The dark, anisotropic region of a sarcomere, spanning the full length of the thick (myosin) filaments; its width does not change during contraction, which is the observation that first suggested the sliding filament model.
abduction
Movement of a limb away from the midline of the body, or of a digit away from the midline of the hand or foot. The opposite of adduction.
absorption
The movement of digested nutrients from the lumen of the gut across the epithelium into blood or lymph; distinct from digestion, which is the chemical breakdown that precedes it.
accommodation
The change in lens shape produced by ciliary muscle contraction that focuses near objects on the retina; its loss with age is presbyopia.
acetylcholine (ACh)
The neurotransmitter of the neuromuscular junction, of all autonomic preganglionic neurons, and of parasympathetic postganglionic neurons.
acid
A substance that donates hydrogen ions in solution, lowering pH.
acidosis
A process that lowers blood pH below 7.35; respiratory if driven by retained CO₂, metabolic if driven by lost bicarbonate or gained fixed acid.
acinus
A small, rounded, grape-like secretory unit of an exocrine gland, as in the pancreas or salivary glands.
acromegaly
Growth hormone excess arising after the epiphyseal plates have closed, causing enlargement of hands, feet, jaw, and soft tissue rather than increased height.
actin
The protein of the thin filament; its binding sites for myosin heads are covered at rest by tropomyosin.
action potential
A brief, all-or-none, self-propagating reversal of membrane potential produced by voltage-gated ion channels; the signal of excitable cells.
active transport
Movement of a solute across a membrane against its electrochemical gradient, requiring energy — directly from ATP (primary) or from another solute's gradient (secondary).
adaptive immunity
The specific, learned arm of defense, mediated by lymphocytes, that recognizes particular antigens and generates memory; slower than innate immunity on first exposure and far faster on the second.
adduction
Movement of a limb toward the midline; the opposite of abduction.
adenosine triphosphate (ATP)
The cell's immediate energy currency; energy is released when the terminal phosphate bond is hydrolyzed to ADP.
adipose tissue
Connective tissue dominated by fat-storing adipocytes; also an endocrine organ secreting leptin and adiponectin.
adrenal cortex
The outer region of the adrenal gland, in three zones producing aldosterone, cortisol, and adrenal androgens respectively.
adrenal medulla
The inner adrenal region; a modified sympathetic ganglion whose chromaffin cells secrete epinephrine and norepinephrine into blood.
aerobic respiration
Oxygen-requiring catabolism through the citric acid cycle and electron transport chain, yielding about 30–32 ATP per glucose.
afferent
Carrying toward a structure — sensory neurons carry impulses toward the CNS, and the afferent arteriole carries blood toward the glomerulus. Contrast efferent, which carries away; the pair is a constant source of error, and the mnemonic that survives is A for arriving, E for exiting.
afterload
The pressure a ventricle must exceed to open its semilunar valve and eject blood; approximated by aortic pressure for the left ventricle. Contrast preload.
agglutination
Clumping of cells caused by antibodies cross-linking surface antigens; the basis of blood typing.
agonist (muscle)
The prime mover of a given action. In pharmacology the same word means a molecule that activates a receptor.
albumin
The most abundant plasma protein, made by the liver; the principal determinant of plasma colloid osmotic pressure and a carrier for many hormones and drugs.
aldosterone
The mineralocorticoid of the zona glomerulosa; increases renal sodium reabsorption and potassium and hydrogen ion secretion, expanding extracellular volume.
alkalosis
A process that raises blood pH above 7.45; respiratory if driven by excess CO₂ loss, metabolic if driven by gained bicarbonate or lost acid.
allele
One of the alternative versions of a gene at a given locus.
alveolus (lung)
The thin-walled terminal air sac where gas exchange occurs; about 480 million of them provide roughly 70 m² of surface.
amino acid
The monomer of proteins; twenty are used in human proteins, nine of them essential in the diet.
amphiarthrosis
A slightly movable joint, such as the pubic symphysis or an intervertebral disc.
anabolism
Metabolic reactions that build larger molecules from smaller ones, consuming energy. The opposite of catabolism.
anaphase
The stage of mitosis in which sister chromatids separate and move to opposite poles.
anastomosis
A natural connection between two vessels or tubes, providing alternative flow; the coronary circulation has relatively few, which is why coronary occlusion causes infarction.
anatomical position
The standard reference posture — erect, feet parallel, arms at the sides, palms facing forward — against which every directional term is defined.
anatomy
The study of body structures and their relationships.
anemia
A reduction in the oxygen-carrying capacity of blood, from low red cell number, low hemoglobin, or abnormal hemoglobin.
aneurysm
A localized, permanent dilation of an artery caused by weakening of its wall.
angiotensin II
The active peptide of the renin–angiotensin–aldosterone system; a potent vasoconstrictor that also stimulates aldosterone, thirst, and proximal sodium reabsorption.
anion
A negatively charged ion. Contrast cation, which is positive.
anion gap
Sodium minus the sum of chloride and bicarbonate; normally 8–12 mEq/L, and raised when unmeasured acids such as lactate or ketoacids accumulate.
antagonist (muscle)
A muscle that opposes the action of the prime mover; in pharmacology, a molecule that blocks a receptor.
anterior (ventral)
Toward the front of the body.
antibody (immunoglobulin)
A Y-shaped protein secreted by plasma cells that binds a specific antigen, neutralizing it or marking it for destruction.
anticodon
The three-base sequence on a tRNA that pairs with a codon on mRNA during translation.
antidiuretic hormone (ADH, vasopressin)
A posterior pituitary hormone that inserts aquaporin-2 channels into collecting duct cells, conserving water; released in response to rising plasma osmolality or falling blood volume.
antigen
Any molecule capable of being recognized by an antibody or a lymphocyte receptor.
antigen-presenting cell (APC)
A cell — dendritic cell, macrophage, or B cell — that displays processed antigen on MHC class II to helper T cells.
aorta
The largest artery, carrying oxygenated blood from the left ventricle to the systemic circulation.
apex (heart)
The pointed inferolateral tip of the heart, formed by the left ventricle and directed toward the left fifth intercostal space.
apocrine gland
A gland whose secretion includes a portion of the apical cytoplasm; in humans, the axillary and genital sweat glands.
aponeurosis
A broad, flat, sheet-like tendon, as in the abdominal wall.
apoptosis
Programmed cell death — an orderly, energy-requiring self-dismantling that does not trigger inflammation. Contrast necrosis, which is uncontrolled and inflammatory.
appendicular skeleton
The bones of the limbs and their girdles. Contrast the axial skeleton.
aquaporin
A membrane channel protein that permits rapid water movement; aquaporin-2 in the collecting duct is ADH-regulated.
arachnoid mater
The middle meningeal layer, a delicate web beneath which cerebrospinal fluid circulates.
areolar tissue
Loose connective tissue with a gel-like matrix and scattered fibers; the universal packing material under epithelia.
arteriole
A small artery with a thick smooth-muscle wall relative to its lumen; the chief site of vascular resistance and therefore the principal regulator of blood pressure and of flow distribution.
artery
A vessel carrying blood away from the heart — a direction, not an oxygen content, which is why the pulmonary artery carries deoxygenated blood.
articulation
A joint; any point where two bones meet.
astrocyte
The most abundant glial cell; supports neurons metabolically, buffers extracellular potassium, and forms part of the blood–brain barrier.
atherosclerosis
Accumulation of lipid, inflammatory cells, and fibrous tissue in the arterial intima, forming plaques that narrow the lumen and may rupture.
atom
The smallest unit of an element retaining its chemical properties.
ATP synthase
The inner mitochondrial membrane enzyme that uses the proton gradient built by the electron transport chain to phosphorylate ADP.
atrial natriuretic peptide (ANP)
A hormone released by stretched atrial myocytes that promotes sodium and water excretion and opposes the renin–angiotensin–aldosterone system.
atrioventricular (AV) node
The only normal electrical connection between atria and ventricles; its conduction delay of about 100 ms allows the atria to finish emptying before the ventricles contract.
atrium
An upper receiving chamber of the heart.
atrophy
A decrease in cell or organ size from disuse, denervation, or loss of hormonal or nutritional support. Contrast hypertrophy.
auscultation
Listening to body sounds, usually with a stethoscope.
autonomic nervous system
The involuntary motor division of the PNS, comprising sympathetic and parasympathetic subdivisions plus the enteric nervous system.
autoregulation
The ability of an organ to keep its own blood flow relatively constant across a range of perfusion pressures, prominent in brain, heart, and kidney.
axial skeleton
The skull, vertebral column, sternum, and ribs.
axon
The single long process of a neuron that conducts action potentials away from the cell body. Contrast dendrite, which receives input.
axon hillock
The cone-shaped junction of cell body and axon; usually the trigger zone where summed potentials either reach threshold or do not.
