Contents · 12 sections
In a nutshell. Blood = plasma (55%) + formed elements (45%). Erythrocytes (4–5 × 10¹²/L) transport O₂/CO₂, live ~120 days, anucleated, biconcave. Leukocytes (4–9 × 10⁹/L) provide immunity: neutrophils (phagocytosis), lymphocytes (T/B/NK), monocytes → macrophages, eosinophils (parasites, allergies), basophils (histamine). Platelets (150–400 × 10⁹/L) mediate hemostasis, live 7–10 days. Bone marrow is the primary site of hematopoiesis in adults; prenatally: yolk sac, liver, spleen.
1. Blood composition
- Plasma — 55%: water, proteins (albumin 60%, globulins, fibrinogen), electrolytes, glucose.
- Formed elements — 45%: erythrocytes, leukocytes, platelets.
- Adult blood volume — 4.5–5 L (7% of body weight); in children — 8%.
- Hematocrit — volume fraction of erythrocytes: M 40–50%, F 36–46%.
2. Hematopoiesis
- Pluripotent hematopoietic stem cell (HSC) → myeloid and lymphoid lineages.
- Myeloid: erythropoiesis, granulopoiesis, monocytopoiesis, megakaryocytopoiesis.
- Lymphoid: T lymphocytes (thymus), B lymphocytes (bone marrow), NK cells.
- Regulation: EPO (erythropoietin — kidneys in hypoxia), G-CSF (granulocytes), TPO (platelets), IL-3, IL-7, IL-5.
- Fetal hematopoiesis sites: yolk sac (3–8 wk) → liver and spleen (6 wk–7 mo) → bone marrow (from 5 mo).
- In adults — red bone marrow (sternum, vertebrae, ribs, pelvic bones, epiphyses).
3. Erythrocytes
| Parameter | Significance |
|---|---|
| Count (M/F) | 4.0–5.5 / 3.8–5.0 × 10¹²/L |
| Diameter | 7.2–7.5 µm |
| Morphology | Biconcave disc |
| Nucleus | Absent (in mammals) |
| Lifespan | ~120 days |
| Hemoglobin | M 130–170; F 120–150 g/L |
| MCV | 80–100 fL |
| MCH | 27–33 pg |
| MCHC | 320–360 g/L |
| Reticulocytes | 0,5–2 % |
- Function — transport of O₂ (bound to Hb) and CO₂ (as HCO₃⁻ and carbamino-Hb).
- Metabolism — anaerobic glycolysis (lack of mitochondria) + pentose phosphate pathway.
- Destruction — in the spleen (by macrophages), iron is recycled to heme; bilirubin to the liver.
- Pathology: anisocytosis, poikilocytosis, hypochromia, Heinz bodies (oxidative damage), basophilic stippling (lead poisoning, thalassemia).
4. Hemoglobin: types and functions
- HbA (α₂β₂) — main adult type (97%).
- HbA2 (α2δ2) — 2,5 %.
- HbF (α₂γ₂) — fetal and newborn type (up to 6 mo); high O₂ affinity.
- Pathological: HbS (sickle cell), HbC, HbE, HbH (thalassemia).
- Oxyhemoglobin dissociation curve: right shift — ↑ CO₂, ↑ temperature, ↓ pH (Bohr effect), ↑ 2,3-BPG — enhanced O₂ release to tissues.
5. Leukocytes: general characteristics
| Type | Norm (%) | Abs. (×10⁹/L) | Main function |
|---|---|---|---|
| Neutrophils | 47–72 | 2,0–7,5 | Bacterial phagocytosis |
| Eosinophils | 0,5–5 | 0,02–0,3 | Anti-parasitic immunity, allergy |
| Basophils | 0–1 | 0–0,065 | Histamine, heparin, immediate hypersensitivity |
| Monocytes | 3–11 | 0,09–0,6 | → Tissue macrophages (phagocytosis, APC) |
| Lymphocytes | 19–37 | 1,0–3,0 | Adaptive immunity |
6. Neutrophils
- Produced in the bone marrow, maturation cycle ~14 days.
- Circulate in the blood for 6–12 h, then migrate to tissues (lifespan 1–4 days).
- Granules: primary (myeloperoxidase, lysozyme, elastase) and secondary (lactoferrin, collagenase).
