arguably the most predictable physiological measurement in the body. The near point recedes from roughly 9 cm at 20 to 25 cm at 45 (the age reading glasses are typically first needed, because 25 cm is about reading distance) to a metre or more by 70. The mechanism is lens stiffening and continued growth (see the Development sidebar in §30.2), with a contribution from a weakened ciliary muscle and altered zonular geometry.

Two other visual changes matter as much and are discussed less. Senile miosis: resting pupil diameter falls, so retinal illumination at 60 is roughly a third of that at 20, and dark adaptation is much slower. And the lens yellows, absorbing short wavelengths — impairing blue-violet discrimination and reducing the blue light that entrains circadian rhythm. Add cataract (present to some degree in most people over 75), reduced contrast sensitivity, and a smaller functional visual field, and you have most of the visual contribution to falls.

Presbycusis. Age-related sensorineural hearing loss begins at the highest frequencies and progresses downward, because the outer hair cells at the base of the cochlea — which transduce high frequencies — are the most metabolically stressed and are lost first (Chapter 15). Threshold elevation at 4 kHz of 40–60 dB HL by the seventies is typical, more in men and more with noise exposure history. Loss of outer hair cells removes the cochlear amplifier, so the tuning of the basilar membrane broadens and frequency selectivity degrades. The practical consequence is disproportionate difficulty understanding speech in noise, because distinguishing overlapping sounds requires precisely the frequency resolution that has been lost. Add reduced central auditory temporal processing, and you get the characteristic complaint: "I can hear you, I just can't understand you." Consonants — which carry most of the information in speech and live at high frequencies — are lost preferentially, so speech becomes audible but unintelligible. Shouting does not help; speaking slowly, facing the person, and reducing background noise do.

Smell and taste. Olfactory decline is far greater than most people realize: measurable impairment in over half of adults aged 65–80 and in three-quarters of those over 80. Causes include cumulative damage to olfactory receptor neurons, reduced turnover from the olfactory stem cell pool, and narrowing of the cribriform plate foramina. Taste bud number falls only modestly; detection thresholds for salt and bitter rise more than for sweet and sour. Because flavor is mostly olfactory, the complaint that "food has no taste" is usually an olfactory complaint — which explains why older adults over-salt food and why unexplained weight loss should prompt a question about smell. New olfactory loss is also an early feature of Parkinson disease and Alzheimer disease, sometimes preceding motor or cognitive signs by years.

Proprioception and balance. This one closes the loop to falls. Muscle spindle afferent density and sensitivity fall; large myelinated fibers carrying vibration and joint position sense are preferentially lost, so ankle vibration threshold rises markedly; cutaneous mechanoreceptors on the sole decline; and vestibular hair cells and vestibular ganglion neurons fall by roughly 20–40% by the seventies. Postural control depends on three inputs — vision, vestibular, and proprioception — and normally any two can compensate for the loss of the third. Aging degrades all three at once. Standing sway increases; recovery from a perturbation requires a faster corrective step than aged type II fibers can produce; and the whole system takes longer to compute because central processing has slowed. Every element of the fall in §30.8 is already in this paragraph.

30.5 Cardiovascular, Respiratory, and Hematologic Aging

Arteries stiffen — and that one change explains most of the rest

The large elastic arteries — aorta, carotid, iliac — depend on elastin for the Windkessel function described in Chapter 19: they distend during systole, storing energy, and recoil during diastole, converting the heart's intermittent output into continuous flow.

Elastin is laid down almost entirely before adulthood and has a half-life measured in decades. Across a lifetime it fragments mechanically (roughly 3 billion cardiac cycles), is degraded by matrix metalloproteinases from senescent cells and inflammation, and is not replaced. Collagen increases and becomes crosslinked by advanced glycation end-products. The wall becomes stiffer and the arterial lumen dilates.

The measurable consequence is pulse wave velocity, which rises from about 5–6 m/s at 20 to 10–14 m/s at 80. Two things follow:

1. Systolic pressure rises because the same stroke volume ejected into a stiffer conduit generates a higher peak pressure.
2. Diastolic pressure peaks around 50–60 and then falls, because a stiff artery stores less elastic energy to maintain pressure between beats — and because the reflected pressure wave from the periphery now returns so fast that it arrives during systole, augmenting the peak, rather than during diastole, where in a young artery it helps maintain coronary filling pressure.

The result is isolated systolic hypertension — a systolic pressure ≥130–140 mm Hg with a normal or low diastolic pressure — which is the dominant form of hypertension after 60. Pulse pressure widens (from ~40 mm Hg in youth to 60–80 mm Hg), and pulse pressure is a better predictor of cardiovascular events in older adults than either number alone. Notice that a falling diastolic pressure is not reassuring here: it is the marker of the stiffness that is raising the systolic number, and it simultaneously reduces coronary perfusion pressure, since coronary flow occurs in diastole.

The heart

Three of these interlock into one of the most clinically important facts in geriatrics. Because beta-adrenergic responsiveness is blunted, the aged heart cannot raise output much by beating faster or harder. It is therefore forced to rely on the remaining mechanism: the Frank-Starling relationship — stretch the ventricle more, eject more. That makes the older heart preload-dependent.

Follow the implication. A young person who becomes volume-depleted compensates with tachycardia and increased contractility and maintains output. An older person, deprived of both of those levers, is left dependent on filling volume — which is the very thing volume depletion removes. So dehydration, diuresis, blood loss, and vasodilator drugs are all tolerated far worse. At the same time, the stiff ventricle means that volume overload is also poorly tolerated: a small excess of filling volume produces a large rise in filling pressure and pulmonary congestion. The aged heart operates on a narrow ledge with a cliff on both sides — homeostenosis in one picture, and a direct preview of Chapter 31.

The lungs

Aging does two opposite things to the respiratory pump and they compound.

The lung gets floppier. Alveolar walls and the elastic attachments that tether small airways open are progressively lost. Alveolar ducts enlarge and alveolar surface area falls roughly 20% by age 70 (about 75 m² to 60 m²). This is architecturally similar to emphysema — hence "senile emphysema" — but without inflammation or destruction of the alveolar wall itself.

The chest wall gets stiffer. Costal cartilages calcify, costovertebral joints stiffen, intervertebral discs lose height, and thoracic kyphosis increases. Respiratory muscle strength (maximum inspiratory and expiratory pressures) falls about 20–30%.

The last three rows are the dangerous ones. An older adult who becomes hypoxemic or hypercapnic does not breathe as hard in response, so the compensatory sign clinicians rely on to detect deterioration is blunted at exactly the moment it is most needed. Combined with a weaker cough and impaired clearance, this is most of why pneumonia is so lethal after 80 — and why a rising respiratory rate in an older patient, however modest, is a finding to act on rather than to observe.

Blood and marrow

Peripheral blood counts in healthy aging are essentially normal. Hemoglobin drifts down slightly and the lower limit of normal is debated, but anemia in an older adult is not a normal finding and should be investigated — roughly a third is nutritional (iron, B₁₂, folate), a third inflammatory or renal (reduced erythropoietin, Chapter 26), and a third unexplained.

What changes is reserve, again. Marrow cellularity falls from roughly 50% in young adults to about 30% by 70 as hematopoietic tissue is replaced by fat. Hematopoietic stem cell output skews toward myeloid and away from lymphoid lineages, contributing to immunosenescence. Clonal hematopoiesis of indeterminate potential — an expanded, mutation-bearing stem cell clone without any blood count abnormality — is detectable in over 10% of people over 70 and is associated with increased cardiovascular risk as well as hematologic malignancy. A baseline count that looks identical to a 25-year-old's therefore conceals a much reduced capacity to mount a response to hemorrhage, hemolysis, or infection.