B
B lymphocyte (B cell)
The lymphocyte that matures in bone marrow and, when activated, becomes a plasma cell secreting antibody.
baroreceptor
A stretch receptor in the carotid sinus and aortic arch that detects arterial wall distension and therefore, indirectly, blood pressure.
basal metabolic rate (BMR)
Energy expenditure at complete rest in a fasted, thermoneutral state; roughly 1,300–1,800 kcal/day in adults.
basal nuclei (basal ganglia)
Deep gray-matter masses of the cerebrum that modulate movement initiation and suppression; degeneration produces Parkinson and Huntington diseases.
base
A substance that accepts hydrogen ions, raising pH.
basement membrane
The thin extracellular layer anchoring epithelium to underlying connective tissue; its penetration marks the transition from in-situ to invasive carcinoma.
basophil
The least common granulocyte; releases histamine and heparin in allergic and inflammatory responses.
bicarbonate (HCO₃⁻)
The principal extracellular buffer anion, regulated by the kidney and paired with CO₂ regulated by the lung.
bile
The liver secretion containing bile salts, bilirubin, cholesterol, and phospholipids; emulsifies dietary fat, increasing the surface available to lipase.
bilirubin
The yellow pigment produced by heme breakdown; accumulation causes jaundice.
blastocyst
The hollow ball of cells, with inner cell mass and trophoblast, that implants in the endometrium about six days after fertilization.
blood–brain barrier
Tight junctions between brain capillary endothelial cells, reinforced by astrocyte end-feet, that severely restrict passage of solutes into the CNS.
bolus
A rounded mass of chewed food mixed with saliva, ready to be swallowed.
bone remodeling
The lifelong coupled process of osteoclastic resorption and osteoblastic deposition that replaces about 10% of the skeleton each year and adapts it to load.
Bowman's capsule
The double-walled cup surrounding the glomerulus that collects the filtrate.
Boyle's law
At constant temperature, gas pressure varies inversely with volume; the physical basis of ventilation.
bradycardia
A resting heart rate below 60 beats per minute; normal in trained endurance athletes.
bronchiole
A conducting airway less than 1 mm across with no cartilage; its smooth muscle makes it the site of bronchoconstriction in asthma.
bronchus
A cartilage-supported conducting airway branching from the trachea.
brush border
The dense microvillar surface of intestinal and renal proximal tubule epithelium, which amplifies surface area and carries digestive enzymes.
buffer
A conjugate acid–base pair that resists change in pH by absorbing or releasing hydrogen ions.
bulk flow
Movement of fluid down a pressure gradient, carrying dissolved solutes with it; distinct from diffusion, which is driven by concentration.
bursa
A fluid-filled sac reducing friction where a tendon crosses bone.
C
calcitonin
A thyroid parafollicular cell hormone that lowers plasma calcium by inhibiting osteoclasts; physiologically minor in adult humans.
calcitriol (1,25-dihydroxyvitamin D)
The active form of vitamin D, produced by renal 1α-hydroxylase; the only hormone that increases net intestinal calcium absorption.
calmodulin
A calcium-binding regulatory protein; in smooth muscle it activates myosin light-chain kinase, the equivalent of troponin's role in striated muscle.
calorie
The heat needed to raise 1 g of water by 1 °C. The dietary "Calorie" is a.
canaliculi
Tiny channels in bone matrix through which osteocyte processes contact one another, allowing nutrient exchange in a mineralized tissue.
capacitance vessel
A vein; thin-walled and distensible, holding roughly 60–70% of the blood volume as an adjustable reservoir.
capillary
The smallest vessel, one endothelial cell thick, where all exchange between blood and interstitial fluid occurs.
carbaminohemoglobin
Carbon dioxide bound to the globin portion of hemoglobin; carries about 20–25% of transported CO₂.
carbonic anhydrase
The enzyme that interconverts CO₂ and water with carbonic acid; central to CO₂ transport, gastric acid secretion, and renal acid handling.
cardiac cycle
One complete sequence of atrial and ventricular systole and diastole, lasting about 0.8 s at 75 beats per minute.
cardiac output (CO)
Volume of blood ejected by one ventricle per minute; heart rate times stroke volume, about 5 L/min at rest.
carrier protein
A transmembrane protein that binds a solute and changes conformation to move it; saturable, unlike a channel.
cartilage
Avascular connective tissue with a firm gel matrix and cells in lacunae; hyaline, elastic, and fibrocartilage are the three types. Its avascularity is why it heals so poorly.
catabolism
Metabolic reactions that break larger molecules into smaller ones, releasing energy.
catecholamine
An amine hormone or transmitter derived from tyrosine: dopamine, norepinephrine, epinephrine.
cation
A positively charged ion.
cecum
The blind pouch beginning the large intestine, bearing the appendix.
cell cycle
The ordered sequence G₁, S, G₂, and M by which a cell grows, replicates its DNA, and divides.
cellular respiration
The oxidation of fuel molecules to produce ATP, comprising glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.
central nervous system (CNS)
The brain and spinal cord.
centriole
A paired cylindrical organelle of nine microtubule triplets that organizes the mitotic spindle and forms the basal bodies of cilia.
cerebellum
The posterior brain region that coordinates timing, balance, and the accuracy of movement by comparing intended with actual motion.
cerebral cortex
The outer layer of gray matter of the cerebrum, seat of conscious sensation, voluntary movement, language, and reasoning.
cerebrospinal fluid (CSF)
The clear fluid produced by choroid plexus that circulates in the ventricles and subarachnoid space, cushioning the CNS and buffering its chemistry.
chemoreceptor
A receptor responding to chemical stimuli; central chemoreceptors sense CSF pH, peripheral ones in the carotid and aortic bodies sense arterial O₂, CO₂, and pH.
chief cell
In the stomach, the cell secreting pepsinogen; in the parathyroid gland, the cell secreting PTH.
cholecystokinin (CCK)
A duodenal hormone released by fat and protein that contracts the gallbladder, stimulates enzyme-rich pancreatic secretion, and slows gastric emptying.
cholesterol
A steroid lipid stabilizing membranes and serving as the precursor for all steroid hormones and bile salts.
chondrocyte
The cartilage cell, resident in a lacuna.
chorion
The outer fetal membrane; its villi form the fetal portion of the placenta.
chromatin
The complex of DNA and histone proteins in the nucleus; condenses into chromosomes at cell division.
chromosome
A condensed DNA–protein structure; humans have 46 in somatic cells, 23 in gametes.
chyme
The semifluid mixture of partly digested food and gastric secretions delivered to the duodenum.
cilia
Motile hair-like projections that sweep material along an epithelial surface, as in the respiratory tract and uterine tube. Contrast microvilli, which do not move and exist to increase surface area.
circadian rhythm
A roughly 24-hour biological cycle generated by the suprachiasmatic nucleus and entrained by light.
citric acid cycle (Krebs cycle)
The mitochondrial pathway that oxidizes acetyl-CoA, producing NADH, FADH₂, GTP, and CO₂.
clearance
The volume of plasma completely cleared of a substance per unit time; inulin clearance measures GFR.
clone
A population of genetically identical cells derived from one precursor; clonal selection and expansion underlie adaptive immunity.
coagulation
The cascade of plasma protein activation ending in fibrin formation, which stabilizes a platelet plug.
codominance
An inheritance pattern in which both alleles are fully expressed in the heterozygote, as in AB blood type.
codon
A three-nucleotide sequence in mRNA specifying one amino acid or a stop signal.
collagen
The most abundant protein in the body; a triple-helical fiber providing tensile strength to skin, bone, tendon, and ligament.
collecting duct
The final nephron segment, where ADH controls water reabsorption and aldosterone controls sodium, potassium, and hydrogen ion handling.
colloid osmotic pressure (oncotic pressure)
The osmotic pull exerted by plasma proteins, mainly albumin; the force that returns fluid to capillaries.
colon
The large intestine between cecum and rectum; absorbs water and electrolytes and houses the microbiome.
compact bone
Dense bone tissue organized into osteons, forming the outer shell of every bone. Contrast spongy (trabecular) bone.
complement
A cascade of about thirty plasma proteins that opsonize pathogens, recruit inflammatory cells, and form the membrane attack complex.
compliance
The change in volume produced by a given change in pressure; lung compliance falls in fibrosis, and arterial compliance falls with age.
concentration gradient
A difference in solute concentration between two regions; the driving force for diffusion.
conduction system
The specialized cardiac tissue — SA node, AV node, bundle of His, bundle branches, Purkinje fibers — that initiates and distributes each heartbeat.
cone
The retinal photoreceptor for color and high-acuity vision, concentrated in the fovea and requiring bright light. Contrast rod.
connective tissue
The most diverse tissue class: sparse cells in abundant extracellular matrix, providing support, binding, transport, and storage.
contractility
The intrinsic force of cardiac contraction at a given fiber length, independent of preload; raised by sympathetic stimulation.
contralateral
On the opposite side of the body.
control center
The component of a feedback loop that holds the set point, compares it with receptor input, and directs the response.
cornea
The transparent anterior covering of the eye that provides about two-thirds of its total refractive power.
coronary circulation
The arteries and veins supplying the myocardium; fills chiefly during diastole, which is why tachycardia threatens coronary perfusion.
corpus callosum
The great commissure of white matter connecting the two cerebral hemispheres.
corpus luteum
The progesterone-secreting structure formed from the ruptured follicle after ovulation; regresses in about 14 days unless rescued by hCG.
cortisol
The principal human glucocorticoid; raises blood glucose, is permissive for catecholamines, and suppresses inflammation.
countercurrent multiplier
The mechanism by which the loop of Henle's opposing limbs generate the medullary osmotic gradient that permits concentrated urine.
covalent bond
A chemical bond formed by shared electron pairs; strong and the basis of organic molecules.
cranial nerve
One of twelve paired nerves arising from the brain rather than the spinal cord.
creatine phosphate
A muscle energy store that rapidly regenerates ATP for the first ten seconds or so of intense activity.
crossing over
Exchange of segments between homologous chromosomes during prophase I of meiosis, generating new allele combinations.
cytokine
A signaling protein released by one cell to act on another, coordinating immune and inflammatory responses.
cytoplasm
Everything inside the plasma membrane except the nucleus: cytosol plus organelles.
cytoskeleton
The internal protein framework of microfilaments, intermediate filaments, and microtubules that gives a cell shape, movement, and internal transport.
cytotoxic T cell
A CD8-positive lymphocyte that kills virus-infected and tumor cells displaying antigen on MHC class I.
