17. Blood

Composition, Function, Typing, and Hemostasis

Learning Objectives

17.1 Blood Is a Connective Tissue

Blood is the only fluid connective tissue in the body, and calling it connective tissue is not a technicality — it is the correct classification for exactly the reason all connective tissues are grouped together (Chapter 4): it consists of cells suspended in an extensive extracellular matrix that the cells themselves did not build in place. In bone the matrix is mineralized. In cartilage it is a stiff gel. In blood it is a liquid — plasma — and that single material property is why blood can do a job no other tissue can: it can move, carrying its cells and its dissolved cargo to every one of the roughly 37 trillion cells in the body.

There is also a developmental argument. Blood cells arise from the same mesodermal lineage as the other connective tissues, and they are produced inside a connective tissue — bone marrow.

Physical characteristics

Viscosity deserves a note that returns in Chapter 19. Resistance to flow rises steeply with viscosity, and viscosity rises steeply with hematocrit. A person with a hematocrit of 65% (polycythemia) has blood roughly twice as viscous as normal, so their heart must generate far more pressure for the same flow — which is why polycythemia causes hypertension, headache, and thrombosis rather than the improved oxygen delivery its extra hemoglobin might suggest. More red cells is not more oxygen delivered past a certain point, because delivery is content times flow, and viscosity attacks the flow term.

The three layers

        CENTRIFUGED BLOOD — a 10 mL tube spun at 3,000 g for 10 minutes

     ╔═══════════════════════════════╗  ─┐
     ║                               ║   │
     ║        P L A S M A            ║   │  55%  of total volume
     ║   straw-coloured liquid       ║   │  ────────────────────────
     ║                               ║   │  92%  water
     ║   • WATER            92%      ║   │   7%  plasma PROTEINS
     ║   • PROTEINS          7%      ║   │       albumin  ~60%
     ║       albumin  ~60%           ║   │       globulins ~36%
     ║       globulins ~36%          ║   │       fibrinogen ~4%
     ║       fibrinogen ~4%          ║   │   1%  everything else:
     ║   • OTHER SOLUTES     1%      ║   │       electrolytes, nutrients,
     ║                               ║   │       gases, wastes, hormones
     ╠═══════════════════════════════╣  ─┤
     ║ ▪▪▪ BUFFY COAT ▪▪▪            ║   │  < 1%  (about 0.1%)
     ║ leukocytes + platelets        ║   │  ─────────────────────
     ║ a thin greyish-white band     ║   │  4,500-11,000 WBC/µL
     ╠═══════════════════════════════╣  ─┤  150,000-400,000 platelets/µL
     ║                               ║   │
     ║                               ║   │  45%  = the HAEMATOCRIT
     ║    E R Y T H R O C Y T E S    ║   │  ──────────────────────────
     ║      (packed red cells)       ║   │  Female  36-46%
     ║                               ║   │  Male    41-53%
     ║   ~5 million cells per µL     ║   │  Newborn 45-61%
     ║   ~25 trillion in the body    ║   │
     ║                               ║   │  AMARA: 33%  ← LOW
     ╚═══════════════════════════════╝  ─┘

   THE ORDER IS DENSITY.  Red cells are densest and go to the bottom;
   plasma is least dense and stays on top; leukocytes and platelets are
   intermediate and form the thin layer between — which is why it is
   possible to harvest white cells and platelets separately at all.

   FORMED ELEMENTS = erythrocytes + leukocytes + platelets.
   Only leukocytes are complete cells: red cells have no nucleus and
   platelets are cell FRAGMENTS.

The seven functions

Transport (four). 1. Oxygen from lungs to tissues and carbon dioxide from tissues to lungs. 2. Nutrients from the gut and liver to every cell — glucose, amino acids, fatty acids. 3. Metabolic wastes to the organs that excrete them: urea and creatinine to the kidney, bilirubin to the liver, carbon dioxide to the lung. 4. Hormones from endocrine glands to their targets. Every mechanism in Chapter 16 depends on this one.

Regulation (two). 5. Body temperature. Blood absorbs heat from active muscle and liver and carries it to the skin, where it can be dumped. This is why blood at 38 °C is warmer than measured core temperature, and why skin blood flow is the body's principal thermal control. 6. pH and fluid balance. Plasma proteins and bicarbonate buffer acid; plasma osmotic and hydrostatic pressures determine how fluid is distributed between vessels and tissues.

Protection (one, in two parts). 7. Defense against blood loss (hemostasis, §17.7) and defense against pathogens (leukocytes and antibodies, §17.6 and Chapter 20).

17.2 Plasma

Plasma is 92% water by weight, and the remaining 8% is what makes it more than water.

The plasma proteins

Plasma proteins make up 7–9% of plasma and are, with the single exception of the immunoglobulins, made by the liver. They stay in the vessels because they are too large to cross a normal capillary wall in quantity — and that confinement is the whole point.

Albumin — 60% of plasma protein, about 3.5–5.0 g/dL. A single, relatively small protein present in enormous molar quantity. It does three jobs.

It generates colloid osmotic pressure. Because albumin cannot leave the capillary, it exerts an osmotic pull that opposes the hydrostatic pressure pushing fluid out. At the arterial end of a capillary, hydrostatic pressure exceeds colloid osmotic pressure and fluid filters out; at the venous end, colloid osmotic pressure exceeds hydrostatic pressure and most of that fluid is reabsorbed. This is Starling's principle, and it is developed fully in Chapter 19. Albumin accounts for roughly 80% of the total colloid osmotic pressure of plasma — about 25 mm Hg of the 28 mm Hg total.

Therefore: when albumin falls, fluid leaves the vessels and does not come back. In liver failure (albumin not made), nephrotic syndrome (albumin lost in urine), severe malnutrition, or protein-losing enteropathy, the result is edema — and in the abdomen, ascites. This is one of the most direct structure-to-symptom chains in the book: one protein's concentration determines where the body's water sits.

It is the body's general-purpose carrier. Albumin binds and transports free fatty acids, bilirubin, calcium, many hormones (§16.2), and a very large number of drugs. Two consequences: a hypoalbuminemic patient has a lower total calcium without a lower ionized calcium (§16.6), and a hypoalbuminemic patient has a higher free fraction of albumin-bound drugs, which is why doses of warfarin and phenytoin must be reconsidered when albumin is low.

Globulins — about 36%. Three families: - Alpha and beta globulins, made by the liver: transport proteins (transferrin for iron, haptoglobin for free hemoglobin, ceruloplasmin for copper, thyroxine-binding globulin, transcortin), plus clotting factors and complement. - Gamma globulins — the antibodies, made not by the liver but by plasma cells derived from B lymphocytes (Chapter 20). These are the only major plasma proteins with a non-hepatic origin, which is why liver failure spares antibody production.

Fibrinogen — about 4%, 200–400 mg/dL. A soluble precursor that thrombin converts into insoluble fibrin strands, the structural mesh of a blood clot (§17.7).

That last protein defines an important distinction. Plasma is the liquid fraction of blood that has been prevented from clotting, and it contains fibrinogen. Serum is the liquid that remains after blood has clotted, and it therefore contains everything plasma does except fibrinogen and the other consumed clotting factors. Laboratories care intensely about which tube a sample went into, and this is why.

Everything else

17.3 Erythrocytes

An erythrocyte is 7.5 µm across, 2.5 µm thick at its rim, and 1 µm thick at its center. There are about 5 million per microlitre and roughly 25 trillion in the body — some 84% of all the cells you have, by number. Their combined surface area is around 3,800 square meters, close to the area of a soccer pitch, all of it dedicated to gas exchange.

The biconcave disc, argued from first principles

        THE ERYTHROCYTE — FOUR CONSEQUENCES OF ONE SHAPE

   SIDE VIEW                          FACE VIEW
      2.5 µm                              ┌──── 7.5 µm ────┐
   ┌─────────┐                            ╭────────────────╮
   ╰─╮     ╭─╯  ← 1.0 µm at centre        │   ╭────────╮   │
     ╰─────╯                              │   │ pale   │   │  the pale centre
   ╰─────────╯                            │   │ centre │   │  on a smear IS
                                          │   ╰────────╯   │  the thin part —
   biconcave: thin in the middle,         ╰────────────────╯  1/3 of diameter
   thick at the rim

   ① SURFACE AREA                      ② DIFFUSION DISTANCE
   ────────────────────────────        ────────────────────────────
   Sphere of same volume:              No point in the cytoplasm is
      ~ 98 µm²                         more than ~0.5 µm from the
   Biconcave disc:                     membrane.
      ~ 136 µm²                        O₂ diffusion time ∝ distance²
   = 20-40% MORE surface for           so halving the distance cuts
   the SAME volume of cytoplasm        the time FOURFOLD.

   ③ DEFORMABILITY                     ④ EXCESS MEMBRANE
   ────────────────────────────        ────────────────────────────
   Must pass 3-5 µm capillaries        The disc has ~40% more membrane
   and 1-3 µm splenic slits.           than a sphere of its volume
   Spectrin-actin cytoskeleton         needs → it can change shape
   anchored to band 3 and              WITHOUT stretching the membrane.
   glycophorin lets it fold into       A sphere cannot deform at all
   a parachute/bullet shape and        without tearing: this is why
   spring back, ~170,000 times.        SPHEROCYTES are destroyed early.