30.6 Digestive, Metabolic, and Endocrine Aging

The gut

Most of the digestive tract has enormous functional reserve and ages gracefully. The clinically important changes are concentrated in a few places, and several problems commonly attributed to aging are actually drug effects.

• Saliva. Healthy aging reduces unstimulated salivary flow only modestly. The great majority of clinically significant xerostomia in older adults is caused by medications — over 400 drugs, led by anticholinergics, antidepressants, antihistamines, diuretics, and opioids. This distinction matters because one is untreatable and the other is fixable. Dry mouth impairs bolus formation, taste (tastants must be dissolved to reach receptors), swallowing, and dental health.
• Stomach. Atrophic gastritis — loss of parietal and chief cells, often after Helicobacter pylori infection or autoimmune — affects roughly 20–50% of adults over 60. Two consequences follow. Reduced acid impairs the release of food-bound cobalamin from protein, and reduced intrinsic factor impairs its absorption, so vitamin B₁₂ deficiency occurs in 5–15% of older adults — a genuinely reversible cause of cognitive change, neuropathy, and megaloblastic anemia that is worth actively excluding. Reduced acid also impairs absorption of non-heme iron and of calcium carbonate (which requires acid to dissolve; calcium citrate does not). Proton pump inhibitors reproduce the same effects pharmacologically.
• Motility. Esophageal peristalsis becomes slightly less coordinated and the upper esophageal sphincter weakens, raising aspiration risk. Colonic transit slows modestly. But as with saliva, most constipation in older adults is not primary aging: it is drugs (opioids, anticholinergics, calcium channel blockers, iron), immobility, dehydration, and low fiber intake.
• Liver. Liver mass falls 20–40% by the eighth decade and hepatic blood flow falls roughly 35–40%. This is where a general physiological change becomes a specific, daily clinical hazard.

Drug handling — the most consequential section in the chapter

   ┌──────────────────────────────────────────────────────────────────────┐
   │ A · ABSORPTION                                     effect: SMALL     │
   ├──────────────────────────────────────────────────────────────────────┤
   │ ↓ gastric acid, ↓ splanchnic flow, ↓ gut motility, ↓ surface area    │
   │ → RATE of absorption often slower; EXTENT usually unchanged          │
   │ BUT ↓ first-pass metabolism (↓ liver mass and flow)                  │
   │ → ↑ bioavailability of high-extraction drugs (propranolol, morphine, │
   │   nitrates, verapamil)  ── CONSEQUENCE: greater effect per dose      │
   └──────────────────────────────────────────────────────────────────────┘
   ┌──────────────────────────────────────────────────────────────────────┐
   │ D · DISTRIBUTION                                   effect: LARGE     │
   ├──────────────────────────────────────────────────────────────────────┤
   │ TOTAL BODY WATER ↓ 10–15%   ── lipid-soluble drugs ↑ Vd              │
   │ LEAN MASS ↓ 20–30%             water-soluble drugs ↓ Vd              │
   │ BODY FAT ↑ (15→30% ♂, 25→40% ♀)                                      │
   │ SERUM ALBUMIN ↓ (esp. in illness) → ↑ free fraction of acidic,       │
   │   highly bound drugs (warfarin, phenytoin, naproxen)                 │
   │                                                                      │
   │ CONSEQUENCE 1: water-soluble drugs (digoxin, lithium, aminoglycoside)│
   │   reach HIGHER peak levels for the same mg/kg dose                   │
   │ CONSEQUENCE 2: fat-soluble drugs (diazepam, amiodarone) have a much  │
   │   LONGER half-life — diazepam t½ ≈ 20 h at 20 y, ≈ 90 h at 80 y      │
   └──────────────────────────────────────────────────────────────────────┘
   ┌──────────────────────────────────────────────────────────────────────┐
   │ M · METABOLISM                                     effect: LARGE     │
   ├──────────────────────────────────────────────────────────────────────┤
   │ Liver mass ↓ 20–40% · hepatic blood flow ↓ 35–40%                    │
   │                                                                      │
   │ PHASE I  (CYP450 oxidation, reduction, hydrolysis)   ↓ SUBSTANTIALLY │
   │          diazepam, chlordiazepoxide, amitriptyline, many opioids     │
   │          → metabolites often ACTIVE and accumulate                   │
   │ PHASE II (glucuronidation, sulfation, acetylation)   ↓ LITTLE        │
   │          lorazepam, oxazepam, temazepam → inactive conjugates        │
   │                                                                      │
   │ CONSEQUENCE: among equivalent drugs, prefer the Phase II route.      │
   │   "LOT" — Lorazepam, Oxazepam, Temazepam — is the classic mnemonic.  │
   └──────────────────────────────────────────────────────────────────────┘
   ┌──────────────────────────────────────────────────────────────────────┐
   │ E · EXCRETION                                      effect: LARGEST   │
   ├──────────────────────────────────────────────────────────────────────┤
   │ GFR ↓ ~0.75–1.0 mL/min/1.73 m² per year after ~30–40                 │
   │ Tubular secretion ↓ in parallel                                      │
   │                                                                      │
   │ ★ THE TRAP: muscle mass falls too, so CREATININE PRODUCTION falls.   │
   │   Serum creatinine can stay 0.9 mg/dL while true GFR is 45.          │
   │   A "NORMAL CREATININE" IN AN 80-YEAR-OLD IS NOT A NORMAL KIDNEY.    │
   │                                                                      │
   │ CONSEQUENCE: renally cleared drugs accumulate — digoxin, metformin,  │
   │   gabapentin, lithium, DOACs, many antibiotics, morphine metabolites.│
   │   Always estimate GFR; never read creatinine alone.                  │
   └──────────────────────────────────────────────────────────────────────┘
       ▼
   ┌──────────────────────────────────────────────────────────────────────┐
   │ PLUS PHARMACODYNAMICS: the same blood level does more.               │
   │   ↑ CNS sensitivity to benzodiazepines, opioids, anticholinergics    │
   │   ↑ sensitivity to warfarin at the same INR-determining level        │
   │   ↓ baroreflex buffering of antihypertensives → orthostatic falls    │
   └──────────────────────────────────────────────────────────────────────┘

Metabolism and body composition

Basal metabolic rate falls roughly 1–2% per decade in adulthood — but most of that is explained by the loss of lean mass rather than by a change in the metabolic rate of tissue itself. Body composition shifts even at stable weight: fat mass rises (from roughly 15% to 30% in men and 25% to 40% in women between 25 and 75), and it redistributes from subcutaneous to visceral and intramuscular depots. Sarcopenic obesity — low muscle mass with high fat mass — is the worst combination for function and for metabolic risk, and it is invisible on a scale and nearly invisible on a BMI.

Glucose tolerance declines: fasting glucose rises about 1–2 mg/dL per decade and two-hour post-load glucose rises 5–10 mg/dL per decade. The causes are the ones you would predict from Chapter 24 — less muscle mass means less of the body's principal glucose sink; more visceral fat means more free fatty acid flux and inflammatory adipokines; inactivity reduces GLUT4 translocation; and beta-cell insulin secretory response is slightly blunted. Note that Adwoa's fasting glucose of 101 mg/dL is an age-typical drift, whereas Amara's 118 at 45 is a disease finding — same direction, different distance from the age-matched mean, which is question 1's method in a single line.

