General characteristics of blood
Blood is a fluid tissue circulating within vessels, consisting of plasma (55–60%) and formed elements (40–45%). It performs respiratory, trophic, protective, excretory, and homeostatic functions.
Plasma is the intercellular substance, consisting of 90–93% water. The dry residue (7–10%) includes albumins, globulins, fibrinogen, amino acids, carbohydrates, lipids, hormones, enzymes, cofactors, vitamins, and ions. Plasma pH is approximately 7.36. Formed elements include erythrocytes, leukocytes, and platelets.
Erythrocytes and hemoglobin
Erythrocytes are post-cellular structures that have lost their nucleus and some organelles during development; they are highly differentiated and incapable of division. Their count is 3.9–5.5 × 10¹²/L in men and 3.7–4.9 × 10¹²/L in women.
- Up to 90% of erythrocytes are discocytes (biconcave); planocytes, echinocytes, stomatocytes, and spherocytes are also encountered.
- Aging occurs via crenation (projections on the plasmalemma) or invagination, culminating in hemolysis with the formation of «ghosts».
- Reticulocytes are immature forms; their elevation indicates pathology or blood loss.
- Poikilocytosis is a change in shape (e.g., sickle cells in sickle cell anemia); anisocytosis is a change in size (micro-, macro-, normocytes).
The cytoplasm contains up to 40% dry matter, predominantly hemoglobin, a complex protein composed of four globin polypeptide chains and heme (an iron-containing porphyrin). HbA predominates in adults, whereas HbF with a higher oxygen affinity predominates in the fetus. Binding of O₂ yields oxyhemoglobin, while transport of CO₂ yields carboxyhemoglobin.
Blood groups and Rhesus factor
The erythrocyte glycocalyx contains glycolipids and glycoproteins with oligosaccharides that determine the antigenic profile. Agglutinogens A and B are located on the surface, while agglutinins α and β are present in the plasma. Based on their combinations, four blood groups are distinguished; the key rule is that agglutinogen A must not meet agglutinin α, and B must not meet β.
Rhesus factor is present in 86% of people; in the event of antigen mismatch, agglutination occurs with the formation of "rouleaux". An important laboratory parameter is ESR (erythrocyte sedimentation rate): 4–8 mm/h in men, 7–10 mm/h in women; it increases in inflammation.
Leukocytes: granulocytes
Leukocytes (4–9 × 10⁹/L) are capable of active movement via pseudopodia, passing through the endothelium and basal lamina via chemotaxis. They are divided into granulocytes and agranulocytes. The percentage ratio of the types is the leukocyte differential count.
- Neutrophils (48–78%) are segmented microphages. Immature, band, and segmented forms are distinguished; an increase in immature forms is a «left shift». They contain azurophilic, specific, and tertiary granules (myeloperoxidase, lysozyme, lactoferrin, alkaline phosphatase). A respiratory burst is characteristic; their lifespan is 8–12 hours in the blood and 5–7 days in tissues.
- Eosinophils (0.5–5%) have a bilobed nucleus and crystalloid granules containing major basic protein, histaminase, and peroxidase. They bear FcεR, markers of allergy; they perform an anti-parasitic function and inactivate histamine.
- Basophils (0–1%) have an S-shaped nucleus; their granules exhibit metachromasia, contain heparin and histamine, and participate in immediate-type hypersensitivity.
Agranulocytes: lymphocytes and monocytes
Lymphocytes make up 20–35% of leukocytes; small (light and dark), medium, and large forms are distinguished. They are morphologically indistinguishable, but functionally divided into:
- B-lymphocytes mediate humoral immunity; upon antigenic stimulation, they transform into antibody-producing plasma cells.
- T-lymphocytes (≈70%) mature in the thymus and mediate cellular immunity; they are subdivided into T-helper, T-suppressor, and T-killer cells. Recirculation, which serves as immunological surveillance, is characteristic.
- Null lymphocytes are NK and K cells (up to 10%), natural killers.
Monocytes have a kidney-shaped or horseshoe-shaped nucleus, with a well-developed system of lysosomes and mitochondria. They belong to the mononuclear phagocyte system: they circulate in the blood for 12–32 hours, then migrate into tissues and transform into macrophages, living for up to a month.
Platelets and blood coagulation
Platelets (2–4 × 10⁹/L) are anucleate cytoplasmic fragments of bone marrow megakaryocytes. In the bloodstream, they appear as biconcave disks with a light hyalomere and a dark granulomere. Young, mature, old, degenerative, and giant forms are distinguished.
- The open canalicular system is connected to plasmalemma invaginations and ensures the release of granule contents.
- The dense tubular system is similar to the smooth ER.
- α-granules contain coagulation proteins and glycoproteins; δ-granules contain serotonin, histamine, and adrenaline; lysosomes contain hydrolytic enzymes.
Upon vascular injury, platelets adhere to the basement membrane, release granules, and trigger the conversion of fibrinogen to fibrin and thrombus formation. The thrombus undergoes retraction (up to 10% of volume) due to the actin-myosin apparatus. Platelets accumulate serotonin, secrete growth factors, vasoactive amines, and hydrolases. Lifespan is 9–10 days.
Key points
- Blood = plasma (55–60%) + formed elements; plasma pH is 7.36.
- Erythrocytes are anucleate discocytes that transport O₂ and CO₂ via hemoglobin.
- Blood groups are determined by agglutinogens A/B and agglutinins α/β; the Rhesus factor is also taken into account.
- The leukocyte differential count and left shift are critical markers of inflammation.
- Neutrophils are microphages; eosinophils are associated with allergies and parasitic infestations; basophils release heparin and histamine.
- B, T, and NK lymphocytes provide humoral, cellular, and nonspecific immunity.
- Monocytes transform into macrophages of the mononuclear phagocyte system within tissues.
- Platelets ensure adhesion, aggregation, and the formation of a fibrin thrombus followed by retraction.