   WHAT WAS DELETED, AND WHY
   ─────────────────────────────────────────────────────────────────────
   NO NUCLEUS       a large rigid organelle would block deformation;
                    removing it also frees ~10% more volume for
                    haemoglobin.  COST: cannot divide, cannot repair,
                    cannot synthesise new protein → lifespan 120 days.
   NO MITOCHONDRIA  a cell delivering O₂ must not BURN it.  ATP comes
                    from anaerobic glycolysis only.  COST: no oxidative
                    metabolism, total dependence on glucose.
   NO RIBOSOMES/ER  no protein renewal → enzymes decay with age → the
   (in the mature   cell slowly stiffens and its membrane deteriorates
    cell)           → the spleen removes it.  Senescence is BUILT IN.

   PACKED INSIDE:  ~250 million haemoglobin molecules per cell
                   = ~33 g/dL of cytoplasm (the MCHC) — near the
                   solubility limit.  The cell is essentially a
                   membrane bag of concentrated haemoglobin.

Hemoglobin

Hemoglobin is a tetramer: four globin polypeptide chains, each cradling one heme group, each heme holding one iron (Fe²⁺) atom at its center. Adult hemoglobin (HbA) is two alpha and two beta chains. One iron binds one O₂ molecule, so one hemoglobin carries four oxygen molecules, and one red cell with 250 million hemoglobins carries about a billion.

The four-subunit design is not redundancy. Binding of oxygen to the first subunit changes the conformation of the others so that they bind oxygen more readily — cooperative binding — and this is what produces the sigmoid oxygen–hemoglobin dissociation curve that lets the molecule load almost completely in the lung and unload substantially in a tissue. A single-chain oxygen carrier could not do that. The full argument belongs to Chapter 22; note here that the shape of the molecule dictates the shape of the curve, which dictates how much oxygen a tissue gets.

Hemoglobin also carries about 20% of the body's carbon dioxide (bound to globin's amino groups as carbaminohemoglobin, not to iron) and binds nitric oxide.

Normal values: 12.0–15.5 g/dL (women), 13.5–17.5 g/dL (men). Each gram of hemoglobin binds about 1.34 mL of oxygen, so at 15 g/dL, 100 mL of fully saturated blood carries about 20 mL of oxygen. Amara at 10.8 g/dL carries about 28% less oxygen per unit volume than she would at 15 — in a woman whose coronary arteries are already failing to deliver enough. That number is not a laboratory curiosity; it is a direct subtraction from the oxygen reaching her myocardium.

Iron: too dangerous to waste, too dangerous to leave loose

Free iron catalyzes the production of hydroxyl radicals, so the body never lets any circulate unescorted. Every stage of iron handling is a chaperoned transfer.

• Absorption. Only in the duodenum, and only about 1–2 mg/day — roughly 10% of dietary intake, and much less from plant sources than from heme iron in meat. The rate-limiting control is the liver hormone hepcidin, which blocks iron export from enterocytes and macrophages. Hepcidin rises with inflammation. Remember that sentence; it answers Amara's first question.
• Transport. Bound to transferrin in plasma. Transferrin saturation is normally 20–50%; Amara's is 11%.
• Storage. As ferritin (soluble, readily mobilized) and hemosiderin (insoluble aggregates) in liver, spleen, and macrophages. Serum ferritin normally reflects stores — except that ferritin is also an acute-phase protein that rises in inflammation regardless of iron. Amara's ferritin is 186 ng/mL, apparently generous, while her transferrin saturation says she is functionally iron-deficient. Both are true.
• Recycling. About 25 mg/day, more than twenty times what is absorbed, comes from breaking down old red cells. The body has no route for excreting iron other than losing cells — desquamated skin and gut epithelium, menstrual blood, hemorrhage. This is why iron overload from repeated transfusion is a real and serious problem, and why the whole system is built around recycling rather than intake.

Lifespan, destruction, and where jaundice comes from

Without ribosomes, a red cell cannot replace its enzymes. Over about 120 days its antioxidant defenses fail, its membrane stiffens, and it can no longer squeeze through the narrow slits between endothelial cells in the splenic red pulp. Macrophages there — and in the liver and marrow — engulf it. About 2.5 million red cells are destroyed every second, and exactly that many are made.

The dismantling is a model of thrift:

1. Globin chains are hydrolyzed to amino acids and returned to the general pool.
2. Iron is stripped from heme, bound to transferrin, and returned to the marrow. Recycled.
3. The heme ring is the only part that cannot be reused. It is opened to biliverdin (green) and reduced to bilirubin (yellow-orange).

Bilirubin is lipid-soluble and toxic, so it is bound to albumin and carried to the liver, where hepatocytes conjugate it with glucuronic acid, making it water-soluble. Conjugated bilirubin is excreted in bile into the gut, where bacteria convert it to urobilinogen; most becomes stercobilin and colors the feces brown, while a small absorbed fraction is excreted by the kidney as urobilin and colors urine yellow.

Jaundice is a yellow discoloration of skin and sclerae that appears when plasma bilirubin exceeds roughly 2–3 mg/dL (normal < 1.2). The pathway above tells you there must be exactly three places it can go wrong, and there are:

Pale stools and dark urine together are a mechanical statement: bile is not reaching the intestine. You can derive that from the pathway without memorizing it.

17.4 Erythropoiesis and Its Control

All formed elements descend from a single ancestor: the hematopoietic stem cell (HSC, or hemocytoblast) in red bone marrow. It is pluripotent — capable of becoming any blood cell — and it is self-renewing, dividing so that one daughter differentiates and one remains a stem cell. That second property is what makes marrow transplantation possible and what makes marrow the most radiation-sensitive tissue in the body.

The lineage splits early into a myeloid line (red cells, platelets, granulocytes, monocytes) and a lymphoid line (lymphocytes). Erythropoiesis proceeds:

hematopoietic stem cell → myeloid progenitor → proerythroblast → early, intermediate, and late erythroblasts (progressive hemoglobin accumulation; the nucleus condenses) → reticulocyte (nucleus extruded; ribosomal RNA remains) → erythrocyte

The whole sequence takes about 15 days, of which the last 1–2 days are spent as a reticulocyte. Reticulocytes are released into blood and mature over roughly 24 hours as their residual ribosomes are degraded. Because they are the newest cells, the reticulocyte count is a direct measurement of marrow output — and it is the single most informative number in the workup of an anemia, because it separates "the marrow is not making enough" from "something is destroying or losing what the marrow makes."

The erythropoietin loop

            THE ERYTHROPOIETIN NEGATIVE FEEDBACK LOOP

    ┌──────────────────────────────────────────────────────────────┐
    │  THE REGULATED VARIABLE:                                     │
    │  OXYGEN DELIVERY TO TISSUE  =  [Hb] × SaO₂ × blood flow      │
    │  (NOT red cell count — note this carefully)                  │
    └──────────────────────────┬───────────────────────────────────┘
                               │ falls because of:
       ┌───────────────────────┼───────────────────────┐
       ▼                       ▼                       ▼
   fewer RBCs             less O₂ available        less flow
   (haemorrhage,          (altitude, lung           (heart failure,
    haemolysis)            disease, sleep apnoea)    arterial stenosis)
       └───────────────────────┼───────────────────────┘
                               ▼
    ╔══════════════════════════════════════════════════════════════╗
    ║  RECEPTOR + CONTROL CENTRE                                   ║
    ║  PERITUBULAR FIBROBLASTS of the RENAL CORTEX (~90%)          ║
    ║  and hepatocytes (~10%)                                      ║
    ║                                                              ║
    ║  Mechanism: HYPOXIA-INDUCIBLE FACTOR (HIF-1α)                ║
    ║   • normal O₂ → HIF-1α is hydroxylated and DESTROYED         ║
    ║   • low O₂    → hydroxylation stops → HIF-1α SURVIVES        ║
    ║                 → enters nucleus → transcribes the EPO gene  ║
    ╚═════════════════════════════┬════════════════════════════════╝
                                  │ EFFERENT SIGNAL
                                  ▼
                  ╔═══════════════════════════════╗
                  ║   ERYTHROPOIETIN (EPO)        ║
                  ║   a glycoprotein hormone      ║
                  ║   plasma level can rise       ║
                  ║   1,000-fold in severe hypoxia║
                  ╚═══════════════┬═══════════════╝
                                  ▼
    ╔══════════════════════════════════════════════════════════════╗
    ║  EFFECTOR:  RED BONE MARROW                                  ║
    ║   • rescues erythroid progenitors from apoptosis             ║
    ║   • speeds maturation and haemoglobin synthesis              ║
    ║   • releases reticulocytes early                             ║
    ║   REQUIRES RAW MATERIALS:  IRON · VITAMIN B12 · FOLATE       ║
    ║                            + protein, B6, copper             ║
    ║   LATENCY: reticulocytes rise in 3-5 days;                   ║
    ║            haemoglobin rises over 2-4 WEEKS                  ║
    ╚═════════════════════════════┬════════════════════════════════╝
                                  ▼
                    MORE ERYTHROCYTES → ↑ [Hb]
                                  ▼
                    ↑ OXYGEN DELIVERY TO TISSUE
                                  │
      NEGATIVE FEEDBACK ──────────┘
      Restored delivery → HIF-1α destroyed again → EPO falls

  WHERE THIS LOOP BREAKS
  ─────────────────────────────────────────────────────────────────
  KIDNEY DISEASE     control centre destroyed → EPO not made
                     → anaemia despite normal marrow and iron
  IRON / B12 / FOLATE effector cannot execute → EPO HIGH, output LOW
  INFLAMMATION       hepcidin locks iron in stores; EPO response blunted
  SENSOR FOOLED      a renal artery stenosis makes ONE kidney think the
                     whole body is hypoxic → EPO high → polycythaemia

Two features of this loop are worth stating explicitly because students routinely get them wrong.