D
dead space
Airway volume that is ventilated but not perfused, so no gas exchange occurs; anatomical dead space is about 150 mL.
deamination
Removal of an amino group from an amino acid, producing ammonia that the liver converts to urea.
deep
Away from the body surface; internal. Contrast superficial.
defecation
Expulsion of feces, involving involuntary internal and voluntary external anal sphincters.
dehydration synthesis
A reaction joining two monomers with the removal of a water molecule; the reverse of hydrolysis.
dendrite
A branched neuronal process that receives signals and conducts them toward the cell body. Contrast axon.
dendritic cell
The most effective antigen-presenting cell; captures antigen in tissue and migrates to lymph nodes to activate naive T cells.
depolarization
A shift of membrane potential toward zero or positive values, making an excitable cell more likely to fire.
dermis
The connective tissue layer beneath the epidermis, containing vessels, nerves, glands, and follicles; papillary and reticular layers.
desmosome
A rivet-like cell junction of cadherins anchored to intermediate filaments, resisting mechanical stress in skin and heart muscle.
diapedesis
The squeezing of leukocytes between endothelial cells to leave a capillary and enter tissue.
diaphragm
The dome-shaped skeletal muscle separating thorax from abdomen; the principal muscle of inspiration, innervated by the phrenic nerve from C3–C5.
diaphysis
The shaft of a long bone. Contrast epiphysis, the expanded end.
diarthrosis
A freely movable joint; synonymous in practice with synovial joint.
diastole
The phase of the cardiac cycle during which a chamber relaxes and fills. Contrast.
diencephalon
The region containing thalamus, hypothalamus, and epithalamus.
differentiation
The process by which an unspecialized cell acquires a specialized structure and function.
diffusion
Net movement of particles from higher to lower concentration by random thermal motion; requires no energy.
digestion
The chemical and mechanical breakdown of food into absorbable units.
diploid
Having two sets of chromosomes (2n = 46 in humans); all somatic cells. Contrast.
disaccharide
A two-sugar carbohydrate: sucrose, lactose, maltose.
distal
Farther from the trunk or from the point of origin. Contrast proximal.
distal convoluted tubule
The nephron segment after the loop of Henle, site of PTH-regulated calcium reabsorption and the thiazide-sensitive NaCl cotransporter.
diuresis
Increased urine production.
DNA (deoxyribonucleic acid)
The double-helical molecule storing genetic information in the sequence of its four bases.
dopamine
A catecholamine neurotransmitter of movement, reward, and motivation; also the hypothalamic inhibitor of prolactin release.
dorsal body cavity
The cranial cavity and vertebral canal, containing the CNS.
dorsiflexion
Movement of the foot such that the toes rise toward the shin. Contrast.
duodenum
The first and shortest segment of the small intestine, receiving chyme, bile, and pancreatic juice; the main site of chemical digestion.
dura mater
The tough outermost meningeal layer.
dyspnea
The subjective sensation of difficult or uncomfortable breathing.
E
eccrine gland
The common sweat gland, secreting a dilute salt solution directly onto the skin for thermoregulation.
ectoderm
The outer primary germ layer, giving rise to epidermis and the entire nervous system.
edema
Excess fluid in the interstitial space, arising from raised capillary hydrostatic pressure, lowered plasma oncotic pressure, increased permeability, or blocked lymphatic drainage.
effector
The muscle or gland that carries out the response in a feedback loop.
efferent
Carrying away from a structure — motor neurons carry impulses away from the CNS, and the efferent arteriole carries blood away from the glomerulus. Contrast afferent.
ejection fraction
The percentage of end-diastolic volume ejected per beat; normally 55–70%.
elastin
A connective tissue protein that stretches and recoils, abundant in arteries, lung, and skin; its degradation with age explains both wrinkles and rising pulse pressure.
electrocardiogram (ECG)
A body-surface recording of the summed electrical activity of the heart; P wave for atrial depolarization, QRS for ventricular depolarization, T for ventricular repolarization.
electrolyte
A substance that dissociates into ions in solution and conducts electricity; clinically, the plasma cations and anions.
electron transport chain
The series of inner-mitochondrial-membrane complexes that pass electrons to oxygen while pumping protons to build the gradient driving ATP synthase.
emergence
The appearance at one level of organization of properties absent from the level below.
emulsification
The mechanical breakup of fat into small droplets by bile salts, increasing the surface area available to lipase.
endocardium
The endothelial lining of the heart chambers and valves.
endochondral ossification
Bone formation in which a hyaline cartilage model is replaced by bone; forms most of the skeleton. Contrast intramembranous ossification.
endocrine gland
A ductless gland secreting hormones into blood. Contrast exocrine gland, which secretes through a duct onto a surface.
endocytosis
Uptake of material by inward folding of the plasma membrane; phagocytosis, pinocytosis, and receptor-mediated endocytosis.
endoderm
The inner primary germ layer, giving rise to the gut epithelium and its derivatives — liver, pancreas, lungs.
endometrium
The uterine lining that thickens and is shed each menstrual cycle; its functional layer is lost and its basal layer regenerates it.
endoplasmic reticulum
A membrane network for synthesis; rough ER studded with ribosomes makes secreted and membrane proteins, smooth ER makes lipids and detoxifies.
endothelium
The simple squamous epithelium lining all blood vessels and the heart; an active regulator of tone, coagulation, and permeability.
enzyme
A biological catalyst, nearly always a protein, that lowers activation energy without being consumed.
eosinophil
A granulocyte that attacks parasites and participates in allergic reactions.
epidermis
The avascular keratinized stratified squamous epithelium forming the skin surface, in four or five layers.
epiglottis
The elastic cartilage flap that closes the laryngeal inlet during swallowing.
epinephrine (adrenaline)
The chief adrenal medullary catecholamine; raises heart rate and contractility, dilates bronchi, and mobilizes fuel.
epiphyseal plate
The hyaline cartilage growth plate between epiphysis and diaphysis; responsible for longitudinal growth and closed by sex steroids at puberty.
epithelial tissue
Sheets of tightly packed, polarized, avascular cells covering surfaces and forming glands, resting on a basement membrane.
equilibrium potential
The membrane voltage at which an ion's electrical and chemical driving forces exactly balance; about −97 mV for K⁺ and +67 mV for Na⁺.
erythrocyte
The red blood cell; a biconcave, anucleate, hemoglobin-filled disc surviving about 120 days.
erythropoietin (EPO)
The renal hormone that stimulates red cell production in response to tissue hypoxia.
esophagus
The muscular tube conducting food from pharynx to stomach by peristalsis.
estrogen
A class of steroid sex hormones, chiefly estradiol, driving female secondary characteristics, endometrial proliferation, and restraint of bone resorption.
eversion
Turning the sole of the foot laterally. Contrast inversion.
excitation–contraction coupling
The sequence linking a muscle action potential to cross-bridge cycling, via calcium release from the sarcoplasmic reticulum.
exocytosis
Fusion of a secretory vesicle with the plasma membrane to release contents outside the cell.
expiration
Breathing out; passive at rest, driven by elastic recoil.
extension
Increasing the angle at a joint, usually returning it toward anatomical position. Contrast flexion.
extracellular fluid (ECF)
Fluid outside cells — interstitial fluid plus plasma; the internal environment homeostasis defends.
extrinsic pathway
The coagulation route triggered by tissue factor from damaged tissue; faster than the intrinsic pathway and assessed by the prothrombin time.