Endocrine

Note the pattern across the table: the aged endocrine system is not uniformly "low." Some signals fall (estradiol, IGF-1, DHEA-S, aldosterone), some rise (FSH, PTH, TSH, ADH), and several show preserved signal with reduced target response — which is the endocrine version of homeostenosis. Reading an older adult's hormone panel against a young-adult reference range without knowing which pattern applies is a reliable way to create disease that is not there.

30.7 Renal, Fluid, and Immune Aging

The kidney

Renal aging matters out of proportion to its symptoms, because the kidney sets the chemical conditions every other system operates in (Chapter 26) and because it clears most drugs.

Structure. Kidney mass falls from roughly 250–270 g in young adulthood to 180–200 g by 80, with loss concentrated in the cortex. The proportion of globally sclerosed glomeruli rises from under 5% at age 40 to 10–30% by 80. Functioning nephron number falls by roughly 30–50%. Surviving nephrons hyperfiltrate to compensate — which maintains total GFR for a time and simultaneously accelerates their own sclerosis, a small positive feedback loop.

Function.

• GFR falls about 0.75–1.0 mL/min/1.73 m² per year after roughly age 30–40 in most people — though roughly a third of healthy older adults show no measurable decline at all, which is a useful reminder that "age-related" is a statement about a distribution.
• Serum creatinine does not rise correspondingly, because creatinine is generated from muscle at a rate proportional to muscle mass, and muscle mass is falling in parallel. This is the single most dangerous number in geriatric medicine. Adwoa's admission creatinine of 0.9 mg/dL is squarely "normal" and her estimated GFR is 52 — stage 3a chronic kidney disease. Every renally cleared drug she takes must be dosed to 52, not to 0.9.
• Concentrating ability falls: maximum urine osmolality declines from about 1,100–1,200 mOsm/kg at 20 to 700–800 mOsm/kg at 80. The mechanisms are a reduced medullary interstitial osmotic gradient (partly from reduced urea recycling and loss of long juxtamedullary nephrons) and reduced collecting duct responsiveness to ADH at the V2 receptor and aquaporin-2 level, despite ADH concentrations that are normal or high.
• Diluting ability also falls, so free water cannot be excreted as rapidly either — which is why older adults are at high risk of both dehydration and hyponatremia, sometimes within the same admission.
• Sodium conservation is impaired: reduced renin and aldosterone (both down 30–50%) mean the kidney takes longer to reduce urinary sodium in response to depletion. A young person can drop urinary sodium to under 10 mEq/L within about 24 hours; an older person may take two to three days.
• Potassium handling is impaired in the other direction: reduced GFR and reduced aldosterone make hyperkalemia more likely, particularly with ACE inhibitors, ARBs, or spironolactone.

Thirst

The final piece, and the one that converts all of the above into a clinical emergency: thirst is blunted. In controlled water-deprivation studies, older adults reach a higher plasma osmolality than young adults before reporting thirst, drink less when water is offered, and stop drinking sooner — with intact ADH release. The deficit is in the osmoreceptor–thirst pathway itself, not in the hormonal arm.

Now assemble the three findings:

   ↓ ability to CONCENTRATE urine  →  obligatory water loss is larger
                +
   ↓ ability to CONSERVE sodium    →  volume is defended more slowly
                +
   ↓ THIRST                        →  the loss is not replaced
                =
   DEHYDRATION IS THE COMMONEST FLUID DISORDER OF OLD AGE
                +
   ↓ GFR with a NORMAL creatinine  →  DRUGS ACCUMULATE SILENTLY

That block is the mechanistic core of Adwoa's day 2. She is on a thiazide, which increases sodium and water loss. She has not been drinking. She cannot concentrate her urine efficiently to defend against the loss, and she does not feel thirsty enough to correct it. Her sodium rises to 148 mEq/L and her BUN to 38 with a creatinine of 1.4 — a BUN:creatinine ratio of 27, the signature of volume depletion. Her diphenhydramine, cleared partly by declining Phase I metabolism and acting on a brain with increased sensitivity, is now present in higher effective concentration than the dose implies. Chapter 31 takes this apart quantitatively.

Immunosenescence

The aged immune system is simultaneously weaker at responding and more inflamed at baseline — a combination that sounds contradictory and is not.

Thymic involution. The thymus begins involuting at puberty, not in old age. Functional thymic epithelial tissue declines roughly 3% per year until about 35–45 and about 1% per year thereafter, replaced by fat. By 60–70, thymopoiesis is minimal. The consequence is that the supply of naive T cells — those that have never met their antigen and are therefore the only cells capable of responding to a genuinely new pathogen or vaccine — depends increasingly on peripheral homeostatic proliferation of existing cells rather than on new production.

That last row is the hinge of Case File question 3. Fever is a regulated response requiring pyrogen production, an intact hypothalamic set-point shift, and effectors (vasoconstriction, shivering, brown fat) capable of raising temperature. Aging degrades production, transduction, and effector capacity at once. And because the baseline is lower, a temperature of 37.6 °C — "normal" on any chart — can represent a rise of more than a degree in an older adult. Many geriatric guidelines therefore define fever in this population as a single reading ≥37.8 °C, a persistent temperature ≥37.2 °C, or a rise of ≥1.1 °C above the person's own baseline. Adwoa's temperature of 36.9 °C is 0.5 °C above her admission 36.4 °C, with a nitrite-positive urine and 50–100 white cells per high-power field. She has an infection. She does not have a fever.

30.8 The Geriatric Syndromes as Multi-System Failure

Sections 26.3 to 26.7 walked one system at a time. That is a useful way to learn and a misleading way to think, because older patients do not present with one system. They present with a geriatric syndrome: a clinical state produced by accumulated impairment in several systems, in which the presenting problem is a final common pathway rather than the name of a diseased organ. Before assembling them, here is the whole book condensed.