The kidney does not measure red cells. It measures its own oxygenation. That is why anemia, altitude, chronic lung disease, obstructive sleep apnea, and a narrowed renal artery all raise EPO, and why a normal red cell count does not guarantee a normal EPO. It is the same design principle as renin (§16.7): the kidney is a very good local oxygen and perfusion sensor that the body uses as a proxy for systemic conditions — and, as with renin, the proxy can be wrong.

Latency is set by biology, not by the hormone. Even a maximal EPO signal cannot produce hemoglobin faster than the marrow can build cells. Reticulocytes appear in 3–5 days; hemoglobin takes 2–4 weeks. This is why transfusion, not EPO, is the treatment for acute severe anemia.

The raw materials

The mechanistic reason the two megaloblastic deficiencies produce large cells is worth a sentence, because it makes the classification make sense. B12 and folate are required for DNA replication. Hemoglobin and cytoplasmic protein synthesis, however, do not need them. So the cell keeps growing while its nuclear divisions are delayed — the result is a cell that has enlarged more than it has divided. Iron deficiency is the exact opposite: divisions proceed normally but each daughter has too little hemoglobin, so cells come out small. The MCV is therefore a direct readout of which process failed, which is why it organizes the entire diagnostic approach in §17.5.

17.5 Anemia and Polycythemia

Anemia is a reduction in the oxygen-carrying capacity of blood — defined operationally as hemoglobin below 12.0 g/dL in women or 13.5 g/dL in men. It is a sign, not a diagnosis, and the clinical task is always to find out which mechanism produced it.

The symptoms are the same regardless of cause, and they are all consequences of reduced oxygen delivery plus the body's compensation: fatigue, pallor (best seen in the conjunctivae and palmar creases), dyspnea on exertion, tachycardia, palpitations, headache, and — where the circulation is already compromised — angina. That last one matters here. In a patient with coronary disease, anemia converts a borderline oxygen supply into an inadequate one, and correcting anemia can relieve chest pain that no cardiac intervention touched.

Classification by mechanism

1 · Blood loss. Acute — hemorrhage. Note that immediately after acute bleeding the hematocrit is normal, because whole blood was lost: cells and plasma in the same proportion. The hematocrit only falls over the following hours as interstitial fluid moves in to restore volume. A normal hemoglobin in the first hour after major trauma is meaningless. Chronic — slow gastrointestinal or menstrual loss, which drains iron faster than it can be absorbed and therefore becomes an iron-deficiency (microcytic) anemia.

2 · Insufficient production. Missing raw materials (iron, B12, folate); missing signal (renal failure, low EPO); missing or replaced marrow (aplastic anemia, leukemia, metastatic infiltration, myelofibrosis); or defective globin synthesis (thalassemia).

3 · Excessive destruction (hemolysis). Intrinsic — inherited defects of the membrane (hereditary spherocytosis), of enzymes (G6PD deficiency), or of hemoglobin itself (sickle cell, thalassemia). Extrinsic — autoimmune hemolysis, mechanical destruction on a prosthetic heart valve, malaria, transfusion reaction. The laboratory signature of hemolysis is consistent and derivable from §17.3: high reticulocytes (marrow compensating), high unconjugated bilirubin and jaundice (heme being broken down), high lactate dehydrogenase (an intracellular enzyme spilling out), and low haptoglobin (the plasma protein that mops up free hemoglobin is consumed).

The MCV approach

Anemia of chronic disease — and Amara

The commonest anemia in hospitalized patients, and the second commonest worldwide after iron deficiency, is anemia of chronic disease (also called anemia of inflammation). It is typically normocytic, moderate (hemoglobin 9–11 g/dL), and it appears in the setting of any sustained inflammatory state: infection, autoimmune disease, malignancy, chronic kidney disease, heart failure, and — importantly for this book — atherosclerosis and metabolic syndrome, which are themselves chronic low-grade inflammatory conditions (Chapter 16, Chapters 19 and 20).

Three mechanisms operate together, and all three are driven by inflammatory cytokines, principally interleukin-6:

1. Iron is sequestered. IL-6 stimulates hepatic production of hepcidin, which degrades ferroportin — the only iron export channel — on enterocytes and macrophages. Dietary iron absorption falls and recycled iron is locked inside macrophages. The result is a functional iron deficiency: total body iron is normal or high, but the marrow cannot get any. Serum ferritin is normal or high (it reflects trapped stores, and is itself an acute-phase protein), while transferrin saturation is low. This is an evolutionary defense — most bacteria require iron to grow, so withholding iron from plasma is an antimicrobial strategy. It becomes maladaptive when the inflammation is sterile and chronic.
2. Erythropoietin response is blunted. Inflammatory cytokines suppress both renal EPO production and the marrow's responsiveness to it.
3. Red cell survival is modestly shortened. Activated macrophages clear red cells slightly more aggressively.

This is Amara's anemia, and the laboratory pattern in her Case File table fits it exactly: hemoglobin 10.8, normocytic at MCV 88, an inappropriately unimpressive reticulocyte count of 0.9%, a high ferritin of 186 ng/mL alongside a low transferrin saturation of 11%. The full argument is assembled in the Case File Resolution, along with the second contributor — her kidneys.

Polycythemia

The opposite condition: an abnormally high red cell mass, hematocrit above roughly 49% in men or 48% in women, raising viscosity and thrombotic risk.

• Primary (polycythemia vera). A myeloproliferative neoplasm, usually driven by a JAK2 mutation that makes erythroid progenitors respond as though EPO were present when it is not. Hematocrit can exceed 60%. EPO is low, because the loop is trying to switch off a process that is no longer listening to it.
• Secondary. An appropriate response to genuine hypoxia — altitude, chronic lung disease, cyanotic heart disease, obstructive sleep apnea, heavy smoking (carbon monoxide occupies hemoglobin binding sites). EPO is high, appropriately.
• Inappropriate secondary. EPO-secreting tumours, renal artery stenosis, or exogenous EPO. EPO is high, inappropriately.
• Relative. Plasma volume is reduced by dehydration or diuretics; red cell mass is normal. The hematocrit is a ratio, and this raises it by shrinking the denominator.

Note the diagnostic logic: measure the hormone alongside the variable it controls, and the pattern localizes the lesion. That is §16.10 again, in a different system.

17.6 Leukocytes

Leukocytes are the only complete cells in blood — nucleated, organelle-bearing, capable of protein synthesis and movement. There are 4,500–11,000 per microlitre, roughly one for every 700 red cells, and they are the least numerous formed element and the most consequential per cell.

Two properties define what they do. Diapedesis — they squeeze between endothelial cells to leave the bloodstream. Chemotaxis — they migrate along chemical gradients toward damaged tissue or invading organisms. Together these mean that blood is not where leukocytes work; it is how they commute. A neutrophil in a blood sample is a neutrophil in transit. This single fact explains most of what a white count means, and it is the key to Amara's second question.

They divide into granulocytes (visible cytoplasmic granules, lobed nuclei) and agranulocytes (no visible granules, unlobed nuclei).

        THE FIVE LEUKOCYTES — "Never Let Monkeys Eat Bananas"
        (order of decreasing abundance)