F
facilitated diffusion
Passive movement of a solute through a channel or carrier down its gradient; requires no ATP but is saturable.
fascia
Sheets of connective tissue that wrap and separate muscles and organs.
fascicle
A bundle of muscle fibers or nerve axons enclosed by connective tissue.
fatty acid
A hydrocarbon chain with a terminal carboxyl group; saturated if it has no double bonds, unsaturated if it does.
feedback inhibition
Suppression of a pathway by its own product; the general principle behind most hormone control.
fertilization
Fusion of sperm and secondary oocyte to form a diploid zygote, normally in the ampulla of the uterine tube.
fetus
The developing human from the ninth week until birth; the embryonic period precedes it.
fibrin
The insoluble protein mesh formed from fibrinogen by thrombin that stabilizes a clot.
fibroblast
The principal cell of connective tissue proper, secreting collagen and ground substance; the cell that makes scar.
fibrocartilage
Cartilage with thick collagen bundles resisting compression and tension, as in intervertebral discs and menisci.
filtration
Movement of fluid and small solutes across a membrane driven by a hydrostatic pressure difference.
flagellum
A long motile projection; in humans, only the sperm tail.
flexion
Decreasing the angle at a joint.
follicle (ovarian)
An oocyte with its surrounding granulosa and theca cells; the functional unit of the ovary.
follicle-stimulating hormone (FSH)
An anterior pituitary glycoprotein driving follicular development in females and spermatogenesis in males.
fontanelle
An unossified fibrous membrane between infant skull bones, permitting molding at birth and brain growth after it.
foramen
A hole in a bone through which vessels or nerves pass.
fovea centralis
The cone-packed retinal pit of maximal visual acuity.
Frank–Starling law
The greater the ventricular filling, the greater the force of the next contraction, within limits; the mechanism that automatically matches output between the two ventricles.
frontal (coronal) plane
A vertical plane dividing the body into anterior and posterior parts.
functional residual capacity
The air remaining in the lungs after a normal quiet expiration; about 2,400 mL.
G
gallbladder
The sac that stores and concentrates bile, contracting in response to cholecystokinin.
gamete
A haploid reproductive cell — sperm or oocyte.
ganglion
A cluster of neuron cell bodies in the PNS. The CNS equivalent is a nucleus.
gap junction
A channel of connexons directly linking the cytoplasm of adjacent cells, permitting electrical coupling in cardiac and smooth muscle.
gastrin
An antral hormone that stimulates gastric acid secretion and motility; inhibited by falling antral pH.
gene
A DNA sequence encoding a functional product, usually a polypeptide.
gene expression
The transcription and translation of a gene into a functional product; the step most hormones and drugs regulate.
genotype
An individual's genetic makeup. Contrast phenotype, the expressed traits.
gestation
Pregnancy; about 40 weeks from the last menstrual period in humans.
ghrelin
The gastric hunger hormone; rises before meals and stimulates appetite and growth hormone release.
gland
An epithelial structure specialized for secretion.
glial cell (neuroglia)
Non-neuronal CNS and PNS support cells: astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells, and satellite cells.
glomerular filtration rate (GFR)
The volume of filtrate formed by all glomeruli per minute; about 125 mL/min, or 180 L/day.
glomerulus
The tuft of fenestrated capillaries within Bowman's capsule where filtration occurs.
glucagon
The alpha cell hormone that raises blood glucose by hepatic glycogenolysis and gluconeogenesis.
gluconeogenesis
Synthesis of glucose from non-carbohydrate precursors — lactate, glycerol, amino acids — chiefly in the liver.
glycocalyx
The sugar coat of membrane glycoproteins and glycolipids that mediates cell recognition and adhesion.
glycogen
The branched storage polysaccharide of liver and muscle; liver glycogen buffers blood glucose, muscle glycogen serves only that muscle.
glycogenolysis
Breakdown of glycogen to glucose-1-phosphate.
glycolysis
The cytosolic pathway splitting glucose into two pyruvate molecules with a net gain of 2 ATP; does not require oxygen.
goblet cell
A unicellular mucous gland in respiratory and intestinal epithelium.
Golgi apparatus
The stack of membranous cisternae that modifies, sorts, and packages proteins from the rough ER.
gonad
An ovary or testis; produces gametes and sex hormones.
gonadotropin-releasing hormone (GnRH)
The pulsatile hypothalamic peptide driving FSH and LH release; pulse frequency determines which predominates.
graded potential
A local, decremental change in membrane potential whose size varies with the stimulus; summation of graded potentials at the axon hillock determines whether an action potential fires.
gray matter
CNS tissue composed of neuron cell bodies, dendrites, and unmyelinated axons. Contrast white matter, which is myelinated tracts.
growth hormone (GH)
The anterior pituitary peptide that drives linear growth largely through hepatic IGF-1 and shifts fuel use toward fat.
gyrus
A ridge on the cerebral cortex. The groove between two gyri is a sulcus.
H
hair cell
The mechanoreceptor of the cochlea and vestibular apparatus; bends its stereocilia to open ion channels, and is not replaced when lost.
haploid
Having one set of chromosomes (n = 23); gametes only.
haversian system (osteon)
The cylindrical structural unit of compact bone: concentric lamellae around a central canal carrying vessels and nerves.
heart block
Impaired conduction between atria and ventricles, from first-degree delay to complete dissociation.
heart rate
Beats per minute; normally 60–100 at rest, set by the SA node and modulated by autonomic tone.
helper T cell
A CD4-positive lymphocyte that recognizes antigen on MHC class II and coordinates the entire adaptive response through cytokines.
hematocrit
The percentage of blood volume occupied by red cells; about 42% in men, 38% in women.
hematopoiesis
Blood cell formation, occurring in adult red bone marrow from a common multipotent stem cell.
hemoglobin
The four-subunit iron-containing protein of red cells that reversibly binds up to four oxygen molecules.
hemostasis
The arrest of bleeding: vascular spasm, platelet plug formation, and coagulation.
heparin
A natural anticoagulant released by mast cells and basophils that potentiates antithrombin.
hepatic portal system
The venous system that carries nutrient-rich blood from the gut to the liver before it reaches the systemic circulation, allowing first-pass processing.
hepatocyte
The liver cell; performs metabolism, detoxification, protein synthesis, and bile production.
heterozygous
Carrying two different alleles at a locus. Contrast homozygous.
hilum
The indented region of an organ where vessels, nerves, and ducts enter and leave.
histamine
An inflammatory mediator from mast cells and basophils causing vasodilation, increased capillary permeability, and bronchoconstriction; also stimulates gastric acid.
histology
The microscopic study of tissues.
homeostasis
Active maintenance of a relatively stable internal environment despite external change; a dynamic equilibrium, not a static one.
homologous chromosomes
The matched maternal and paternal copies of a chromosome, carrying the same genes at the same loci.
hormone
A chemical messenger released into blood by one tissue to alter the activity of distant cells bearing its receptor.
human chorionic gonadotropin (hCG)
The trophoblast hormone that rescues the corpus luteum in early pregnancy; the molecule detected by pregnancy tests.
hydrogen bond
A weak attraction between a hydrogen already bonded to an electronegative atom and another electronegative atom; individually weak but collectively responsible for water's properties and protein folding.
hydrolysis
Splitting a molecule by adding water; the chemistry of digestion.
hydrophilic
Water-attracting; polar or charged. Contrast hydrophobic.
hydrostatic pressure
The pressure a fluid exerts against a wall; the force driving filtration out of capillaries.
hypercapnia
An elevated arterial PCO₂, usually from inadequate alveolar ventilation.
hyperkalemia
A plasma potassium above 5.0 mEq/L; dangerous because it depolarizes excitable membranes and destabilizes cardiac conduction.
hyperplasia
An increase in tissue size caused by an increase in cell number. Contrast.
hypertonic
Having a higher effective osmotic concentration than the reference solution, so cells placed in it shrink.
hypertrophy
an increase caused by larger cells; the distinction matters because tissues with no capacity for division, such as cardiac muscle, can only hypertrophy.
hyperventilation
Ventilation in excess of metabolic need, lowering arterial PCO₂ and raising pH.
hypodermis (subcutaneous layer)
The fatty layer beneath the dermis; not technically part of the skin, but the layer that anchors it and stores energy.
hyponatremia
A plasma sodium below 135 mEq/L; usually a problem of excess water rather than lost sodium.
hypothalamus
The small diencephalic region controlling autonomic output, endocrine function through the pituitary, temperature, hunger, thirst, and circadian rhythm; the principal link between nervous and endocrine systems.
hypoxia
Inadequate oxygen delivery to tissue; distinguish from hypoxemia, a low arterial oxygen level, since anemia and cyanide poisoning cause hypoxia without hypoxemia.