- Effector mechanisms: phagocytosis, oxidative burst (NADPH oxidase → O₂•⁻, H₂O₂), NETs (neutrophil extracellular traps).
- Leukocyte formula shift to the left (band cells, metamyelocytes) is a sign of bacterial infection.
- Neutropenia < 0.5 × 10⁹/L — risk of sepsis (febrile neutropenia).
7. Lymphocytes
- T lymphocytes (70%): CD4+ (helpers — Th1, Th2, Th17, Treg), CD8+ (cytotoxic).
- B lymphocytes (10–20%) → plasma cells → antibodies (IgG, IgA, IgM, IgD, IgE).
- NK cells (5–15%) — innate antitumor and antiviral immunity.
- Granules B and T — enter lymph nodes, spleen, Peyer's patches to interact with antigens.
- Lymphocytosis — viral infections, pertussis, chronic lymphocytic leukemia.
- Lymphopenia — HIV, stress, glucocorticoids, radiation sickness.
8. Monocytes and macrophages
- Circulate in blood for 1–3 days → migrate to tissues → macrophages (lifespan months to years).
- Tissue macrophages: Kupffer cells (liver), alveolar (lungs), microglia (brain), osteoclasts, Langerhans cells (skin).
- Functions: phagocytosis, antigen presentation (MHC II), cytokine production (IL-1, TNF-α, IL-6).
- Monocytosis: chronic infections (tuberculosis, brucellosis, syphilis), sarcoidosis, autoimmune diseases, CMML.
9. Eosinophils and basophils
- Eosinophils: granules with cationic protein, peroxidase, MBP — against helminths and in allergy.
- Eosinophilia > 0.5 × 10⁹/L: ascariasis, echinococcosis, allergy, drug reactions (DRESS), EGPA (Churg-Strauss), hypereosinophilic syndrome, CEL.
- Basophils and mast cells: granules with histamine, heparin, proteases; FcεRI — IgE receptor.
- Basophilia: CML, other MPNs; hypersensitivity reactions.
10. Platelets
- Anuclear fragments of megakaryocytes; size 2–4 µm.
- Normal range 150–400 × 10⁹/L; lifespan 7–10 days.
- Production regulation — thrombopoietin (TPO), synthesized in the liver.
- Functions: primary hemostasis (adhesion via GPIb to vWF, aggregation via GPIIb/IIIa to fibrinogen), secretion (ADP, serotonin, TxA2), secondary hemostasis (surface for the coagulation cascade).
- Thrombocytopenia: ITP, HIT (heparin-induced), TTP, DIC, hypersplenism, drug-induced.
- Thrombocytosis: essential thrombocythemia, reactive (inflammation, iron deficiency anemia, post-splenectomy).
11. Clinical indicators
- Neutrophil-to-lymphocyte ratio (NLR) — marker of systemic inflammation and prognosis (> 4 is poor).
- RDW — red cell distribution width; ↑ in iron deficiency anemia, mixed anemias, prognostic marker in cardiac patients.
- MPV (mean platelet volume) — ↑ in activation, ↓ in ITP (aplastic).
- Absolute count is more important than percentage (neutropenia ≠ low %, it is < 1.5 × 10⁹/L).
12. FAQ
Why do mammalian erythrocytes lack a nucleus?
Evolutionary compromise: maximum space for Hb and flexibility. In exchange — inability to divide and a short lifespan (120 days).
How long do blood cells live?
Erythrocytes — 120 days, neutrophils — 1–4 days (in tissues), monocytes — 1–3 days in blood, then months to years as macrophages, lymphocytes — days to years (memory cells — lifelong), platelets — 7–10 days.
Why are erythrocytes smaller in iron deficiency anemia?
Iron deficiency → decreased heme synthesis → decreased Hb per cell → additional mitoses in the bone marrow → smaller and paler erythrocytes (microcytic hypochromic anemia).
What does a "left shift" of the leukocyte formula mean?
An increase in the proportion of immature neutrophil forms (band cells, metamyelocytes, myelocytes) — a sign of acute bacterial infection or active regeneration.
Why are lymphocytes increased and neutrophils decreased during a viral infection?
Viruses replicate inside cells → cellular immunity (T lymphocytes) is activated. Some viruses (CMV, EBV) cause characteristic atypical monocytosis with wide-cytoplasm lymphocytes.
See also: Anemias · Leukemias · Thrombocytopenias · Coagulopathies · Hematology catalog