 ╔══════════════════════════════════════════════════════════════════════╗
 ║ NEUTROPHIL              50-70%   (2,000-7,000/µL)   10-12 µm         ║
 ║  ╭──────────╮   nucleus: 3-5 LOBES joined by thin threads            ║
 ║  │ ⊂⊃-⊂⊃-⊂⊃ │   granules: FINE, PALE LILAC (hence "neutro-")         ║
 ║  ╰──────────╯   cytoplasm: pale pink, looks almost empty             ║
 ║  JOB: first responder to BACTERIA. Phagocytosis + respiratory burst  ║
 ║       + NETs.  Dies doing it — dead neutrophils = PUS.               ║
 ║  LIFE: 6-8 HOURS in blood, 1-2 days in tissue.  ~100 billion made/day║
 ╠══════════════════════════════════════════════════════════════════════╣
 ║ LYMPHOCYTE              20-40%   (1,000-4,000/µL)   6-14 µm          ║
 ║  ╭──────────╮   nucleus: LARGE, ROUND, DARK — fills the cell,        ║
 ║  │  ●●●●●●  │            leaving only a THIN RIM of blue cytoplasm   ║
 ║  ╰──────────╯   granules: none visible                               ║
 ║  JOB: ADAPTIVE immunity.  B cells → antibodies; T cells → kill       ║
 ║       infected cells and direct the response; NK cells → kill        ║
 ║       tumour and virus-infected cells (Ch. 20)                       ║
 ║  LIFE: HOURS to DECADES — memory cells can outlive the infection     ║
 ║        by a lifetime.  The only leukocyte that RECIRCULATES.         ║
 ╠══════════════════════════════════════════════════════════════════════╣
 ║ MONOCYTE                 2-8%    (100-700/µL)       12-20 µm LARGEST ║
 ║  ╭──────────╮   nucleus: KIDNEY-SHAPED or horseshoe, indented        ║
 ║  │  ⌒⌒⌒⌒⌒   │   cytoplasm: ABUNDANT, dull blue-grey, often vacuolated║
 ║  ╰──────────╯   granules: none visible                               ║
 ║  JOB: leaves blood → becomes a MACROPHAGE or dendritic cell.         ║
 ║       Phagocytosis of large debris; ANTIGEN PRESENTATION; chronic    ║
 ║       infection; clears dead neutrophils and dead myocytes.          ║
 ║  LIFE: ~1-3 days in blood; MONTHS to YEARS as a tissue macrophage.   ║
 ╠══════════════════════════════════════════════════════════════════════╣
 ║ EOSINOPHIL               1-4%    (30-350/µL)        10-14 µm         ║
 ║  ╭──────────╮   nucleus: BILOBED — "spectacles" or a telephone       ║
 ║  │ ⊂⊃══⊂⊃   │   granules: COARSE, BRIGHT RED-ORANGE, uniform         ║
 ║  ╰──────────╯   (eosin is an acidic red dye; these granules are      ║
 ║                  basic, so they take it up avidly)                   ║
 ║  JOB: PARASITES too big to phagocytose — releases toxic proteins     ║
 ║       onto them.  Also mediates allergic inflammation and asthma.    ║
 ║  LIFE: ~8-12 hours in blood; days in tissue.                         ║
 ╠══════════════════════════════════════════════════════════════════════╣
 ║ BASOPHIL              0.5-1%     (10-60/µL)  RAREST  8-10 µm         ║
 ║  ╭──────────╮   nucleus: bilobed but usually HIDDEN by granules      ║
 ║  │ ██████   │   granules: COARSE, VERY DARK BLUE-PURPLE, obscure     ║
 ║  ╰──────────╯            the whole cell                              ║
 ║  JOB: releases HISTAMINE (vasodilation, ↑ permeability) and          ║
 ║       HEPARIN (anticoagulant).  Tissue counterpart = MAST CELL.      ║
 ║       Immediate hypersensitivity and anaphylaxis.                    ║
 ║  LIFE: hours to a few days.                                          ║
 ╚══════════════════════════════════════════════════════════════════════╝

  GRANULOCYTES = neutrophil, eosinophil, basophil (lobed nuclei, granules)
  AGRANULOCYTES = lymphocyte, monocyte (unlobed nuclei, no visible granules)

  AMARA'S DIFFERENTIAL:  WBC 11.4 K/µL · Neutrophils 78% (8.9 K) ↑
                         Lymphocytes 14% ↓ · no infection anywhere

Leukopoiesis and the differential

All leukocytes arise from the same hematopoietic stem cell, under the control of colony stimulating factors and interleukins released by macrophages and T cells during infection — which is a feedback loop of the familiar shape: the infection stimulates the cells that stimulate production of more cells to fight the infection. Granulocyte colony-stimulating factor (G-CSF) is used clinically to accelerate neutrophil recovery after chemotherapy.

Reading a differential requires one discipline: always convert percentages to absolute counts. A neutrophil percentage of 78% in a patient with a white count of 11.4 K/µL means 8,900 neutrophils per microlitre; the same 78% in a patient with a white count of 2.0 K/µL means 1,560, which is neutropenia. The percentage alone is a ratio and can move because another cell line moved.

Neutrophilia without infection: demargination and necrosis

Two mechanisms raise the neutrophil count within hours, and neither requires a pathogen.

1 · Demargination. At any moment, roughly half the neutrophils in the vasculature are not circulating freely: they are rolling slowly along the endothelium of small vessels, loosely adherent, in the marginal pool. A blood sample draws only from the circulating pool, so the count reported is about half the total present. Anything that increases blood flow velocity or reduces adhesion — catecholamines, cortisol, exercise, acute stress, pain — sweeps the marginated cells back into the circulating pool. The white count can double within minutes, with no new cell made and no infection anywhere. This is why a frightened child, a patient in pain, or anyone in the first hours of a myocardial infarction can show a leukocytosis that means nothing about bacteria. Cortisol also demarginates neutrophils while sequestering lymphocytes into lymphoid tissue, which is why the steroid pattern is neutrophilia with lymphopenia — exactly the pattern on Amara's differential.

2 · Sterile inflammation from necrosis. When cells die by necrosis rather than apoptosis they spill their contents, and those contents are damage-associated molecular patterns — DNA, ATP, uric acid, HMGB1, mitochondrial fragments — that innate immune receptors read as danger. Resident macrophages and injured endothelium release IL-1, IL-6, TNF-α, and CXCL8, which up-regulate endothelial adhesion molecules and recruit neutrophils from the marrow within hours. The response is indistinguishable, in its first day, from the response to bacteria, because it uses the same receptors and the same mediators. It just has no organism in it.

Infarcted myocardium is a large mass of necrotic tissue, and the neutrophil influx that follows is the opening phase of an inflammatory sequence — neutrophils to debride, then monocytes and macrophages to clear debris and coordinate repair, then fibroblasts to lay down collagen scar — that Chapter 20 develops in full and Chapter 18 evaluates in terms of what it does to the ventricle. This is the answer to Amara's second question, and the full argument is in the Resolution.

17.7 Platelets and Hemostasis

Hemostasis is the arrest of bleeding. It must be fast, local, and self-limiting — and those three requirements explain everything about its design. Fast, because a breach in a high-pressure system empties quickly. Local, because a clot that spread would occlude the vessel and infarct the tissue. Self-limiting, because runaway coagulation is as lethal as hemorrhage.

Platelets (thrombocytes) are not cells. They are anucleate fragments, 2–4 µm across, shed from the cytoplasm of enormous marrow cells called megakaryocytes — each of which produces 2,000 to 3,000 platelets by extending long proplatelet processes into marrow sinusoids. Normal count is 150,000–400,000/µL; lifespan is 8–10 days; production is regulated by thrombopoietin from the liver. About a third of the body's platelets are held in reserve in the spleen at any moment, which is why splenomegaly lowers the platelet count without any production problem.

Despite having no nucleus, a platelet is metabolically busy. Its granules are pre-loaded: dense granules contain ADP, ATP, serotonin, and calcium; alpha granules contain fibrinogen, von Willebrand factor, factor V, platelet-derived growth factor, and adhesion molecules. It also carries the enzymes to synthesize thromboxane A2 on demand.

        THE THREE STEPS OF HAEMOSTASIS, IN SEQUENCE

  ═══ STEP 1 · VASCULAR SPASM ═══════════════════ seconds ═══════════
     Vessel is cut.  Smooth muscle in the wall contracts immediately.
        TRIGGERS: • direct injury to smooth muscle
                  • pain reflexes (local nervous)
                  • ENDOTHELIN + serotonin + thromboxane A2 released
                    by damaged endothelium and activated platelets
        EFFECT:   lumen narrows → flow falls → less blood lost and
                  more time for steps 2 and 3.  Lasts minutes only.
                  Most effective in SMALL vessels.

  ═══ STEP 2 · PLATELET PLUG ════════════════════ seconds-minutes ═══
     ① ADHESION   Injury exposes SUBENDOTHELIAL COLLAGEN.
                  VON WILLEBRAND FACTOR bridges collagen to the
                  platelet GPIb receptor.  (vWF is essential in fast
                  arterial flow, where platelets would be swept past.)
                          collagen ══ vWF ══ GPIb ══ ●platelet
     ② ACTIVATION Platelet changes shape (disc → spiky sphere),
                  and DEGRANULATES:
                     ADP  ──────► recruits + activates more platelets
                     THROMBOXANE A2 ─► recruits + VASOCONSTRICTS
                     serotonin, Ca²⁺, fibrinogen released
                  ◄── ASPIRIN blocks thromboxane A2 synthesis
                  ◄── CLOPIDOGREL blocks the ADP (P2Y12) receptor
     ③ AGGREGATION Activation flips GP IIb/IIIa receptors into their
                  active form; FIBRINOGEN bridges IIb/IIIa on one
                  platelet to IIb/IIIa on the next.
                     ●═══fibrinogen═══●═══fibrinogen═══●
                  ◄── GP IIb/IIIa INHIBITORS block this final step
                  THIS IS POSITIVE FEEDBACK: activated platelets
                  release agonists that activate more platelets.
                  It stops because intact endothelium either side
                  releases NITRIC OXIDE and PROSTACYCLIN, which
                  INHIBIT platelets — so the plug cannot spread.

  ═══ STEP 3 · COAGULATION ══════════════════════ 1-6 minutes ═══════
     A cascade of plasma protein enzymes converts soluble FIBRINOGEN
     into insoluble FIBRIN, which polymerises into a mesh that traps
     red cells and cements the platelet plug into a true CLOT.
     (See Figure 17.6.)
     Then CLOT RETRACTION: platelet actin-myosin contracts, squeezing
     out serum and pulling the wound edges together over 30-60 min.
     Platelet-derived growth factor summons fibroblasts; VEGF summons
     endothelium.  Repair begins inside the clot.