I
I band
The light, isotropic region of a sarcomere containing only thin filaments; it narrows during contraction.
ileum
The final and longest segment of the small intestine; absorbs bile salts and vitamin B₁₂. Do not confuse with the ilium, a bone of the pelvis — the spelling differs by one letter and the organ system by two.
immunoglobulin
An antibody; the five classes are IgG, IgA, IgM, IgE, and IgD.
implantation
Embedding of the blastocyst in the endometrium, about six to seven days after fertilization.
inferior (caudal)
Below; away from the head.
inflammation
The innate vascular and cellular response to injury, producing redness, heat, swelling, pain, and loss of function; it isolates damage and initiates repair.
inhibin
A gonadal glycoprotein that selectively suppresses FSH without affecting LH, allowing gamete production to be regulated separately from steroid production.
innate immunity
The fast, non-specific, memoryless first line of defense: barriers, phagocytes, natural killer cells, complement, inflammation, and fever.
insertion
The muscle attachment that moves during contraction. Contrast origin, the attachment that stays relatively fixed.
inspiration
Breathing in; always active, driven by diaphragm and external intercostal contraction.
insulin
The beta cell hormone that lowers blood glucose by promoting cellular uptake via GLUT4 and by driving glycogenesis, lipogenesis, and protein synthesis.
insulin resistance
Reduced tissue response to insulin at normal concentrations, compensated initially by higher insulin output; the central lesion of type 2 diabetes and metabolic syndrome.
integral protein
A membrane protein embedded within the lipid bilayer, often spanning it and serving as a channel, carrier, or receptor.
integumentary system
The skin and its derivatives — hair, nails, and cutaneous glands.
intercalated disc
The junction between cardiac muscle cells, containing desmosomes for mechanical strength and gap junctions for electrical coupling.
interneuron
A CNS neuron connecting sensory and motor neurons; the great majority of all neurons.
interphase
The portion of the cell cycle between divisions, comprising G₁, S, and G₂.
interstitial fluid
The fluid between cells; about three-quarters of extracellular fluid.
intracellular fluid (ICF)
Fluid within cells; about two-thirds of total body water and potassium-rich.
intramembranous ossification
Bone formation directly within a fibrous membrane, forming the flat bones of the skull and the clavicles.
intrapleural pressure
The pressure in the pleural cavity, normally about −4 mm Hg relative to atmospheric; its negativity holds the lungs expanded.
intrinsic factor
A glycoprotein from gastric parietal cells required for vitamin B₁₂ absorption in the ileum; its loss causes pernicious anemia.
intrinsic pathway
The coagulation route triggered by contact with exposed collagen; assessed by the partial thromboplastin time.
inversion
Turning the sole of the foot medially.
ion
An atom or molecule with a net electrical charge from gaining or losing electrons.
ionic bond
An attraction between oppositely charged ions formed by electron transfer; dissociates in water.
ipsilateral
On the same side of the body. Contrast contralateral.
iris
The pigmented muscular diaphragm of the eye that controls pupil size.
ischemia
Inadequate blood flow to a tissue, causing both oxygen and substrate deprivation and waste accumulation.
islets of Langerhans
The endocrine clusters of the pancreas containing alpha, beta, delta, and F cells.
isometric contraction
Muscle tension rises without a change in muscle length. Contrast.
isotonic
Having the same effective osmotic concentration as the reference solution, so cells neither swell nor shrink.
isotonic contraction
in which length changes at constant tension.
isotope
An atom of an element with the usual number of protons but a different number of neutrons.
J
jaundice
Yellow discoloration of skin and sclera caused by accumulated bilirubin.
jejunum
The middle segment of the small intestine and the principal site of nutrient absorption.
joint (articulation)
The junction of two bones, classified structurally as fibrous, cartilaginous, or synovial and functionally by degree of movement.
juxtaglomerular apparatus
The structure where the distal tubule contacts its own afferent arteriole; the macula densa senses sodium delivery and granular cells release renin.
K
keratin
The tough fibrous protein of epidermis, hair, and nails that waterproofs and protects.
keratinocyte
The dominant epidermal cell, which produces keratin and migrates upward, dying to form the protective surface layer.
ketone body
Acetoacetate, β-hydroxybutyrate, or acetone; produced by the liver from fatty acids during fasting and usable by the brain when glucose is scarce.
kidney
The retroperitoneal organ that filters plasma and regulates fluid volume, electrolytes, acid–base status, blood pressure, red cell production, and vitamin D activation.
kilo
calorie, a thousand times larger.
kilocalorie
One thousand calories; the unit written as a capital-C Calorie on food labels.
Kupffer cell
The resident macrophage of the liver sinusoids, clearing bacteria and debris arriving from the gut.
L
lacteal
The lymphatic capillary in an intestinal villus that absorbs chylomicrons, which are too large to enter blood capillaries.
lactic acid (lactate)
The product of anaerobic glycolysis; can be reconverted to glucose in the liver via the Cori cycle.
lacuna
A small cavity in bone or cartilage matrix housing a single cell.
lamella
A concentric layer of bone matrix in an osteon.
larynx
The cartilaginous voice box between pharynx and trachea; houses the vocal folds and guards the airway during swallowing.
lateral
Away from the midline. Contrast medial.
leptin
The adipocyte hormone signaling long-term energy stores to the hypothalamus, suppressing appetite and permitting reproductive function.
leukocyte
A white blood cell; neutrophil, lymphocyte, monocyte, eosinophil, or basophil.
ligament
A band of dense regular connective tissue joining bone to bone. Contrast.
limbic system
The interconnected structures — amygdala, hippocampus, cingulate gyrus and others — governing emotion and the formation of new declarative memories.
lipase
A fat-digesting enzyme; pancreatic lipase, acting with bile salts and colipase, does most of the work.
lipid
A nonpolar biological molecule: triglycerides, phospholipids, steroids, and eicosanoids.
lipoprotein
A protein–lipid particle transporting insoluble lipids in plasma: chylomicron, VLDL, LDL, and HDL.
liver
The largest internal organ; processes portal blood, synthesizes plasma proteins, stores glycogen, detoxifies, and secretes bile.
loop of Henle
The hairpin nephron segment that generates the medullary osmotic gradient, making concentrated urine possible.
lower esophageal sphincter
The functional sphincter preventing reflux of gastric contents; its incompetence causes gastroesophageal reflux disease.
lumen
The interior space of a tube or hollow organ.
luteinizing hormone (LH)
The anterior pituitary glycoprotein whose mid-cycle surge triggers ovulation and which stimulates Leydig cell testosterone production.
lymph
The interstitial fluid that has entered lymphatic vessels; returns about 3 L of fluid and its protein to the bloodstream each day.
lymph node
A bean-shaped lymphoid organ that filters lymph and provides the site where antigen-presenting cells meet naive lymphocytes.
lymphatic system
The vessels, nodes, and lymphoid organs that return interstitial fluid to blood, absorb dietary fat, and house immune surveillance.
lymphocyte
The agranular leukocyte of adaptive immunity: B cells, T cells, and natural killer cells.
lysosome
The membrane-bound organelle containing acid hydrolases that digest ingested material and worn organelles.
lysozyme
An enzyme in tears, saliva, and sweat that breaks bacterial cell walls.
M
macrophage
A tissue phagocyte derived from a monocyte; clears debris and pathogens and presents antigen to helper T cells.
macula densa
The distal tubule cells that sense sodium chloride delivery and signal the granular cells to adjust renin release.
mast cell
A connective tissue cell releasing histamine and heparin; the trigger cell of immediate hypersensitivity.
matrix (extracellular)
The non-cellular material of connective tissue: ground substance plus fibers. Its character defines the tissue — fluid in blood, mineralized in bone, gel in cartilage.
mean arterial pressure (MAP)
The average arterial pressure over a cardiac cycle, approximated as diastolic pressure plus one third of the pulse pressure; the pressure that actually drives tissue perfusion.
mechanoreceptor
A sensory receptor responding to physical deformation — touch, stretch, pressure, vibration, or sound.
mediastinum
The central thoracic compartment between the pleural cavities, containing heart, great vessels, esophagus, trachea, and thymus.
medulla oblongata
The lowest brainstem region, containing the cardiovascular, respiratory, and vomiting centers; damage here is rapidly fatal.
meiosis
The two-division reduction process producing four haploid, genetically unique gametes. Contrast mitosis, one division producing two identical diploid cells.
melanin
The pigment made by melanocytes that colors skin and hair and absorbs ultraviolet radiation.
melatonin
The pineal hormone of biological night, secreted in darkness and suppressed by light.
membrane potential
The voltage difference across a plasma membrane, negative inside at rest.
memory cell
A long-lived lymphocyte generated in a primary response that permits the faster, larger secondary response.
meninges
The three connective tissue coverings of the CNS: dura, arachnoid, and pia mater.
menopause
The permanent cessation of menses following follicular depletion, diagnosed after twelve months of amenorrhea; estrogen falls and gonadotropins rise.
menstrual cycle
The roughly 28-day sequence of endometrial change driven by ovarian hormones: menstrual, proliferative, and secretory phases.
mesoderm
The middle germ layer, giving rise to muscle, bone, connective tissue, blood, kidney, and gonads.
metabolic rate
The energy consumed per unit time, usually measured as oxygen consumption.
metabolic syndrome
The clustering of abdominal obesity, hypertension, dyslipidemia, and impaired glucose regulation that multiplies cardiovascular risk.
metabolism
The sum of all chemical reactions in the body: catabolism plus anabolism.
metaphase
The mitotic stage in which chromosomes align at the cell equator.