 SYSTEM        CHANGE (20 → 80)                        TYPICAL RATE      RESERVE
 ───────────────────────────────────────────────────────────────────────────────
 INTEGUMENT    dermal thickness ↓ ~20%                 collagen ↓1%/y    ▓▓▓░░░░
               vitamin D synthesis ↓ ~75%              —                 ▓▓░░░░░
               wound healing time ↑ 25–50%             —                 ▓▓▓░░░░
 ───────────────────────────────────────────────────────────────────────────────
 SKELETAL      bone mass ↓ 30–40% (♀), 20–25% (♂)      0.5–1%/y;         ▓▓░░░░░
                                                       ♀ 2–3%/y × 5–10 y
                                                       post-menopause
 ───────────────────────────────────────────────────────────────────────────────
 MUSCULAR      muscle MASS ↓ 30–50%                    0.5–1%/y          ▓▓░░░░░
               muscle STRENGTH ↓ more                  1.5–3%/y          ▓░░░░░░
               muscle POWER ↓ most                     3–4%/y            ▓░░░░░░
               motor units ↓ ~50% by 75                —                 ▓▓░░░░░
 ───────────────────────────────────────────────────────────────────────────────
 NERVOUS       brain volume ↓ 10–15%                   0.2→0.7%/y        ▓▓▓░░░░
               processing speed ↓ markedly             continuous        ▓▓░░░░░
               vocabulary / semantic knowledge         NO DECLINE        ▓▓▓▓▓▓▓
               N3 slow-wave sleep ↓                    ~2%/decade        ▓▓░░░░░
 ───────────────────────────────────────────────────────────────────────────────
 SENSES        accommodation 14 D → <1 D               linear from ~10 y ░░░░░░░
               4 kHz threshold ↑ 40–60 dB              accelerating      ▓░░░░░░
               vestibular hair cells ↓ 20–40%          —                 ▓▓░░░░░
 ───────────────────────────────────────────────────────────────────────────────
 CARDIOVASC.   maximum heart rate                      −0.7 beats/min/y  ▓▓░░░░░
               maximum cardiac output ↓ 20–30%         —                 ▓▓░░░░░
               pulse wave velocity 5–6 → 10–14 m/s     —                 ▓▓░░░░░
               RESTING heart rate & cardiac output     NO CHANGE         ▓▓▓▓▓▓▓
 ───────────────────────────────────────────────────────────────────────────────
 RESPIRATORY   FEV₁                                    −25–30 mL/y       ▓▓░░░░░
               residual volume ↑ ~50%                  —                 ▓▓░░░░░
               PaO₂ ≈ 100 − 0.3 × age                  −0.3 mm Hg/y      ▓▓▓░░░░
               hypoxic ventilatory response ↓ ~50%     —                 ▓░░░░░░
 ───────────────────────────────────────────────────────────────────────────────
 RENAL         GFR ↓ 30–50%                            −0.75–1 mL/min/y  ▓▓░░░░░
               max urine osmolality 1200 → 700–800     —                 ▓▓░░░░░
               THIRST                                  blunted           ▓░░░░░░
               serum CREATININE                        NO CHANGE ← trap  ▓▓▓▓▓▓▓
 ───────────────────────────────────────────────────────────────────────────────
 IMMUNE        naive T cells ↓ markedly                thymus −3%/y      ▓░░░░░░
                                                       from puberty
               vaccine effectiveness ↓ ~half           —                 ▓▓░░░░░
               fever response                          blunted           ▓░░░░░░
 ───────────────────────────────────────────────────────────────────────────────
 ENDOCRINE     estradiol ↓ >90% (♀, over ~2 y)         menopause ~51 y   ░░░░░░░
               IGF-1 ↓ ~50%                            —                 ▓▓░░░░░
               fasting glucose ↑                       1–2 mg/dL/decade  ▓▓▓░░░░
 ───────────────────────────────────────────────────────────────────────────────
  ▓ = reserve retained    ░ = reserve lost   (schematic, not measured units)
  NOTE how many RESTING values are marked "NO CHANGE". That is the whole point.

Frailty

Frailty is the syndrome that names the state of exhausted reserve itself. Two operationalizations dominate.

The phenotype model (Fried) counts five criteria: unintentional weight loss of ≥4.5 kg or 5% in a year; self-reported exhaustion; weakness (grip strength in the lowest sex- and BMI-adjusted quintile); slow walking speed (lowest quintile, roughly <0.8 m/s); and low physical activity. Three or more defines frail; one or two defines prefrail. Prevalence is about 10–15% over 65 and 25–50% over 85. Adwoa meets weakness (15 kg), slow gait (0.68 m/s), and low activity — three criteria, and therefore frail before she ever fell.

The deficit accumulation model (Rockwood) counts the proportion of a long list of possible deficits — diseases, symptoms, abnormal findings, functional limitations — that a person has. The resulting frailty index behaves remarkably regularly: it rises roughly exponentially with age, predicts mortality better than chronological age, and appears to saturate near 0.7, as though no one survives having accumulated more than about two-thirds of the available deficits.

                    ╔══════════════════════════════════════╗
                    ║        THE FRAILTY CYCLE             ║
                    ╚══════════════════════════════════════╝

              ┌──────────────► SARCOPENIA ◄──────────────┐
              │              (muscle mass ↓,             │
              │               type II fibers ↓)          │
              │                    │                     │
              │                    ▼                     │
              │        ↓ STRENGTH and ↓ POWER            │
              │                    │                     │
              │                    ▼                     │
   NEGATIVE   │        ↓ WALKING SPEED, ↓ BALANCE        │  ANABOLIC
   NITROGEN   │                    │                     │  RESISTANCE
   BALANCE    │                    ▼                     │  + INFLAMMAGING
              │      ↓ ACTIVITY  ──►  FEAR OF FALLING ───┤  (IL-6, TNF-α)
              │      (and social withdrawal)             │
              │                    │                     │
              │                    ▼                     │
              │      ↓ TOTAL ENERGY EXPENDITURE          │
              │                    │                     │
              │                    ▼                     │
              │        ↓ APPETITE ("anorexia of aging")  │
              │         + ↓ smell/taste + ↓ dentition    │
              │         + drugs + dry mouth              │
              │                    │                     │
              │                    ▼                     │
              └───── ↓ ENERGY and PROTEIN INTAKE ────────┘
                              │
                              ▼
                       UNDERNUTRITION
                              │
              ╭───────────────┴────────────────╮
              ▼                                ▼
     ↓ IMMUNE FUNCTION                ↓ BONE and ↓ REPAIR
              │                                │
              ▼                                ▼
        INFECTION ──────► HOSPITALIZATION ◄──── FALL / FRACTURE
                                 │
                                 ▼
                  ██ BED REST · SEDATIVES · DELIRIUM ██
                                 │
                                 ▼
              ══► RE-ENTERS THE CYCLE AT SARCOPENIA ══►
                  (10 days of bed rest ≈ 1 kg leg lean mass)

   ENTRY POINTS FOR INTERVENTION (each breaks one arrow):
     ▪ resistance training ────────────► sarcopenia, strength, power
     ▪ protein 1.0–1.2 g/kg/d ─────────► anabolic resistance
     ▪ balance/gait training ──────────► fall risk, fear of falling
     ▪ deprescribing ──────────────────► delirium, falls, appetite
     ▪ early mobilization in hospital ─► the bed-rest shortcut
     ▪ hearing/vision correction ──────► social withdrawal, falls

Falls

Roughly one in three community-dwelling adults over 65 falls each year, and one in two over 80. About 10% of falls cause serious injury and 1–2% cause a hip fracture. Falls are the leading cause of injury death in this age group.

Read the risk factor list as a reserve audit rather than a list:

The number of risk factors matters more than which ones: the probability of falling rises from under 10% with none to over 60–70% with four or more. That is the reserve model expressed epidemiologically, and it is why single-factor interventions underperform multifactorial ones.

Delirium, revisited as a model

Inouye's multifactorial model is the cleanest formal statement of everything in this chapter. Delirium occurs when precipitating factors exceed the person's reserve, which is described by their predisposing factors.

   HIGH RESERVE (few predisposing factors)   LOW RESERVE (many)
   ─────────────────────────────────────     ──────────────────────────────
   Requires a SEVERE insult:                 Requires only a TRIVIAL insult:
     · major surgery                           · one dose of an antihistamine
     · septic shock                            · an unlit unfamiliar room
     · multi-organ failure                     · mild dehydration
                                               · constipation, urinary retention
                                               · a night of broken sleep

Predisposing: pre-existing cognitive impairment (the strongest single factor), sensory impairment, dehydration, illness burden, functional dependence, prior delirium, advanced age. Precipitating: drugs (especially anticholinergics, benzodiazepines, opioids), infection, surgery and anesthesia, pain, hypoxia, metabolic derangement, urinary retention, constipation, restraints and catheters, sleep deprivation, unfamiliar environment.