  ═══ AFTERWARDS · FIBRINOLYSIS ═════════════════ 2 days onward ═════
     Plasminogen is trapped inside the fibrin mesh as it forms.
     Tissue plasminogen activator (t-PA), released slowly by
     endothelium, converts it to PLASMIN, which digests fibrin.
     The clot dissolves as the vessel is repaired.
     ◄── THROMBOLYTIC DRUGS are t-PA, given as a drug, all at once.

Coagulation: a cascade, and why it is built that way

Coagulation converts fibrinogen to fibrin through a series of proteolytic activations in which each activated factor activates the next. Factors circulate as inactive zymogens; activation means a specific cleavage.

Why a cascade rather than one enzyme? For the same reason as the endocrine cascade of §16.3: amplification and control. A few molecules of initiating factor generate thousands of molecules of thrombin, because each enzyme in the chain activates many copies of the next. And a multi-step chain provides many points at which the process can be regulated, inhibited, or targeted by a drug — which is exactly what §17.8 exploits.

            THE COAGULATION CASCADE AND WHERE DRUGS ACT

  INTRINSIC (contact) PATHWAY          EXTRINSIC (tissue factor) PATHWAY
  slower · all components already      FAST · requires a factor from
  in the blood · amplifies             OUTSIDE the blood · initiates
        │                                        │
  damaged surface / collagen           TISSUE INJURY exposes
  exposed collagen                     TISSUE FACTOR (factor III)
        ▼                              on subendothelial cells
     XII → XIIa                                  ▼
        ▼                                   VII → VIIa
     XI  → XIa                                   │
        ▼                              TF-VIIa complex
     IX  → IXa  + VIIIa ────────┐               │
        (haemophilia B) (haem. A)│               │
                                 ▼               ▼
                              ┌──────────────────────┐
                              │  X ──────► Xa        │  ◄── DOACs
                              │  (+ Va, Ca²⁺, PL)    │      "-xaban"
                              └──────────┬───────────┘      block Xa
                        C O M M O N      │
                        P A T H W A Y    ▼
                              PROTHROMBIN (II) ──► THROMBIN (IIa)
                                                       │  ◄── dabigatran
                                                       │      blocks IIa
                                          ┌────────────┤
                                          ▼            ▼
                              FIBRINOGEN (I) ──► FIBRIN (Ia)
                                                       │  + XIIIa
                                                       ▼
                                          CROSS-LINKED FIBRIN MESH

  REQUIRED THROUGHOUT:  Ca²⁺ (factor IV) and PHOSPHOLIPID surface
                        (supplied by activated platelets — which is
                        WHY the platelet plug must come first)

  VITAMIN K is required to carboxylate factors II, VII, IX, X
  and proteins C and S.   "1972" = 10, 9, 7, 2.
     ◄── WARFARIN blocks vitamin K epoxide reductase
  HEPARIN activates ANTITHROMBIN III ×1000, which inactivates
     thrombin (IIa) and factor Xa.
     ◄── acts INSTANTLY; unfractionated heparin hits IIa and Xa,
         low-molecular-weight heparin mainly Xa.

  ═══ LABORATORY TESTS — each reads ONE arm ══════════════════════════
    PT / INR   reads EXTRINSIC + COMMON  (VII, X, V, II, fibrinogen)
               → monitors WARFARIN.  VII has the SHORTEST half-life
                 (~6 h), so PT moves first when vitamin K is blocked.
    aPTT       reads INTRINSIC + COMMON  (XII, XI, IX, VIII, X, V,
               II, fibrinogen)
               → monitors UNFRACTIONATED HEPARIN; prolonged in
                 haemophilia A and B.
    Both prolonged → the COMMON pathway, or liver failure, or DIC.

Three points about the cascade repay attention.

The extrinsic pathway initiates; the intrinsic amplifies. In vivo, coagulation almost always begins when tissue factor — normally never in contact with blood — is exposed by injury. The small amount of thrombin generated then feeds back to activate factors XI, VIII, and V, recruiting the intrinsic pathway to produce the thrombin burst that actually makes a clot. The "two separate pathways" model is a description of the laboratory tests, not of the body. That is why a patient with factor XII deficiency has a markedly prolonged aPTT and no bleeding tendency at all — factor XII matters in a test tube and hardly at all in a person.

Calcium and phospholipid are required at multiple steps. Calcium is factor IV. This is why blood collection tubes for coagulation studies contain citrate, which chelates calcium and prevents clotting in the tube, and it is why massive transfusion of citrated blood can cause hypocalcemia. The phospholipid surface comes from activated platelets, which is the structural reason hemostasis has to happen in the order it does: the platelet plug is not just a plug, it is the catalytic surface on which coagulation assembles.

Coagulation is restrained continuously. Intact endothelium is actively antithrombotic: it presents heparan sulfate (activating antithrombin III), thrombomodulin (which redirects thrombin to activate protein C, which then degrades factors Va and VIIIa), and tissue factor pathway inhibitor, and it releases nitric oxide and prostacyclin to inhibit platelets. Clotting is not the default state that must be triggered; it is a constantly suppressed state that must be disinhibited. Injury does both at once — it exposes activators and removes the inhibitory endothelial surface.

17.8 Hemostasis Disorders and the Logic of "Blood Thinners"

Bleeding disorders, by the step they break

DIC is worth pausing on because it is the clearest demonstration that hemostasis is a balance rather than a switch. Patients bleed from every puncture site while simultaneously infarcting fingers and kidneys, and both findings have the same cause: the system has been driven so hard that it has exhausted its own substrate.

The three drug classes act at three different steps

This is the section that answers Amara's third question, and the single organizing idea is this: "blood thinner" is not a pharmacological category. No drug in common use makes blood thinner in the sense of reducing viscosity. What they do is inhibit different, sequential steps of the hemostatic process — which is precisely why combining them is rational rather than redundant.

Two further principles follow from that table and are worth stating plainly.

Arterial clots are platelet-rich; venous clots are fibrin-rich. An arterial thrombus forms on a ruptured atherosclerotic plaque under high shear, where platelets adhere and aggregate rapidly — it is a "white clot," and antiplatelet drugs are its natural counter. A venous thrombus forms in slow or stagnant flow, where activated coagulation factors are not washed away, trapping red cells in fibrin — a "red clot," and anticoagulants are its natural counter. This is why aspirin prevents heart attacks and warfarin prevents strokes from atrial fibrillation, and why swapping them would be a serious error.

Warfarin takes days to work, and that fact is derivable. Warfarin does not remove existing clotting factors; it prevents the synthesis of new functional ones. The factors already circulating must be cleared first, and the delay is therefore set by their half-lives: factor VII about 6 hours, factor IX about 24, factor X about 40, prothrombin about 60–72. The PT/INR rises within a day, because it reads factor VII — but full anticoagulation waits for prothrombin, three to five days later. Worse, proteins C and S, the natural anticoagulants, are also vitamin-K-dependent and have short half-lives, so the first day or two of warfarin can be transiently procoagulant. Both facts have the same practical consequence: a patient started on warfarin for an acute clot is bridged with heparin, which works instantly, until the INR is therapeutic.

17.9 Blood Groups and Transfusion

Red cell membranes carry inherited surface antigens — glycoproteins and glycolipids — of which more than 30 systems and hundreds of antigens are known. Two systems account for almost all clinically significant transfusion reactions: ABO and Rh.

ABO

The ABO gene encodes a glycosyltransferase that adds a terminal sugar to a precursor called the H antigen. The A allele adds N-acetylgalactosamine; the B allele adds galactose; the O allele is non-functional and adds nothing. A and B are codominant; O is recessive.

The critical peculiarity of ABO is that the corresponding antibodies are present from infancy without prior transfusion. Environmental bacteria carry sugars closely resembling the A and B antigens, so an infant makes antibodies against whichever of those structures it does not possess. By six months, a person of blood group A has anti-B antibodies; group B has anti-A; group O has both; group AB has neither. These are IgM antibodies, large and highly efficient at agglutination and complement activation, which is exactly why an ABO-incompatible transfusion is an immediate catastrophe rather than a delayed one.

Rh

The Rh system's most important antigen is D. About 85% of people are RhD-positive. Unlike ABO, anti-D antibodies are not naturally present; an RhD-negative person makes them only after exposure to RhD-positive red cells, through transfusion or pregnancy. Anti-D is IgG — smaller, and critically, able to cross the placenta.

              ABO AND Rh — ANTIGENS, ANTIBODIES, COMPATIBILITY

  ┌───────┬──────────────┬────────────────┬───────────┬──────────────┐
  │ GROUP │ ANTIGEN on   │ ANTIBODY in    │ US freq.  │ Can receive  │
  │       │ the RED CELL │ the PLASMA     │ (approx.) │ RED CELLS    │
  ├───────┼──────────────┼────────────────┼───────────┼──────────────┤
  │   A   │  A           │  anti-B        │   ~40%    │ A, O         │
  │   B   │  B           │  anti-A        │   ~11%    │ B, O         │
  │  AB   │  A and B     │  NEITHER       │    ~4%    │ A,B,AB,O ALL │
  │   O   │  NEITHER     │  anti-A + B    │   ~45%    │ O only       │
  └───────┴──────────────┴────────────────┴───────────┴──────────────┘

     O NEGATIVE  = UNIVERSAL RED CELL DONOR
                   (no A, no B, no D antigen for the recipient's
                    antibodies to attack)
     AB POSITIVE = UNIVERSAL RED CELL RECIPIENT
                   (no antibodies in the plasma to attack anything)

     REVERSED FOR PLASMA:  AB plasma is the universal donor plasma,
     because AB plasma contains NO anti-A and NO anti-B.
     The rule is always: DONOR ANTIGEN vs RECIPIENT ANTIBODY.