MHC (major histocompatibility complex)
Cell-surface proteins that display peptide antigen; class I on all nucleated cells is read by cytotoxic T cells, class II on antigen-presenting cells is read by helper T cells.
micelle
A tiny bile-salt aggregate that ferries lipid digestion products to the enterocyte brush border.
microglia
The resident immune cells of the CNS; phagocytic and derived from the same lineage as macrophages.
microvilli
Non-motile membrane projections that multiply absorptive surface area, forming the brush border.
micturition
Urination; a reflex arc with voluntary override at the external urethral sphincter.
midbrain
The upper brainstem region containing the substantia nigra and the visual and auditory reflex centers of the colliculi.
mineralocorticoid
An adrenal steroid regulating sodium and potassium, principally aldosterone.
mitochondrion
The double-membraned organelle of oxidative phosphorylation, containing its own DNA and inherited exclusively from the mother.
mitosis
Nuclear division producing two genetically identical diploid daughter cells.
mitral (bicuspid) valve
The left atrioventricular valve.
molarity
Moles of solute per liter of solution. Contrast molality, moles per kilogram of solvent.
monocyte
The largest leukocyte; leaves blood to become a tissue macrophage.
monosaccharide
A single-sugar unit: glucose, fructose, galactose.
motor unit
One somatic motor neuron and all the muscle fibers it innervates; small units give fine control, large units give power.
mucosa
The moist epithelial membrane lining cavities open to the exterior, with its own underlying connective tissue and often a thin muscle layer.
muscle fiber
A single multinucleated skeletal muscle cell.
muscle spindle
The intrafusal stretch receptor that senses muscle length and triggers the stretch reflex.
muscle tone
The continuous low-level tension in resting muscle maintained by asynchronous motor unit firing.
mutation
A change in DNA sequence; may be silent, missense, nonsense, or a frameshift.
myelin
The lipid-rich insulating sheath formed by oligodendrocytes in the CNS and Schwann cells in the PNS, which speeds conduction by forcing saltatory propagation.
myocardium
The cardiac muscle layer of the heart wall, thickest in the left ventricle.
myofibril
A rod-like bundle of contractile filaments running the length of a muscle fiber, made of repeating sarcomeres.
myoglobin
The oxygen-binding pigment of muscle; higher affinity than hemoglobin, so it takes up oxygen from blood and releases it only at very low PO₂.
myosin
The thick filament motor protein whose heads bind actin, pivot, and release in the cross-bridge cycle.
N
natural killer (NK) cell
An innate lymphocyte that kills cells displaying too little MHC class I, such as virus-infected and tumor cells.
necrosis
Uncontrolled cell death from injury, releasing contents and provoking inflammation. Contrast apoptosis.
negative feedback
A control loop in which the response opposes the stimulus, returning the variable toward its set point; the basis of nearly all physiological regulation.
nephron
The functional unit of the kidney: renal corpuscle plus tubule, about one million per kidney.
nerve
A bundle of axons with connective tissue wrappings in the PNS. The CNS equivalent is a tract.
neurogenesis
Formation of new neurons; limited in adults to a few regions, notably the hippocampal dentate gyrus.
neuromuscular junction
The synapse between a somatic motor neuron and a skeletal muscle fiber, using acetylcholine on nicotinic receptors.
neuron
The excitable cell of the nervous system, specialized to generate and transmit electrical signals.
neurotransmitter
A chemical released from an axon terminal that changes the membrane potential or metabolism of a postsynaptic cell.
neutrophil
The most abundant leukocyte and the first phagocyte to arrive at infection; short-lived and forms the bulk of pus.
nociceptor
A receptor for tissue-damaging stimuli; the origin of pain.
node of Ranvier
A gap between myelin segments where voltage-gated sodium channels cluster, allowing saltatory conduction.
norepinephrine
The principal postganglionic sympathetic neurotransmitter and a CNS transmitter of arousal.
nucleic acid
DNA or RNA; a polymer of nucleotides carrying genetic information.
nucleolus
The nuclear region where ribosomal RNA is transcribed and ribosome subunits are assembled.
nucleotide
The monomer of nucleic acids: a five-carbon sugar, a phosphate, and a nitrogenous base.
nucleus (cell)
The membrane-bound organelle containing DNA and directing protein synthesis.
nucleus (CNS)
A cluster of neuron cell bodies within the CNS.
O
olfaction
The sense of smell; the only sensory modality that reaches the cortex without first relaying in the thalamus.
oligodendrocyte
The CNS glial cell that myelinates axons, each wrapping segments of many axons. Contrast the Schwann cell, which myelinates one segment of one axon.
oocyte
The developing female gamete, arrested in meiosis I from before birth until ovulation.
oogenesis
Formation of the ovum; produces one functional gamete and up to three polar bodies per primary oocyte.
opsonization
Coating a pathogen with antibody or complement to make phagocytosis far more efficient.
organ
A structure of at least two tissue types performing a specific function.
organ system
A group of organs cooperating toward a shared outcome; eleven in the human body.
organelle
A specialized subcellular structure with a defined function.
origin
The relatively fixed muscle attachment. Contrast insertion.
osmolality
Osmoles of solute per kilogram of water; what laboratories actually measure, and unaffected by the volume occupied by proteins and lipids.
osmolarity
Osmoles of solute per liter of solution; differs from osmolality by about 1% in body fluids, which is why the words are often used loosely and occasionally should not be.
osmoreceptor
A hypothalamic receptor detecting plasma osmolality and driving ADH release and thirst.
osmosis
Net diffusion of water across a selectively permeable membrane toward the higher solute concentration.
osmotic pressure
The pressure that would just prevent osmosis into a solution; proportional to the number of dissolved particles.
ossification
Bone formation, either endochondral or intramembranous.
osteoblast
The bone-forming cell, which secretes osteoid and then mineralizes it.
osteoclast
The large multinucleated bone-resorbing cell derived from the monocyte lineage; activated indirectly by PTH through RANKL on osteoblasts.
osteocyte
A mature osteoblast trapped in its own matrix; senses mechanical strain and directs remodeling.
osteoporosis
Loss of bone mass with normal mineralization, producing fragility fractures; accelerated by estrogen loss at menopause.
ovary
The female gonad, producing oocytes, estrogen, and progesterone.
ovulation
Release of a secondary oocyte from a mature follicle, triggered by the LH surge about 14 days before the next menses.
oxidative phosphorylation
ATP synthesis driven by the proton gradient generated by the electron transport chain.
oxygen debt
The elevated oxygen consumption after exercise, used to restore creatine phosphate, clear lactate, and replenish myoglobin and hemoglobin.
oxyhemoglobin dissociation curve
The sigmoid relationship between PO₂ and hemoglobin saturation; shifted right by acidity, CO₂, temperature, and 2,3-BPG, which unloads more oxygen exactly where tissue demand is highest.
oxytocin
The posterior pituitary peptide causing uterine contraction and milk ejection.
P
pacemaker potential
The slow spontaneous depolarization of SA nodal cells between action potentials that sets heart rate.
pancreas
The retroperitoneal organ with an exocrine role (digestive enzymes and bicarbonate) and an endocrine role (islet hormones).
parasympathetic division
The "rest and digest" autonomic subdivision, with craniosacral outflow, long preganglionic fibers, and acetylcholine at both synapses.
parathyroid hormone (PTH)
The chief regulator of plasma calcium; raises it by bone resorption, renal calcium reabsorption, and calcitriol activation, and lowers plasma phosphate.
parietal cell
The gastric cell that secretes hydrochloric acid and intrinsic factor.
parietal layer
The outer layer of a serous membrane, lining the cavity wall. Contrast the.
partial pressure
The pressure contributed by one gas in a mixture; the gradient that drives gas diffusion.
passive transport
Movement of a substance across a membrane without cellular energy: simple diffusion, facilitated diffusion, osmosis, and filtration.
pepsin
The gastric protein-digesting enzyme, activated from pepsinogen by acid and active only at low pH.
peptide bond
The covalent link between the amino group of one amino acid and the carboxyl group of the next.
perfusion
Blood flow through a tissue per unit time.
pericardium
The double-layered serous sac around the heart; its fibrous outer layer limits acute overdistension.
perimenopause
The transition of several years before menopause, marked by irregular cycles and fluctuating estrogen.
peripheral nervous system (PNS)
All nervous tissue outside the brain and spinal cord: cranial and spinal nerves and their ganglia.
peristalsis
Progressive waves of circular and longitudinal smooth muscle contraction that propel contents along a tube.
peritoneum
The serous membrane of the abdominal cavity.
pH
The negative logarithm of hydrogen ion concentration; arterial blood is normally 7.35–7.45, and each unit is a tenfold change.
phagocytosis
Engulfment of large particles by a cell, performed chiefly by neutrophils and macrophages.
pharynx
The muscular passage shared by the respiratory and digestive tracts, in naso-, oro-, and laryngopharyngeal parts.
phenotype
The observable expression of a genotype.
phospholipid
An amphipathic lipid with a polar head and two nonpolar tails; the molecule whose properties dictate the bilayer structure of every membrane.
photoreceptor
A retinal rod or cone that converts light into a change in membrane potential — by hyperpolarizing, uniquely among sensory receptors.
physiology
The study of how body structures function.
pia mater
The delicate innermost meningeal layer, adherent to the brain and cord surface.
pineal gland
The small diencephalic gland secreting melatonin in darkness.
pinocytosis
Non-selective uptake of extracellular fluid and its dissolved solutes.