Adwoa has five predisposing factors and at least six precipitants, and neither list contains a single item that would have moved a 24-year-old. That is Case File question 2, and it is answered in full in the Resolution.

Incontinence and polypharmacy

Age-related urinary changes — reduced functional bladder capacity (roughly 600 to 300 mL), increased involuntary detrusor contractions, increased post-void residual, weaker urethral closure (estrogen-dependent in women), prostatic enlargement in men, and increased nocturnal urine production because ADH's circadian rhythm flattens — produce a system with no margin. Incontinence then appears the moment mobility or cognition or drug burden is added: if it takes longer to get to the toilet than the bladder can wait, the person is incontinent regardless of bladder physiology. This is why the mnemonic for reversible causes (DIAPPERS — delirium, infection, atrophic vaginitis, pharmaceuticals, psychological, excess output, restricted mobility, stool impaction) is dominated by items that are not urological. Polypharmacy was covered in §30.6; note only that it sits in the causal path of every other syndrome in this section.

Why older patients present atypically

Resolving the case: three generations, separated

  VARIABLE                NIA 24   AMARA 45  ADWOA 78   VERDICT FOR ADWOA
  ─────────────────────────────────────────────────────────────────────────
  Resting heart rate        48        78        72      AGING (none): resting
                                                        values do not change.
                                                        NIA's 48 = training.
  Max heart rate achieved  191       148       118      AGING for Adwoa
                                                        (208−0.7×78 ≈ 153;
                                                        submaximal effort).
                                                        AMARA's 148 = DRUG
                                                        (beta-blocker).
  VO2max (mL/kg/min)        58        21        15      BOTH. Adwoa ~ expected
                                                        for a sedentary 78.
                                                        Amara's 21 at 45 is
                                                        DISEASE + deconditioning.
  FEV1 absolute (L)        3.6       2.6       1.5      —
  FEV1 % predicted        112%       92%       88%      ← THE DISCRIMINATOR.
                                                        88% = normal FOR AGE.
                                                        AGING, no lung disease.
  eGFR                     118        48        52      Adwoa: AGING.
                                                        Amara at 45: DISEASE
                                                        (CKD 3 from HTN + DM).
  Creatinine               0.9       1.4       0.9      ★ Adwoa's normal
                                                        creatinine with eGFR 52
                                                        = the muscle-mass trap.
  BMD T-score             +0.8      −0.6      −3.1      DISEASE. Median for a
                                                        78-y ♀ ≈ −1.8. Adwoa is
                                                        far below → osteoporosis.
  Lean mass index          7.9       6.4       4.9      DISEASE (sarcopenia;
                                                        cutoff 5.5) on a base of
                                                        normal age-related loss.
  Grip strength (kg)        34        26        15      DISEASE (below 16 cutoff).
  Gait speed (m/s)        1.52      1.21      0.68      DISEASE (<0.8 = severe
                                                        sarcopenia / frailty).
  Dermal thickness (mm)    1.5       1.3       0.9      AGING. Universal.
  Near point (cm)            9        35        98      AGING. Universal,
                                                        schedule-locked.
  4 kHz threshold (dB)       5        20        55      MOSTLY AGING + lifetime
                                                        noise (secondary).
  Simple reaction (ms)     215       255       340      AGING.
  Choice reaction (ms)     340       420       610      AGING — and note the gap
                                                        is bigger for CHOICE,
                                                        i.e. central processing.
  Fasting glucose          82       118       101       Adwoa 101 = AGING drift.
                                                        Amara 118 at 45 = DISEASE.
  ─────────────────────────────────────────────────────────────────────────
  THE METHOD, IN ONE LINE:
    compare against the AGE-MATCHED distribution, not the young-adult one —
    and ask whether the change is UNIVERSAL and SCHEDULE-LOCKED (aging)
    or FAR OUT IN THE TAIL and MODIFIABLE (disease).

30.9 What Actually Modifies Aging

Aging cannot yet be stopped. It can be shifted, and the interventions that shift it are not evenly matched — one of them dwarfs the others.

Exercise: the strongest evidence in the field

If exercise were a drug it would be the most valuable medication ever developed, and the comparison is worth making because the effect sizes are of the same order as the best pharmacological interventions in medicine while the mechanism is broader than any of them.

The mechanism reaches almost every hallmark in Figure 30.2: exercise activates AMPK and suppresses excess mTOR signaling, stimulates mitochondrial biogenesis through PGC-1α, improves mitophagy and autophagy, lowers circulating inflammatory cytokines, improves insulin sensitivity, and — in animal models — reduces senescent cell burden. It is not a symptomatic treatment; it acts upstream.

What to prescribe, from the physiology. Aerobic work for the cardiorespiratory and metabolic reserve; progressive resistance training at 70–85% of one-repetition maximum for sarcopenia, because type II fibers respond to load and not to volume; power training (fast concentric phase) for the corrective step that prevents falls; and balance and gait work, because postural control is a trainable skill and is the only one of the three sensory inputs to balance that improves with practice.

Protein, sleep, connection, and correction

Protein. The RDA of 0.8 g/kg/day was derived largely in young adults and is probably inadequate for older ones. Consensus recommendations for healthy older adults are 1.0–1.2 g/kg/day, rising to 1.2–1.5 g/kg/day in acute or chronic illness (with caution in advanced renal disease). The reason is anabolic resistance: an older muscle requires a larger bolus of amino acids to reach the same rate of muscle protein synthesis — roughly 25–40 g of high-quality protein per meal, containing about 2.5–3 g of leucine, versus perhaps 20 g in a young adult. The implication is practical: distribution matters as much as total. Three meals each containing 30 g beats one containing 60 g and two containing 10 g. And protein without loading builds little; the two are multiplicative.

Sleep. Adequate, consolidated sleep supports memory consolidation, glucose regulation, and immune function, and the glymphatic clearance of interstitial solutes including amyloid-β is enhanced during slow-wave sleep. Treating sleep-disordered breathing is worthwhile on its own merits. Note the asymmetry in the evidence: treating a sleep disorder is well supported; sedating a person who simply sleeps like a 78-year-old is harmful (§30.6).

Social connection. Meta-analytic estimates place the mortality risk associated with social isolation and loneliness at roughly a 25–30% increase — a magnitude comparable to several conventional risk factors. Causality is difficult to establish and reverse causation is real (illness causes isolation), but the association is robust across cohorts, and the mechanistic candidates — inflammatory signaling, blood pressure, sleep, physical activity, and health behaviors — are all plausible.

Hearing and vision correction. Hearing loss is identified in the Lancet Commission analyses as the largest single potentially modifiable midlife contributor to dementia risk. Three mechanisms are proposed: increased cognitive load spent on degraded auditory input; social withdrawal and reduced cognitive stimulation; and accelerated auditory-cortical atrophy from reduced input. The intervention evidence must be stated honestly: the ACHIEVE randomized trial found no significant slowing of cognitive decline in the overall cohort, but a substantial effect in the prespecified subgroup at higher baseline risk. Hearing aids are worth recommending; the claim that they prevent dementia is not established. Cataract surgery is associated with fewer falls and hip fractures and with lower dementia incidence in observational data, with the same causal caveats.

Blood pressure and glucose control, smoking cessation, and vaccination belong in the list too, and their evidence is stronger and more specific than most of what follows.