  ═══ THE COMPATIBILITY GRID ═══════════════════════════════════════
      Recipient →      O⁻    O⁺    A⁻    A⁺    B⁻    B⁺   AB⁻   AB⁺
      Donor ↓
      O⁻              YES   YES   YES   YES   YES   YES   YES   YES
      O⁺               no   YES    no   YES    no   YES    no   YES
      A⁻               no    no   YES   YES    no    no   YES   YES
      A⁺               no    no    no   YES    no    no    no   YES
      B⁻               no    no    no    no   YES   YES   YES   YES
      B⁺               no    no    no    no    no   YES    no   YES
      AB⁻              no    no    no    no    no    no   YES   YES
      AB⁺              no    no    no    no    no    no    no   YES

  ═══ WHY AGGLUTINATION HAPPENS ════════════════════════════════════
     Anti-A and anti-B are IgM: PENTAMERIC, ten binding sites each.
     One antibody molecule can bridge SEVERAL red cells at once.

        ●═══Y═══●          Y = IgM antibody bridging two cells
        ║   ║   ║          ● = red cell carrying the antigen
        ●═══Y═══●          → lattice → AGGLUTINATION → complement
                             fixation → intravascular HAEMOLYSIS

  ═══ HAEMOLYTIC DISEASE OF THE NEWBORN ════════════════════════════
     PREGNANCY 1   RhD-NEGATIVE mother, RhD-POSITIVE fetus.
                   At delivery, fetal cells enter maternal blood.
                   Mother is SENSITISED → makes anti-D (IgG).
                   This baby is UNAFFECTED (antibody made too late).
     PREGNANCY 2   Maternal anti-D IgG CROSSES the placenta and
                   attacks fetal RhD-positive red cells →
                   fetal haemolysis, anaemia, jaundice, kernicterus,
                   heart failure, hydrops fetalis.
     PREVENTION    Anti-D immunoglobulin (RhoGAM) at ~28 weeks and
                   within 72 h of delivery.  It clears fetal cells
                   from maternal circulation BEFORE the mother's own
                   immune system can respond — passive antibody used
                   to prevent active immunisation.

Cross-matching and transfusion reactions

Before a transfusion, three steps are performed. Type — determine the recipient's ABO and Rh group. Screen — test the recipient's plasma against panel cells for unexpected antibodies against the many minor antigen systems. Cross-match — mix recipient plasma with the actual donor unit and look for agglutination. In an emergency, uncross-matched O-negative cells can be given, which is why every emergency department keeps some.

TACO deserves a note for this book, because Amara is exactly the patient at risk: a woman with an ischemic, stiff ventricle and impaired renal handling of sodium and water (Chapter 16). Transfusing her rapidly would raise preload on a ventricle that cannot accommodate it. This is one reason transfusion thresholds in stable cardiac patients are conservative — typically a hemoglobin of 7–8 g/dL rather than a "normal" value — and why Amara at 10.8 g/dL is treated by finding and fixing the cause, not by transfusing.

17.10 Advanced Topic · The Hematopoietic Hierarchy, Marrow Failure, and Blood Cancers

The hierarchy

Every blood cell descends from a hematopoietic stem cell through a branching hierarchy in which each step trades potency for commitment:

HSC (self-renewing, pluripotent) → multipotent progenitor (loses self-renewal) → common myeloid or common lymphoid progenitor (lineage committed) → unipotent progenitor → precursor (morphologically identifiable) → mature cell

Two properties of the top of that hierarchy explain most of hematology. Self-renewal means a small number of cells can sustain output for a lifetime, and means a transplanted marrow can permanently reconstitute a recipient. High proliferative rate in the progenitor compartment means that anything toxic to dividing cells — radiation, cytotoxic chemotherapy, benzene, chloramphenicol — hits blood production early and hard. The predictable sequence after marrow injury follows directly from the lifespans in §17.6: neutrophils fall first (6–8 hours in circulation), platelets next (8–10 days), and red cells last (120 days). A nadir at 7–14 days after chemotherapy is that arithmetic, not a coincidence.

Bone marrow failure

Aplastic anemia is failure of the stem cell compartment itself, producing pancytopenia — anemia, neutropenia, and thrombocytopenia together — with a marrow that is hypocellular and replaced by fat. Causes include autoimmune destruction (most cases), drugs, viruses, radiation, and inherited syndromes such as Fanconi anemia. The clinical picture is a direct readout of the three missing lines: fatigue and pallor from anemia, infection from neutropenia, and bleeding from thrombocytopenia. Treatment is immunosuppression or stem cell transplantation.

Distinguish it from myelodysplastic syndromes, where the marrow is cellular but produces defective cells that die before leaving it — ineffective hematopoiesis, with cytopenias despite a busy marrow — and from marrow infiltration by metastatic carcinoma, lymphoma, or fibrosis, which physically crowds out hematopoiesis and characteristically pushes immature cells into the blood, a picture called leukoerythroblastic.

Leukemia and lymphoma in outline

Blood cancers are classified along two axes: which lineage the malignant cell belongs to, and how mature it is. That gives a usable grid.

Leukemia is a malignancy of blood-forming cells that primarily involves marrow and blood. Lymphoma is a malignancy of lymphocytes that primarily forms solid masses in lymph nodes and lymphoid organs — divided into Hodgkin lymphoma (characterized by Reed–Sternberg cells, spreading in an orderly way to contiguous node groups, highly curable) and the many non-Hodgkin lymphomas. The distinction is one of location and behaviour more than of cell type, and the two overlap: CLL and small lymphocytic lymphoma are the same disease named for where it is found.

Two general principles are worth extracting. First, the symptoms of acute leukemia are the symptoms of marrow failure, not of the tumour itself — the blasts do not hurt; they displace. Anemia, infection, and bleeding, all three at once, in a patient who was well a month ago, is the pattern. Second, blood cancers were the proving ground for modern targeted therapy precisely because their cells are easy to sample repeatedly, easy to classify by flow cytometry, and easy to count. Imatinib for BCR-ABL, all-trans retinoic acid for acute promyelocytic leukemia, and CAR-T cells for refractory B-cell malignancies were all developed here first, and diffused outward to solid tumours afterward.

Chapter Summary

§17.1 Blood is the body's only fluid connective tissue: cells suspended in a liquid extracellular matrix. Five litres, 8% of body mass, pH 7.35–7.45, 38 °C, four to five times as viscous as water. Centrifuged, it separates into plasma (55%), a buffy coat of leukocytes and platelets (<1%), and packed erythrocytes (45%, the hematocrit). Its seven functions divide into transport of gases, nutrients, wastes, and hormones; regulation of temperature, pH, and fluid distribution; and protection against blood loss and against pathogens.

§17.2 Plasma is 92% water. Its proteins — nearly all hepatic — are albumin (60%), which generates about 80% of colloid osmotic pressure and carries fatty acids, bilirubin, calcium, and drugs; globulins, which transport and defend; and fibrinogen, the precursor of the fibrin clot. Serum is plasma minus fibrinogen. Low albumin means fluid leaves the vessels and does not return: edema and ascites.

§17.3 The biconcave disc maximizes surface area, minimizes internal diffusion distance, and provides excess membrane for deformation. The nucleus and mitochondria were deleted so the cell can bend and so it does not consume its own cargo — at the cost of a 120-day lifespan and total dependence on glycolysis. Each of the roughly 250 million hemoglobin tetramers per cell binds four oxygen molecules cooperatively. Iron is chaperoned at every step and recycled almost completely; heme becomes bilirubin, whose handling explains the three types of jaundice.

§17.4 Erythropoiesis proceeds from the hematopoietic stem cell through erythroblasts to the reticulocyte in about fifteen days. Renal peritubular fibroblasts sense oxygen through HIF-1α and secrete erythropoietin, whose effector is the marrow and whose raw materials are iron, B12, and folate. Iron deficiency produces small cells; B12 and folate deficiency produce large ones, because they impair DNA replication while cytoplasmic growth continues.

§17.5 Anemia is classified by mechanism (loss, underproduction, destruction) and by MCV, and the reticulocyte count separates marrow failure from destruction or loss. Anemia of chronic disease is normocytic and mediated by hepcidin, which sequesters iron in stores, producing high ferritin with low transferrin saturation. Polycythemia raises viscosity and thrombotic risk; measuring EPO alongside the hematocrit localizes its cause.

§17.6 Leukocytes are the only complete cells in blood, and blood is their commute rather than their workplace. Neutrophils, lymphocytes, monocytes, eosinophils, and basophils are identified on a smear by size, nuclear shape, and granule staining. A high neutrophil count can arise without infection through demargination driven by catecholamines and cortisol, and through sterile inflammation triggered by necrotic tissue.