pituitary gland
The gland in the sella turcica with an anterior lobe of glandular epithelium and a posterior lobe of neural tissue; controlled by the hypothalamus.
placenta
The temporary organ of maternal–fetal exchange and a major endocrine gland of pregnancy.
plasma
The straw-colored liquid matrix of blood, about 55% of blood volume. Contrast.
plasma cell
The antibody-factory descendant of an activated B cell.
plasma membrane
The phospholipid bilayer with embedded proteins that bounds every cell and controls what crosses.
platelet (thrombocyte)
A cell fragment from a megakaryocyte that adheres, aggregates, and provides the surface for coagulation.
pleura
The serous membrane of the lungs; the thin fluid film between its layers couples lung to chest wall.
plexus
A network of interlacing nerves or vessels, as in the brachial and lumbosacral plexuses.
pneumothorax
Air in the pleural cavity, abolishing the negative intrapleural pressure so the lung collapses.
polar covalent bond
A covalent bond with unequal electron sharing, giving partial charges; the reason water is a solvent.
polygenic inheritance
A trait determined by many genes of small effect interacting with the environment; height, blood pressure, and most common diseases.
polypeptide
A chain of amino acids joined by peptide bonds.
polysaccharide
A long carbohydrate polymer: starch, glycogen, cellulose.
pons
The brainstem region between midbrain and medulla; relays to the cerebellum and contains respiratory modulating nuclei.
positive feedback
A loop in which the response amplifies the stimulus until an external endpoint stops it; used in clotting, labor, and the action potential.
posterior (dorsal)
Toward the back of the body.
potential difference
A voltage; the separation of charge across a membrane.
preload
The degree of ventricular stretch at the end of filling, determined by end-diastolic volume and venous return. Contrast afterload.
pressure gradient
A difference in pressure between two points; the driving force for all bulk flow of blood and air.
primary active transport
Solute movement against a gradient powered directly by ATP hydrolysis, as by the Na⁺/K⁺-ATPase.
progesterone
The steroid of the corpus luteum and placenta that converts the endometrium to its secretory state and quiets the myometrium.
prolactin
The anterior pituitary hormone driving milk synthesis; uniquely held down by tonic dopamine inhibition rather than driven up by a releasing hormone.
pronation
Rotation of the forearm so the palm faces posteriorly. Contrast supination.
prone
Lying face down. Contrast supine.
prophase
The first mitotic stage: chromatin condenses, the nuclear envelope breaks down, and the spindle forms.
proprioception
The sense of body position and movement, from receptors in muscles, tendons, and joints.
prostaglandin
A locally acting eicosanoid mediating inflammation, pain, fever, uterine contraction, and gastric mucosal protection.
protein
A folded polypeptide; the structural and functional workhorse molecule of the cell.
proximal
Nearer to the trunk or point of origin.
proximal convoluted tubule
The nephron segment that reabsorbs about 65% of the filtrate, including nearly all glucose and amino acids.
pulmonary circulation
The low-pressure circuit from right ventricle through the lungs to left atrium. Contrast the systemic circulation.
pulse pressure
Systolic minus diastolic pressure; widens as arteries stiffen with age.
Purkinje fibers
The fast-conducting terminal fibers that deliver the impulse to ventricular myocardium from apex upward, so ejection proceeds toward the outflow valves.
Q
QRS complex
The ECG deflection of ventricular depolarization; atrial repolarization is buried within it.
quadrant
One of four abdominal divisions — right and left upper and lower — used for rapid clinical localization.
R
receptor (cellular)
A protein that binds a specific ligand and initiates a cellular response.
receptor (homeostatic)
The component of a feedback loop that monitors the regulated variable.
receptor (sensory)
A structure that detects a stimulus and converts it into a change in membrane potential.
recessive
An allele expressed only when homozygous. Contrast dominant.
red bone marrow
The hematopoietic tissue of flat bones and proximal long bone epiphyses in adults.
referred pain
Visceral pain perceived in a somatic region sharing the same spinal segments, because the cortex has no map for visceral origin; the reason cardiac ischemia is felt in the jaw and left arm.
reflex arc
The five-component pathway of an involuntary response: receptor, sensory neuron, integration center, motor neuron, effector.
refractory period
The interval after an action potential during which another cannot be generated (absolute) or requires a stronger stimulus (relative); the long cardiac refractory period prevents tetanus of the heart.
renal corpuscle
The glomerulus plus Bowman's capsule; the filtering unit.
renin
The enzyme released by juxtaglomerular cells that begins the renin–angiotensin– aldosterone cascade.
renin–angiotensin–aldosterone system (RAAS)
The linked hormonal cascade that defends blood pressure and extracellular volume by vasoconstriction and sodium retention; the loop that connects kidney, vessels, adrenal gland, and heart.
repolarization
Return of membrane potential toward its resting negative value after depolarization.
residual volume
The air remaining in the lungs after maximal expiration; cannot be measured by spirometry.
resistance (vascular)
Opposition to blood flow, determined chiefly by vessel radius, which enters the relationship to the fourth power — halving a radius raises resistance sixteenfold.
respiratory membrane
The 0.2–0.6 µm barrier of alveolar epithelium, fused basement membranes, and capillary endothelium across which gases diffuse.
resting membrane potential
The steady voltage of an unstimulated excitable cell; about −70 mV in neurons and −90 mV in skeletal muscle.
reticular formation
The brainstem network governing arousal, consciousness, and filtering of sensory input.
retina
The neural layer of the eye containing photoreceptors, bipolar cells, and ganglion cells whose axons form the optic nerve.
retroperitoneal
Lying behind the peritoneum rather than within the peritoneal cavity; describes the kidneys, adrenal glands, much of the pancreas and duodenum, and the ascending and descending colon.
ribosome
The RNA–protein machine that translates mRNA into polypeptide.
RNA (ribonucleic acid)
The single-stranded nucleic acid using uracil in place of thymine; messenger, transfer, and ribosomal forms carry out protein synthesis.
rod
The retinal photoreceptor for dim light and peripheral vision; highly sensitive but not color-discriminating. Contrast cone.
rugae
Folds in the wall of a distensible organ such as the stomach or bladder, which flatten as the organ fills.
S
sagittal plane
A vertical plane dividing the body into left and right parts; midsagittal if exactly on the midline.
saltatory conduction
Action potential propagation that jumps from node to node along a myelinated axon, greatly increasing speed.
sarcolemma
The plasma membrane of a muscle fiber.
sarcomere
The contractile unit between two Z discs; the repeating pattern that gives skeletal and cardiac muscle their striations.
sarcoplasmic reticulum
The specialized smooth ER of muscle that stores and releases the calcium controlling contraction.
satellite cell
The muscle stem cell that permits repair and hypertrophy of skeletal muscle; cardiac muscle has no useful equivalent, which is why heart damage is permanent.
Schwann cell
The PNS glial cell that myelinates a single axon segment and supports regeneration.
sebaceous gland
An oil gland, usually opening into a hair follicle, secreting sebum by holocrine secretion.
second messenger
An intracellular signal such as cAMP, IP₃, DAG, or Ca²⁺ generated when a water-soluble hormone binds its surface receptor.
secondary active transport
Solute movement against its gradient powered by another solute's gradient, itself maintained by ATP; symport and antiport.
secretin
The duodenal hormone released by acid that triggers bicarbonate-rich pancreatic secretion.
semen
Sperm plus the secretions of the seminal vesicles, prostate, and bulbourethral glands.
semilunar valve
The aortic or pulmonary valve; closes at the start of diastole, producing the second heart sound.
seminiferous tubule
The site of spermatogenesis within the testis, lined by Sertoli cells.
sensory adaptation
Declining receptor response to a constant stimulus, which is why you stop feeling your clothes.
serous membrane
A double-layered membrane with a lubricating fluid film between its visceral and parietal layers, enclosing a potential space around an organ.
Sertoli cell
The supporting cell of the seminiferous tubule; forms the blood–testis barrier, nourishes developing sperm, and secretes inhibin.
serum
which is plasma after clotting has removed fibrinogen.
set point
The target value a control system defends; deliberately shifted upward in fever.
sex chromosome
The X or Y chromosome; XX is female and XY male, with the SRY gene on Y directing testis formation.
sinoatrial (SA) node
The pacemaker of the heart, in the right atrial wall, with the fastest intrinsic rate at 60–100 per minute.
skeletal muscle
Voluntary, striated, multinucleated muscle attached to bone.
sliding filament model
The explanation of contraction in which thick and thin filaments slide past one another without changing length, shortening the sarcomere.
smooth muscle
Involuntary, non-striated, single-nucleus muscle of hollow organs and vessels; uses calmodulin rather than troponin.
sodium–potassium pump (Na⁺/K⁺-ATPase)
The primary active transporter that exports three sodium ions for every two potassium imported, maintaining the gradients on which membrane potential, secondary transport, and cell volume all depend.
solute
The dissolved substance in a solution. The dissolving medium is the solvent.
somatic nervous system
The voluntary motor division innervating skeletal muscle by a single myelinated neuron from CNS to target.
spermatogenesis
Sperm formation in the seminiferous tubules; produces four functional gametes per primary spermatocyte and continues throughout adult life.
sphincter
A ring of muscle that regulates passage through an opening.
spinal nerve
One of thirty-one paired mixed nerves arising from the spinal cord by dorsal sensory and ventral motor roots.
spirometry
Measurement of the volumes and flow rates of air moved in breathing.
spongy (trabecular) bone
Internal bone tissue of trabeculae aligned along lines of stress, housing red marrow and giving strength at low weight.
squamous
Flat and scale-like, of epithelial cell shape.