An honest statement of what is not established

30.10 Advanced Topic · Compression of Morbidity, Biological Age, and Healthspan

Compression of morbidity

In 1980 James Fries proposed that if the age of onset of chronic disability could be postponed faster than the age of death, the period of morbidity at the end of life would compress into a shorter interval. Life would end after a brief terminal decline rather than after a decade of dependency.

The alternative — expansion of morbidity — holds that medicine mostly converts fatal diseases into chronic ones, so people survive longer with disease and the disabled period lengthens. A third possibility is dynamic equilibrium: severity falls even as duration rises.

The data support all three in different places and periods. Roughly, severe disability rates have fallen in many high-income countries while the absolute number of years lived with some morbidity has often risen. The honest summary is that morbidity has been compressed relative to lifespan in some populations and expanded absolutely in others, and that the difference between populations is largely accounted for by the modifiable factors in §30.9.

The theoretical requirement for genuine compression is worth stating precisely, because it is the argument for studying aging as a process rather than disease by disease: eliminating any one disease adds only modestly to life expectancy. Completely eliminating all cancer would add roughly 3 years to life expectancy at birth; eliminating all heart disease roughly 4. Slowing the underlying rate of aging modestly would add more than either — and, crucially, would add healthy years rather than dependent ones, because it delays every age-related condition simultaneously. This is the "longevity dividend" argument.

Measuring biological age

If aging is to be modified, it must be measured on a timescale shorter than a lifetime. Hence the search for biomarkers.

Two observations tie the table together. First, the simplest measures perform astonishingly well: usual gait speed below about 0.8 m/s, or grip strength below sex-specific cutoffs, predicts hard outcomes about as well as far more elaborate assays. That is because they are integrated measures — walking requires the cardiovascular, respiratory, neurological, musculoskeletal, and sensory systems all to work simultaneously, so gait speed is a functional assay of total reserve. Second, no biomarker has yet been shown to be a valid surrogate endpoint — that is, a marker whose improvement reliably predicts improved outcomes. Until one is, trials of aging interventions must measure function and survival, which is slow and expensive, and which is the principal practical obstacle in the field.

Lifespan is not the point

The Gompertz law describes human mortality with unsettling regularity: after about age 30, the probability of dying in the next year doubles roughly every 8 years. Public health advances across the twentieth century mostly shifted this curve rather than flattening it — rectangularizing the survival curve, so that a far higher proportion of people survive to old age, but with the terminal slope little changed.

Rectangularization is a triumph and it is not the same as extending healthspan. The last decade of a rectangularized life can be spent independent or dependent, and which one it is depends overwhelmingly on reserve — the vertical distance in Figure 30.1 — rather than on the horizontal position of the endpoint.

That is the argument of this whole chapter, and it is why §30.9 exists. Adwoa's problem is not that she is 78. It is that her grip strength is 15 kg and her gait speed is 0.68 m/s, and both of those are trainable.

Chapter Summary

§30.1 Chronological age is time; biological age is functional drift. Primary aging is universal, intrinsic, progressive, and deleterious; secondary aging is disease- and lifestyle-driven and largely preventable. Life expectancy exceeds healthspan by a decade or more, which is the actual problem. The organizing idea is physiological reserve — the gap between resting and maximum function. Most systems lose roughly 1% of maximal capacity per year after 30 while resting function is preserved, so nothing appears wrong until the falling maximum meets the demands of daily life. Homeostenosis is the narrowing of the range across which homeostasis can still be defended.

§30.2 The hallmarks of aging form an interacting network: genomic instability, telomere attrition, epigenetic alteration, and loss of proteostasis generate damage; deregulated nutrient sensing, mitochondrial dysfunction, and cellular senescence are protective responses that become harmful when they persist; stem cell exhaustion and altered intercellular communication — inflammaging — produce the phenotype. The SASP explains how a few percent of senescent cells alter a whole tissue. Aging is the continuation of the developmental program past the age at which selection could act on it (antagonistic pleiotropy).

§30.3 Skin thins in every layer, flattens at the dermo-epidermal junction, loses thermoregulatory and vitamin D capacity, and heals more slowly; senile purpura is a connective-tissue sign, not a hematologic one. Bone peaks at 25–30, falls 0.5–1%/y, and accelerates to 2–3%/y for 5–10 years after menopause. Sarcopenia loses mass at ~1%/y, strength two to three times faster, and power faster still, with preferential type II fiber loss and motor unit remodeling — and it is substantially preventable and partly reversible.

§30.4 Brain volume falls regionally (prefrontal and hippocampal most), white matter integrity falls, and processing speed slows — but crystallized knowledge and vocabulary do not decline. Delirium is acute brain failure with inattention as its core feature, affects 20–30% of hospitalized older adults, is missed in over half of cases because the hypoactive form predominates, and is a medical emergency requiring a search for a cause. Presbyopia, presbycusis (with disproportionate loss of speech-in-noise discrimination), olfactory loss, and reduced proprioception together account for most of the sensory contribution to falls.

§30.5 Elastin fragmentation stiffens arteries, raising systolic pressure and lowering diastolic pressure — isolated systolic hypertension with a wide pulse pressure. Maximum heart rate falls ~0.7 beats/min per year and beta-adrenergic responsiveness declines, leaving the aged heart preload-dependent and intolerant of both volume depletion and volume overload. Loss of lung elastic recoil raises residual volume and closing volume, lowering PaO₂; ventilatory responses to hypoxia and hypercapnia are blunted by 40–50%. Marrow cellularity falls, so baseline counts are normal but the response capacity is not.

§30.6 Atrophic gastritis impairs B₁₂ absorption; most xerostomia and constipation in older adults are drug effects rather than aging. Hepatic Phase I metabolism declines while Phase II is preserved. Reduced GFR with an unchanged serum creatinine, altered volume of distribution, and increased CNS sensitivity together make drug handling the most consequential single topic in geriatric physiology. Menopause is abrupt; andropause, somatopause, and adrenopause are gradual; several axes show preserved signal with reduced target response.

§30.7 Nephron number falls 30–50% and GFR by 0.75–1 mL/min/y, while serum creatinine stays normal because muscle mass falls in parallel. Reduced concentrating ability, impaired sodium conservation, and blunted thirst combine to make dehydration the commonest fluid disorder of old age and drug accumulation the dominant iatrogenic risk. Immunosenescence — thymic involution from puberty, contracted naive T cell repertoire, impaired antibody affinity maturation, blunted fever — coexists with inflammaging: inflamed but ineffective.

§30.8 The geriatric syndromes are reserve exhaustion across several systems presenting through a common final pathway. Frailty can be defined by phenotype (weight loss, exhaustion, weakness, slowness, low activity) or by deficit accumulation. The frailty cycle is self-reinforcing and can be interrupted at six points. Atypical presentation follows from four mechanisms: the least-reserved system fails first, afferent signaling is blunted, effector responses are blunted, and baseline noise is high. In an older adult, a change in function is a vital sign.

§30.9 Exercise is by a wide margin the best-evidenced intervention: ~20–35% lower all-cause mortality, 10–25% gains in VO₂max, 25–100%+ gains in strength even in nonagenarians, and about a quarter reduction in fall rate. Protein at 1.0–1.2 g/kg/day distributed across meals addresses anabolic resistance. Sleep, social connection, and sensory correction have supportive evidence of varying strength. Antioxidant supplements are refuted; growth hormone is not recommended; and senolytics, metformin, rapamycin, and NAD⁺ precursors are promising and unproven in humans.