§17.7 Hemostasis proceeds in three steps: vascular spasm, platelet plug formation (adhesion via von Willebrand factor to GPIb, activation with release of ADP and thromboxane A2, aggregation via fibrinogen bridging GPIIb/IIIa), and coagulation, in which extrinsic and intrinsic pathways converge on factor X and thrombin converts fibrinogen to fibrin. Calcium and a platelet phospholipid surface are required throughout; the process is confined by nitric oxide, prostacyclin, antithrombin III, and protein C from intact endothelium, and reversed later by plasmin.

§17.8 Bleeding disorders are classified by the step they break: platelet-step failures bleed early and superficially, coagulation-step failures bleed late and deep. The three drug classes act at three different points — antiplatelets at step 2, anticoagulants at step 3, thrombolytics after the clot — which is why they are combined rather than substituted. PT/INR reads the extrinsic and common pathways and monitors warfarin; aPTT reads the intrinsic and common pathways and monitors unfractionated heparin.

§17.9 ABO antibodies are naturally occurring IgM; anti-D is acquired IgG that crosses the placenta. Compatibility is always donor antigen against recipient antibody, making O-negative the universal red cell donor and AB the universal plasma donor. Transfusion reactions range from febrile to fatal, and the commonest cause of the fatal one is misidentification.

§17.10 All blood cells descend from a self-renewing hematopoietic stem cell, and the order in which counts fall after marrow injury follows the cells' lifespans. Marrow failure produces pancytopenia; leukemias are classified by lineage and maturity, and their symptoms are the symptoms of the marrow they displace.

Case File 17 · Resolution

Question 1 — Why is Amara anemic when she has not bled, and why is her MCV normal?

Two processes are operating together, and neither involves losing blood.

The dominant one is anemia of chronic disease (anemia of inflammation). Amara has atherosclerosis, metabolic syndrome, and — since her infarct — a substantial mass of necrotic myocardium. All three are inflammatory states, and inflammatory cytokines, chiefly interleukin-6, drive hepatic production of hepcidin. Hepcidin degrades ferroportin, the only channel by which iron leaves an enterocyte or a macrophage. Two consequences follow immediately: dietary iron absorption falls, and the roughly 25 mg of iron recycled every day from senescent red cells is trapped inside macrophages instead of being handed back to transferrin.

The result is a functional iron deficiency. Her total body iron is adequate — her ferritin is 186 ng/mL, which is high — but almost none of it is available to the marrow, which is why her transferrin saturation is only 11%. That pair of values, high ferritin with low saturation, is the signature of the condition and is exactly what distinguishes it from true iron deficiency, where ferritin would be low. Inflammation also blunts renal EPO production and the marrow's responsiveness to it, which is why her reticulocyte count of 0.9% is inappropriate: at a hemoglobin of 10.8 g/dL a healthy marrow should be running at two to three times baseline output. Nominally normal, physiologically inadequate.

The second contributor is her kidneys, and it foreshadows Chapter 26. Twenty years of hypertension, several years of hyperglycemia, and sustained angiotensin II exposure (Chapter 16) damage glomeruli and the peritubular interstitium — and the peritubular fibroblasts are the cells that make erythropoietin. As they are lost, the control centre of the loop in Figure 17.3 weakens. Her EPO is very likely low-normal, which is to say inappropriately low for her hemoglobin. Her creatinine has been drifting, and this anemia is one of the earliest clinical manifestations of chronic kidney disease, often appearing before anyone has labelled the kidney disease at all.

Why is the MCV normal? Because neither mechanism affects the ratio between cytoplasmic growth and nuclear division. Cell size is set by how much hemoglobin accumulates per division cycle. In absolute iron deficiency, iron is genuinely scarce over months, hemoglobin synthesis falls behind the division schedule, and cells come out small. In anemia of inflammation the marrow simply makes fewer cells of normal size: production is reduced, not distorted. The same is true of EPO deficiency — the order to produce is missing, but the cells that are produced are built correctly. So both mechanisms are normocytic, and Amara's MCV of 88 fL with a normal red cell distribution width is exactly what the mechanism predicts.

What follows clinically. Giving her oral iron would largely fail, because hepcidin is blocking absorption at the enterocyte and would sequester whatever arrived. The effective interventions are treating the inflammation and the underlying disease. And her anemia is not a cosmetic finding: at 10.8 g/dL she carries roughly 28% less oxygen per unit of blood than she would at 15 g/dL, in a woman whose coronary arteries already cannot deliver enough. Anemia is a genuine aggravator of angina, and correcting it can relieve chest pain that no stent touched.

Question 2 — Why is her white count up when she has no infection?

Because neutrophils respond to tissue damage, not specifically to bacteria — and because a white cell count measures where cells are, not how many exist.

Two mechanisms are operating, on two timescales.

Demargination, within minutes to hours. About half of the neutrophils in Amara's vasculature are normally rolling slowly along endothelium in the marginal pool, invisible to a venous blood draw. Since her arrival she has had high sympathetic tone, circulating catecholamines, pain, and physiological stress — and, from Chapter 16, a cortisol axis that is elevated on top of an already flattened rhythm. Catecholamines increase flow velocity and reduce neutrophil adhesion; cortisol demarginates neutrophils while simultaneously sequestering lymphocytes into lymphoid tissue. That combination produces neutrophilia with lymphopenia — which is precisely her differential: neutrophils 78%, lymphocytes 14%. Not one new cell need have been made.

Sterile inflammation from necrotic myocardium, over hours to days. A myocardial infarction kills cardiac muscle cells by necrosis, and necrotic cells spill their contents. Those contents — mitochondrial DNA, ATP, uric acid, HMGB1, heat shock proteins — are damage-associated molecular patterns, read by the same innate immune receptors that detect bacteria. Resident cardiac macrophages and injured endothelium respond by releasing IL-1, IL-6, TNF-α, and CXCL8, which up-regulate adhesion molecules on the local endothelium and recruit neutrophils out of the blood and into the infarct within hours. The marrow is simultaneously instructed to increase output and to release its reserve pool, which is why a left shift often appears.

Her body is not confused. It is running the correct, evolutionarily ancient program for tissue injury, and the program does not require an organism to be present. The reason it exists is that the neutrophil influx is the first phase of repair: neutrophils debride dead tissue, monocytes arrive at 3–7 days to clear debris and switch the response toward resolution, and fibroblasts then lay down the collagen scar that will replace her dead myocardium. Chapter 20 develops this sequence in full, and Chapter 18 evaluates its consequence — because that scar is not muscle, it does not contract, and its stiffness is part of why her ventricle will fill poorly.

One clinical caution follows. Because a leukocytosis of 11–15 K/µL is expected after an infarct, it cannot by itself be used to diagnose an infection in this patient. Fever, a rising count over several days rather than a falling one, a focal source, and a rising C-reactive protein or procalcitonin are what distinguish a complication from the expected physiology. The magnitude of the post-infarct leukocytosis also correlates with infarct size and with worse outcomes, which is why the number is noted rather than ignored.

Question 3 — Why two antiplatelet drugs plus an anticoagulant? Aren't they all blood thinners?

None of them thins blood. They inhibit three distinct molecular targets across two different steps of hemostasis, and Amara needs all three because she has two different problems at once.

The problem being treated. An acute coronary syndrome begins when an atherosclerotic plaque ruptures, exposing subendothelial collagen and tissue factor to flowing blood. That single event triggers both arms of hemostasis simultaneously: collagen and von Willebrand factor recruit platelets (step 2), and tissue factor initiates the coagulation cascade (step 3). A thrombus forming under high arterial shear is platelet-rich — a "white clot" — but it is stabilized by fibrin. Blocking only one arm leaves the other running.

Aspirin — step 2, target one. Irreversibly acetylates cyclooxygenase-1 in the platelet, abolishing thromboxane A2 synthesis. Because the platelet has no nucleus it cannot replace the enzyme, so the effect lasts that platelet's entire 8–10 day life.

Clopidogrel — step 2, target two. Irreversibly blocks the platelet P2Y12 receptor for ADP. Thromboxane A2 and ADP are two separate autocrine amplifiers released by activated platelets, and both converge on activating GPIIb/IIIa. Blocking only one leaves the other able to drive aggregation. This is why dual antiplatelet therapy reduces recurrent events substantially more than aspirin alone, and it is not redundancy — it is the same reasoning that puts two drugs on two receptors in any other system. It also matters intensely once a coronary stent is placed: a bare metal strut in an artery is a thrombogenic surface until endothelium grows over it, and stent thrombosis is frequently fatal.

Heparin — step 3, a different arm entirely. Binds antithrombin III and increases its activity roughly a thousandfold, so that it inactivates thrombin and factor Xa within seconds. This attacks the fibrin arm of the clot, which the antiplatelet drugs do not touch at all. Its onset is immediate, its half-life short, and its effect monitored by aPTT and reversible with protamine — all of which matter in a patient going to the catheterization laboratory tomorrow, where the level of anticoagulation must be controllable hour by hour.

Why not a thrombolytic as well? Because thrombolytics belong to a fourth category acting at a fourth point — they dissolve fibrin that has already formed, by converting trapped plasminogen to plasmin. They are used when a coronary artery is completely occluded and catheterization is not available in time. Amara has a non-ST-elevation event with a planned angiogram; her artery is narrowed, not abruptly and totally occluded, and thrombolysis in that setting adds bleeding risk without benefit. The choice of drug is a choice about which step of Figure 17.5 the problem is at.