Starling forces
The four pressures — capillary and interstitial hydrostatic, capillary and interstitial oncotic — whose balance determines net capillary filtration or reabsorption.
stem cell
An unspecialized cell that can divide indefinitely and differentiate into one or more specialized types.
steroid
A lipid of four fused carbon rings derived from cholesterol; all steroid hormones and bile salts.
stimulus
A detectable change that provokes a response.
stroke volume
The blood ejected by one ventricle per beat; end-diastolic minus end-systolic volume, about 70 mL at rest.
substrate
The molecule an enzyme acts upon.
sulcus
A shallow groove, especially between cerebral gyri.
summation
Addition of stimuli in time (temporal) or space (spatial) — of graded potentials at a neuron, or of muscle twitches to produce stronger contraction.
supination
Rotation of the forearm so the palm faces anteriorly.
surfactant
The phospholipid–protein mixture from type II alveolar cells that lowers alveolar surface tension, preventing collapse and reducing the work of breathing; its absence causes neonatal respiratory distress syndrome.
suture
An immovable fibrous joint between skull bones.
sympathetic division
The "fight or flight" autonomic subdivision, with thoracolumbar outflow, short preganglionic and long postganglionic fibers, and norepinephrine at most target synapses.
synapse
The junction where a neuron communicates with another cell, chemically across a cleft or electrically through gap junctions.
synergist
A muscle that assists the prime mover or stabilizes a joint so the prime mover acts efficiently.
synovial joint
A freely movable joint with a fluid-filled cavity, articular cartilage, and a capsule.
systemic circulation
The high-pressure circuit from left ventricle to all body tissues and back to the right atrium.
systole
the contraction and ejection phase; the two words are constantly confused, and the memory hook is that diastole and dilate both begin with di-.
T
T lymphocyte (T cell)
The lymphocyte that matures in the thymus; helper, cytotoxic, and regulatory subsets direct and execute cell-mediated immunity.
T tubule
An invagination of the sarcolemma carrying the action potential into the fiber's interior so that all myofibrils are triggered together.
tachycardia
A resting heart rate above 100 beats per minute.
telophase
The final mitotic stage: chromosomes decondense and nuclear envelopes re-form.
tendon
which joins muscle to bone; the two look identical histologically and the distinction is purely about what is attached at each end.
testis
The male gonad, producing sperm in the seminiferous tubules and testosterone in the Leydig cells.
testosterone
The principal androgen; drives male secondary characteristics, spermatogenesis, and anabolism of muscle and bone.
tetanus (muscle)
Sustained maximal contraction produced when stimuli arrive faster than the muscle can relax; normal in skeletal muscle and impossible in cardiac muscle.
thalamus
The paired diencephalic relay through which nearly all sensory information passes on its way to the cortex — olfaction excepted.
thermoreceptor
A receptor detecting temperature change, peripherally in skin and centrally in the hypothalamus.
thermoregulation
Maintenance of core temperature near 37 °C by balancing heat production against heat loss.
threshold
The membrane potential, about −55 mV in neurons, at which enough voltage-gated sodium channels open to make an action potential inevitable.
thrombin
The enzyme that converts fibrinogen to fibrin and amplifies its own production.
thrombus
A clot formed within an intact vessel; a fragment that breaks free and travels is an embolus.
thymus
The mediastinal lymphoid organ where T cells mature and are selected; largest before puberty and progressively replaced by fat thereafter.
thyroid gland
The neck gland producing T₃ and T₄, which set metabolic rate, and calcitonin.
thyroid hormone
T₄ and the more active T₃; iodinated, lipid-soluble, and acting on nuclear receptors to raise metabolic rate.
tidal volume
The air moved in one quiet breath, about 500 mL.
tight junction
A continuous seal of transmembrane proteins between adjacent cells that prevents leakage between them; the reason the blood–brain barrier and the gut epithelium are selective.
tissue
A group of similar cells with a common origin performing a shared function; four primary types.
tonicity
The effect of a solution on cell volume, determined only by solutes that cannot cross the membrane.
trachea
The cartilage-ringed windpipe from larynx to the primary bronchi.
tract
A bundle of axons with a common origin and destination within the CNS.
transcription
Synthesis of messenger RNA from a DNA template in the nucleus.
translation
Synthesis of a polypeptide from an mRNA template at a ribosome.
transverse (horizontal, axial) plane
A plane dividing the body into superior and inferior parts; the orientation of a standard CT slice.
tricuspid valve
The right atrioventricular valve.
triglyceride
Three fatty acids esterified to glycerol; the body's principal energy store.
tropomyosin
The rod-shaped protein that covers actin's myosin-binding sites at rest.
troponin
The regulatory protein complex of the thin filament that binds calcium and moves tropomyosin off the myosin-binding sites; cardiac isoforms in blood are the marker of myocardial injury.
tubular reabsorption
Movement of filtered substances from tubule back into peritubular blood; recovers about 99% of the filtrate.
tubular secretion
Movement of substances from peritubular blood into the tubule; the route for many drugs, potassium, and hydrogen ion.
U
umbilical cord
The fetal lifeline of two arteries carrying deoxygenated blood away from the fetus and one vein carrying oxygenated blood toward it — the reverse of the usual adult convention.
urea
The nitrogenous waste made by the liver from ammonia; the principal route of nitrogen excretion and a contributor to the medullary gradient.
ureter
The muscular tube conducting urine from kidney to bladder by peristalsis.
urethra
The tube conducting urine from bladder to exterior; also carries semen in males.
urinary bladder
The distensible muscular reservoir for urine, lined by transitional epithelium that flattens as it fills.
uterine tube (fallopian tube)
The ciliated tube conveying the oocyte toward the uterus and the usual site of fertilization.
uterus
The muscular organ that houses and nourishes the embryo and fetus; endometrium, myometrium, and perimetrium.
V
vasa recta
The hairpin capillaries of the renal medulla that supply it without washing out the osmotic gradient.
vasoconstriction
Narrowing of a vessel lumen by smooth muscle contraction, raising resistance and reducing flow.
vasodilation
Widening of a vessel lumen, lowering resistance and increasing flow.
vein
A vessel carrying blood toward the heart, thin-walled, low-pressure, and often valved.
vena cava
The superior and inferior great veins returning systemic blood to the right atrium.
venous return
The volume of blood returning to the heart per minute; must equal cardiac output over time, and is aided by the skeletal muscle pump, the respiratory pump, and venous valves.
ventilation
Movement of air into and out of the lungs; distinguish from respiration, which in physiology means gas exchange or cellular oxidation.
ventral body cavity
The thoracic and abdominopelvic cavities, containing the viscera.
ventricle (brain)
One of four CSF-filled cavities of the brain.
ventricle (heart)
A lower pumping chamber; the left is about three times thicker-walled than the right because it faces a far higher-resistance circuit.
venule
The smallest vein, receiving blood from capillaries.
vesicle
A small membrane-bounded sac transporting material within or out of a cell.
vestibular apparatus
The inner ear structures — semicircular canals for rotational acceleration, utricle and saccule for linear acceleration and head position — that supply equilibrium.
villus
A finger-like projection of intestinal mucosa; villi, plicae, and microvilli multiply absorptive area roughly 600-fold.
visceral layer
The inner layer of a serous membrane, adhering to the organ surface.
vital capacity
The maximum air exhaled after a maximal inhalation; about 4,800 mL.
vitamin
An organic micronutrient required in small amounts and not synthesized in adequate quantity; fat-soluble A, D, E, and K are stored, water-soluble B and C are not.
voltage-gated channel
An ion channel that opens or closes in response to membrane potential; the basis of the action potential.
W
water
The solvent of life; its polarity and hydrogen bonding give high heat capacity, cohesion, and solvent properties on which all physiology depends.
white matter
CNS tissue of myelinated axon tracts. Contrast gray matter.
Wolff's law
Bone remodels in response to the mechanical loads placed on it, becoming denser where stressed and thinner where unloaded.
X
X-linked inheritance
Inheritance of a gene on the X chromosome; recessive X-linked conditions such as hemophilia A and red–green color blindness appear far more often in males, who have only one X.
Y
yellow bone marrow
The fat-storing marrow that progressively replaces red marrow in the long bones with age; can revert to hematopoietic marrow under severe demand.
Z
Z disc
The protein plate anchoring thin filaments and defining the boundaries of a sarcomere.
zona glomerulosa / fasciculata / reticularis
The three adrenal cortical zones, from outside in, producing aldosterone, cortisol, and adrenal androgens respectively — salt, sugar, and sex.
zygote
The single diploid cell formed at fertilization, from which the entire organism develops.