§30.10 Compression of morbidity requires postponing disability faster than death, which requires acting on aging as a process rather than disease by disease — eliminating all cancer would add ~3 years, all heart disease ~4. Biological age can be estimated by epigenetic clocks, frailty indices, and — remarkably well — by gait speed and grip strength, because these are integrated functional assays of total reserve. No biomarker is yet a validated surrogate endpoint.

Case File 30 · Resolution

Question 1 — Which differences in the three-generation table are aging and which are disease, and how would you tell?

The method has three steps, and Figure 30.6 applies it line by line.

Step 1 — Compare against the age-matched distribution, not the young-adult one. This is the step everyone skips. Adwoa's FEV₁ of 1.5 L is less than half Nia's, which looks alarming until you convert it to 88% of predicted for her age, sex, and height — squarely normal. Her lungs are behaving exactly as a healthy 78-year-old's lungs behave. By contrast her femoral neck T-score of −3.1 sits far below the median for a 78-year-old woman, which is nearer −1.8. Same direction of change, entirely different meaning. The absolute number tells you what she can do; the age-referenced number tells you whether anything is wrong beyond time. You need both, and confusing them is how pathology gets dismissed as aging and aging gets over-investigated as pathology.

Step 2 — Ask whether the change is universal and schedule-locked, or far out in the tail and modifiable. Presbyopia (near point 9 → 35 → 98 cm) is universal and follows a near-linear schedule from childhood: primary aging, in all three women, on time. Dermal thinning is the same. Reaction time is the same — and note that the gap is larger for the four-choice task (270 ms) than for the simple task (125 ms), which localizes the change to central processing speed rather than to nerve conduction or muscle. Bone density, lean mass, grip strength, and gait speed are not universal: Adwoa sits below the diagnostic thresholds for osteoporosis (≤ −2.5), sarcopenia (grip <16 kg, ALMI <5.5 kg/m²), and severe sarcopenia/frailty (gait <0.8 m/s). Those are diseases superimposed on a normal aging process, and all four are modifiable.

Step 3 — Look for the third and fourth explanations: drugs, and training. Two rows of the table are neither aging nor disease. Amara's maximum heart rate of 148 against a predicted 176 is a beta-blocker, prescribed after her infarction. Nia's resting heart rate of 44 is endurance training, not bradycardia. Missing either one produces a diagnostic error in opposite directions.

Three rows deserve individual comment.

• Resting heart rate (48 / 78 / 72). There is no aging signal here at all, and there was never going to be: resting values are preserved until reserve is exhausted (§30.1). A resting measurement is the wrong instrument for detecting age-related change.
• Creatinine (0.9 / 1.4 / 0.9). Adwoa's creatinine is identical to Nia's and her eGFR is 52 against Nia's 118. Creatinine is generated by muscle, and Adwoa has an ALMI of 4.9. Her normal creatinine is produced by her sarcopenia, and it conceals stage 3a chronic kidney disease. This is the single most dangerous number on the sheet.
• Fasting glucose (82 / 118 / 101). Adwoa's 101 is the ordinary age-related drift of 1–2 mg/dL per decade. Amara's 118 at 45 is a disease finding, and the two would be reported identically as "high" by a laboratory using a single reference range.

The one-line version: aging changes are universal, schedule-locked, symmetrical between similar people, and centred on the age-matched mean. Disease changes are in the tail of the age-matched distribution, asymmetrically distributed between people of the same age, and usually modifiable. Every clinical judgement in geriatrics starts here.

Question 2 — Why does a 78-year-old become confused from causes that would not touch a 24-year-old?

Because delirium occurs when precipitating insults exceed physiological reserve, and reserve is precisely what has been lost.

Take the two halves separately.

Her reserve was already low. Adwoa arrived with at least five predisposing factors: mild pre-existing cognitive change (the strongest single predictor), sensory impairment in two modalities (presbycusis and cataract), age itself, functional limitation, and a brain with reduced volume, reduced white matter integrity, and reduced cholinergic tone (§30.4). Her cerebral "margin" — the distance between current function and the threshold at which cognition fails — was small before anything happened.

The insults were individually trivial and collectively large.

Not one of these is a disease of the brain. Every one is a small subtraction, and the sum crossed a threshold that a 24-year-old is nowhere near. Note also the timing: she was oriented at admission and confused by day 2, with a fluctuating course — drowsy at 09:00, agitated at 16:00 — and impaired attention. Acute onset, fluctuation, and inattention are three of the four Confusion Assessment Method features. Her head CT is normal for age, and its value is entirely negative: it excludes a structural cause and thereby confirms that the answer is metabolic, pharmacological, and infectious.

The treatment follows from the mechanism and contains no sedative: stop the diphenhydramine, rehydrate carefully (Chapter 31 explains why "carefully" matters), treat the urinary infection, restore her glasses and hearing aids, control pain with a non-anticholinergic agent, mobilize her, and reorient and re-establish a day-night cycle. Delirium is reversible when its causes are removed, and the causes are almost always on the list above.

Question 3 — Why do older patients present with confusion, falls, or "just not right" instead of the classic signs?

Four reasons, all of them consequences of §30.1.

1 · The presenting complaint is the failure of the least-reserved system, not the diseased one. A young adult with a urinary infection has abundant reserve everywhere, so the only system that shows strain is the infected one — hence dysuria, frequency, fever, flank pain. Adwoa's brain has the least reserve of any of her systems, so a systemic inflammatory insult anywhere produces cerebral failure first. Her presenting complaint is therefore neurological while her disease is urological. The same logic explains why an infarcting myocardium presents as a fall: the postural control system is the second-least-reserved system, and a modest fall in cardiac output takes it below threshold before the chest pain pathway generates a classic signal.

2 · The afferent signal is quieter. Visceral pain perception declines, so myocardial ischemia and peritoneal irritation generate less pain — chest pain accompanies roughly 75–80% of infarcts under 65 but only about 40% over 85, and 30–40% of infarcts after 75 are clinically silent. The fever response is blunted at every level, so up to 30–50% of older adults with serious bacterial infection are afebrile. Adwoa's temperature of 36.9 °C would be called normal by any chart; against her own baseline of 36.4 °C it is a half-degree rise, and against a geriatric fever definition of a ≥1.1 °C rise it is not quite a fever. She has an infection and no fever, and both statements are true.

3 · The efferent response is quieter. Tachycardia may not appear, because beta-adrenergic responsiveness has fallen or a beta-blocker is on board. Leukocytosis may not appear, because marrow reserve has fallen — Adwoa's white count is 9.1. Guarding and rigidity may not appear in an acute abdomen, because the abdominal wall is weak. Tachypnea, which depends on chemoreceptor drive, appears but is muted. The classic signs of illness are largely compensatory responses, and compensation is what reserve buys.

4 · The baseline is noisy. With five chronic conditions and eight medications, the change produced by a new illness is small relative to normal day-to-day variation, and both the patient and the clinician have trouble seeing it. This is why the highest-yield question in geriatric assessment is not "where does it hurt" but "what is different from last week, and who would know?"

The clinical rule that follows is worth memorizing exactly: in an older adult, a change in function is a vital sign. New confusion, a new fall, new incontinence, or a new inability to rise from a chair deserves the same urgency as a fever in a young adult — because it is the same message, delivered by the only system that still has enough margin to shout.