The price, stated honestly. Three agents inhibiting hemostasis at three points multiply bleeding risk, and bleeding is the commonest serious complication of treating an acute coronary syndrome. This is why the duration of dual antiplatelet therapy is finite and debated, why heparin is stopped as soon as the catheterization is done, why proton pump inhibitors are co-prescribed to protect the stomach, and why every subsequent clinician who sees Amara must know all three drugs are on board before performing any procedure.

So Nia's question has a precise answer: they are not three of the same drug. They are three keys turned in three different locks, on a door that has two separate bolts.

Systems Integration Case File · Entry 17

Review

Level 1 · Recall

15.1 The hematocrit measures:

a) the percentage of blood volume occupied by plasma    b) the percentage of blood volume occupied by erythrocytes    c) the grams of hemoglobin per decilitre    d) the number of red cells per microlitre

15.2 Which plasma protein is chiefly responsible for colloid osmotic pressure?

a) fibrinogen    b) gamma globulin    c) albumin    d) transferrin

15.3 A mature erythrocyte lacks mitochondria, which means it:

a) cannot transport oxygen    b) relies entirely on anaerobic glycolysis for ATP    c) has a shortened lifespan of 30 days    d) cannot bind carbon dioxide

15.4 Erythropoietin is produced primarily by:

a) red bone marrow    b) the liver    c) peritubular fibroblasts of the kidney    d) the spleen

15.5 A patient has hemoglobin 9.8 g/dL, MCV 72 fL, ferritin 5 ng/mL. The most likely diagnosis is:

a) B12 deficiency    b) iron deficiency anemia    c) anemia of chronic disease    d) aplastic anemia

15.6 Which leukocyte is most abundant in normal blood, and what is its principal role?

a) lymphocyte; adaptive immunity    b) neutrophil; phagocytosis of bacteria    c) monocyte; antigen presentation    d) eosinophil; parasite defense

15.7 Von Willebrand factor is required for:

a) conversion of fibrinogen to fibrin    b) platelet adhesion to exposed collagen    c) activation of antithrombin III    d) carboxylation of factor VII

15.8 A person with blood group O negative:

a) can receive red cells from any donor    b) has no anti-A or anti-B antibodies    c) is the universal red cell donor    d) carries the D antigen

Level 2 · Comprehension

15.9 Explain why the biconcave shape is a better solution than a sphere for a cell whose job is carrying oxygen through capillaries.

15.10 Distinguish absolute iron deficiency from functional iron deficiency, and explain why oral iron helps one and not the other.

15.11 Warfarin blocks vitamin K on the day it is given, yet a patient is not anticoagulated for several days. Explain, and explain the transient procoagulant risk.

15.12 Why are ABO antibodies present without prior exposure to blood, while anti-D antibodies are not?

Level 3 · Clinical Application

15.13 A 62-year-old man presents with fatigue and a hemoglobin of 9.4 g/dL, MCV 104 fL. He has numbness in both feet, an unsteady gait, and reduced vibration sense to the knees. His smear shows hypersegmented neutrophils. What is the diagnosis, what is the mechanism of the neurological findings, and what single mistake in treatment must be avoided?

15.14 A 24-year-old woman with heavy menstrual bleeding since menarche, frequent nosebleeds, and prolonged bleeding after a dental extraction has a normal platelet count, a normal PT, and a mildly prolonged aPTT. Her father has similar symptoms. What is the likely diagnosis, why is the aPTT abnormal, and which step of hemostasis has failed?

15.15 A 68-year-old woman is admitted with a pulmonary embolism and started on unfractionated heparin. On day 7 her platelet count has fallen from 265,000 to 92,000/µL, and she develops a cold, painful right leg. Explain what has happened, and state three specific management decisions with their reasons.

Level 4 · Integration and Synthesis

15.16 Amara's hemoglobin is 10.8 g/dL. Quantify the effect on oxygen delivery to her myocardium, explain why this matters more for her than for a healthy person with the same hemoglobin, and explain why transfusing her to a "normal" hemoglobin would nonetheless be a poor decision.

15.17 Amara's daughter Nia, 24, runs marathons. Her hemoglobin is 12.1 g/dL, her ferritin is 14 ng/mL, and her MCV is 84 fL. Her mother's hemoglobin is 10.8 g/dL with a ferritin of 186 ng/mL. Both are "anemic" by the reference range. Explain how their conditions differ at every level, and say what each needs.

15.18 Construct the complete causal chain from Amara's ruptured coronary plaque to the number 11.4 on her white cell count, naming every mediator and cell type, and explain why the same process that repairs her heart also permanently weakens it.

Concept Map to Complete

Copy this onto blank paper and fill every bracket from memory before checking the chapter.

                              WHOLE BLOOD
                                   │
             ┌─────────────────────┴────────────────────┐
        PLASMA  [ ___ %]                    FORMED ELEMENTS [ ___ %]
             │                                          │
   ┌─────────┼─────────┐              ┌─────────────────┼─────────────┐
 water    PROTEINS   solutes     ERYTHROCYTES      LEUKOCYTES     PLATELETS
 [ __%]   [ __%]                  [ ____ /µL]      [ ____ /µL]   [ ____ /µL]
             │                          │                │
   ┌─────────┼─────────┐                │      ┌─────────┴────────┐
[ _______ ] [ ______ ] [ _______ ]      │  GRANULOCYTES     AGRANULOCYTES
 ~60%        ~36%       ~4%             │  [ _ ] [ _ ] [ _ ]  [ _ ] [ _ ]
 makes                  becomes         │   __%  __%   __%     __%   __%
 [ __________            [ ______ ]     │
   pressure ]                           │
                                        ▼
                             SHAPE: [ ___________ ] disc
                             NO [ _______ ] · NO [ __________ ]
                             LIFESPAN [ ____ ] days
                             destroyed in the [ ______ ]
                             haem → [ _________ ] → jaundice


                   THE ERYTHROPOIETIN LOOP
   ↓ O₂ delivery → sensed by [ which cells, which organ? ]
       via [ which transcription factor? ]
       → secretes [ ________ ] → acts on [ ________ ]
       → needs [ ___ ], [ ___ ], [ ______ ]
       → ↑ RBC → ↑ O₂ delivery → loop [ _______ ]

   Broken control centre = [ which disease? ]
   Broken effector       = [ give two ]


                        HAEMOSTASIS
   STEP 1 [ ______________ ]  ── seconds
   STEP 2 [ ______________ ]  ── adhesion via [ ____ ] to receptor [ ____ ]
                                 activation releases [ ___ ] and [ _____ ]
                                 aggregation via [ _______ ] bridging [ ____ ]
   STEP 3 [ ______________ ]  ── [ ____ ] pathway initiates,
                                 [ ____ ] pathway amplifies,
                                 converge on factor [ __ ] → [ _______ ]
                                 → fibrinogen → [ ______ ]
   AFTER  [ ______________ ]  ── plasminogen → [ _______ ] digests fibrin

   ANTIPLATELET drugs act at step [ _ ]   monitored by [ ______ ]
   ANTICOAGULANT drugs act at step [ _ ]  warfarin → [ ____ ]
                                          heparin  → [ ____ ]
   THROMBOLYTIC drugs act at [ ______ ]

Lab / Self-Exploration

1. Find your own capillary refill. Press firmly on a fingernail bed for five seconds, release, and count how long the pink colour takes to return. Under two seconds is normal. Repeat after holding your hand in cold water for a minute. You have just watched vasoconstriction change the delivery of red cells to a tissue — the same mechanism that made Amara's skin pale in Chapter 1.
2. Estimate your own hematocrit from a hemoglobin. The rule of three: hematocrit is approximately three times hemoglobin in g/dL. If you have ever donated blood you were given a hemoglobin value; check whether the rule holds against any hematocrit you can find. Then work out what your value implies about oxygen content, at 1.34 mL of oxygen per gram of hemoglobin per 100 mL of blood.
3. Watch hemostasis happen. The next time you get a small cut, time it. Note the moment bleeding slows (step 1, vascular spasm, within seconds), the moment it stops (step 2, platelet plug, typically 1–3 minutes), and the moment a firm dark clot has formed (step 3, coagulation, typically 3–6 minutes). Then note when the scab tightens and the edges draw together — that is clot retraction, at 30–60 minutes. Four distinct events on four distinct timescales, all visible on your own arm.
4. Read a real CBC. Find any complete blood count — your own, a family member's with permission, or a published sample. Convert the differential percentages to absolute counts by multiplying each by the total white count. Then classify any anemia by MCV. You have just performed the two-question algorithm of §17.5.
5. Find your blood type and reason about it. If you know your ABO and Rh type, write out which antigens you carry, which antibodies you have, whom you can donate red cells to, and whom you can receive from. If you do not know it, work out what types your parents' types would permit. Then answer the harder question: why is the donor list for plasma the reverse of the list for red cells?
6. Time your own pulse pressure response to standing. Lie down for three minutes, take your pulse, then stand and take it again at 15 and 60 seconds. The rise is partly the demargination stimulus described in §17.6 in miniature — catecholamine release. Now consider what a blood sample drawn at each of those three moments would report about your white count, and why.

Key Terms

Next: Chapter 18 · The Cardiovascular System I — the central chapter, where the pump that moves everything in this chapter is finally taken apart, and where Amara's ECG, her catheterization, and her ejection fraction are read.