Systems Integration Case File · Entry 30

Review

Level 1 · Recall

26.1 Physiological reserve is best defined as:

a) the resting function of an organ    b) the difference between resting and maximum function    c) the maximum function of an organ    d) the rate of functional decline per year

26.2 Which cognitive ability typically shows no decline, and may improve, into the seventies?

a) processing speed    b) working memory    c) vocabulary and semantic knowledge    d) executive set-shifting

26.3 The core, discriminating clinical feature of delirium is:

a) memory loss    b) hallucinations    c) inattention    d) agitation

26.4 Sarcopenia preferentially affects which fiber type, and what is the functional consequence?

a) type I; reduced endurance    b) type II; reduced power and slower corrective stepping    c) both equally; generalized weakness    d) type II; reduced endurance

26.5 An 82-year-old has a serum creatinine of 0.9 mg/dL. The most accurate statement is:

a) her kidney function is normal    b) her GFR may be substantially reduced despite the normal creatinine    c) she has acute kidney injury    d) creatinine is unaffected by muscle mass

26.6 Which pattern of blood pressure is most characteristic of arterial aging?

a) raised systolic and raised diastolic    b) raised systolic with normal or low diastolic    c) low systolic with raised diastolic    d) narrow pulse pressure

26.7 Up to what proportion of older adults with serious bacterial infection are afebrile?

a) under 5%    b) about 10%    c) 30–50%    d) over 90%

26.8 Which sedative-hypnotic property makes lorazepam, oxazepam, and temazepam preferable to diazepam in older adults?

a) they are shorter-acting stimulants    b) they undergo Phase II conjugation, which is relatively preserved with age    c) they are not lipid-soluble    d) they have no active metabolites because they are renally excreted unchanged

Level 2 · Comprehension

26.9 Explain why an 80-year-old's resting vital signs can be entirely normal while she is unable to climb a flight of stairs. Use the terms reserve and homeostenosis.

26.10 A hospital adds a "sleep protocol" that offers diphenhydramine to any patient who reports difficulty sleeping. Predict the consequences in patients over 75, giving the mechanisms.

26.11 Distinguish primary from secondary aging and explain why the distinction changes what a clinician does.

26.12 Why is gait speed such a strong predictor of survival and disability despite measuring nothing more than how fast someone walks four metres?

Level 3 · Clinical Application

26.13 An 84-year-old woman is admitted with a fractured wrist after a fall. She is normally independent. Over the next 48 hours she becomes intermittently confused, is found to have a sodium of 151 mEq/L and a creatinine of 1.5 mg/dL (baseline 1.0), and is prescribed a benzodiazepine for "agitation." Analyze what has happened and what should be done differently.

26.14 A 79-year-old man on five medications reports new ankle swelling. He was started on amlodipine two months ago. A clinician adds furosemide. Two weeks later he presents having fallen, with a potassium of 3.1 mEq/L. Explain the sequence and identify the error.

26.15 A previously well 86-year-old is brought in "just not right" for two days. She is afebrile at 36.8 °C, white count 8.4 ×10⁹/L, heart rate 88, respiratory rate 26, blood pressure 104/62, SpO₂ 90% on air. Her daughter says she stopped getting up to make tea yesterday. What is your concern, and what does each vital sign contribute?

Level 4 · Integration and Synthesis

26.16 Construct the complete causal network linking Adwoa's hospital admission to a plausible outcome six months later in which she is living in residential care and has lost 6 kg. Name at least six systems, mark every point where the loop is self-reinforcing, and identify the three interventions with the greatest expected effect.

26.17 Argue for or against: "Most of what we call aging is disuse." Use evidence from at least four systems, and state clearly what the claim gets right and what it gets wrong.

26.18 A health system proposes screening every patient over 70 with a full panel of laboratory tests interpreted against standard adult reference ranges. Using at least four examples from this chapter, predict the consequences and propose a better approach.

Concept Map to Complete

Copy this onto blank paper and complete every bracket from memory before checking the chapter. Then, in a second color, add what you missed.

                      PHYSIOLOGICAL RESERVE
                    = [ ___________ ] function
                      minus [ ___________ ] function
                               │
             ┌─────────────────┴──────────────────┐
      falls ~[ ___ ]% per year              [ ___________ ] function
      after age [ ___ ]                     stays essentially FLAT
             │                                     │
             └──────────────┬──────────────────────┘
                            ▼
                  [ ______________ ] = narrowing of the
                  range over which homeostasis holds
                            │
        ┌───────────────────┼───────────────────┐
        ▼                   ▼                   ▼
   [ RECEPTOR ]        [ __________ ]      [ EFFECTOR ]
   e.g. blunted        slowed              e.g. reduced
   [ _________ ]       integration         [ ____-adrenergic ]
   and [ _________ ]                       responsiveness
        │                                        │
        └────────────────┬───────────────────────┘
                         ▼
              GERIATRIC SYNDROMES  =  reserve exhaustion
                         │            across MANY systems
        ┌────────┬───────┼────────┬──────────┐
        ▼        ▼       ▼        ▼          ▼
    [ _____ ] [ ______ ] [ _______ ] [ __________ ] [ ____________ ]
                         │
                         ▼
              ATYPICAL PRESENTATION because:
              1. the [ ______-reserved ] system fails first
              2. [ ________ ] signal is blunted (pain, fever)
              3. [ ________ ] response is blunted (HR, WBC)
              4. baseline [ _______ ] is high
                         │
                         ▼
              STRONGEST MODIFIER = [ ____________ ]
              (mortality ↓ ~[ __ ]–[ __ ]%; VO2max ↑ [ __ ]–[ __ ]%;
               strength ↑ up to [ ____ ]% even in nonagenarians)

Lab / Self-Exploration

1. Measure your own near point. Hold this page at arm's length and bring it slowly toward one eye (cover the other) until the text just blurs. Measure that distance in centimetres. Compare with the chapter's figures for 20, 45, and 70 years. Repeat with a family member at least twenty years older or younger. This is primary aging, measured in thirty seconds.
2. Time a five-times sit-to-stand. Sit in a firm chair, arms folded across the chest, and stand fully and sit fully five times as fast as you safely can. Under 12 seconds is typical for a healthy older adult; over 15 seconds is a validated marker of lower-extremity weakness. This is a power test, not a strength test — notice how much harder it is fast than slow.
3. Measure usual gait speed. Mark 4 metres on a flat floor with 1–2 m of run-up and run-out. Walk it at your normal pace and divide 4 by the time. Compare with the thresholds in §30.10 (≥1.0 m/s typical; <0.8 m/s indicates severe sarcopenia or frailty). Then repeat while counting backwards from 100 by sevens — the dual-task cost is a measure of how much attention your walking requires, and it rises steeply with age.
4. Test your dark adaptation. In a dark room, note how long it takes before you can read the time on a dim clock. Do this with someone at least thirty years older or younger. The difference is pupil size, lens transmission, and rod regeneration together — and it is why Adwoa's hallway was dangerous.
5. Audit a real medication list (your own, with permission a relative's, or a fictional one). For each drug ask: what is the indication, is it still present, is it renally or hepatically cleared, is it anticholinergic or sedating, and could any item on the list be treating the side effect of another item? You have just performed the single highest-yield intervention in geriatric medicine.
6. Find your own reserve. Take your resting heart rate. Estimate your maximum as 208 − 0.7 × age. The difference is your heart rate reserve. Now do the same calculation for yourself at 80. Nothing about your resting number will have changed.

Key Terms

Next: Chapter 31 · Fluid, Electrolyte, and Acid-Base Balance — where Adwoa's sodium of 148 and Amara's potassium of 2.9 become quantitative, and where every thread in this book converges on a single set of numbers.