Components of the Blood

Core Concepts

Plasma: the liquid matrix

Plasma is about 90–92% water (commonly taught) plus dissolved solutes. The plasma proteins dominate function: albumin (the most abundant) exerts colloid osmotic pressure that holds water in the blood vessels — too little albumin (as in severe liver disease or malnutrition) lets fluid leak into tissues, causing edema; globulins include transport proteins and the antibodies (immunoglobulins) of the immune system; fibrinogen is the soluble precursor that clotting converts into insoluble fibrin. The remaining solutes — sodium, potassium, calcium, chloride, bicarbonate, glucose, amino acids, urea, hormones, and dissolved gases — are the working cargo of the transport system. Serum is plasma minus the clotting proteins: the fluid left after blood has clotted.

Erythrocytes: the oxygen shuttles

Erythrocytes (red blood cells) are the most numerous formed elements — commonly taught adult references are roughly 4.2–5.4 million per µL in women and 4.7–6.1 million per µL in men. Their shape is a biconcave disc: thin in the middle, which maximizes surface area for gas exchange and lets the cell deform to squeeze through capillaries narrower than itself. Mature mammalian red cells have no nucleus and no mitochondria — they cannot divide or use the O₂ they carry, and they live about 120 days before being recycled in the spleen and liver. Their cytoplasm is packed with hemoglobin (~250–300 million molecules per cell, commonly taught), the O₂/CO₂ carrier detailed in Chapter 30. Production (erythropoiesis) happens in the red bone marrow under the control of erythropoietin (EPO), a hormone released mainly by the kidneys in response to low blood oxygen — which is why chronic hypoxia (altitude, lung disease, anemia) raises red cell counts, and why EPO misuse is banned in sport.

Leukocytes: the defenders

Leukocytes (white blood cells) are far fewer than red cells — commonly taught reference ~5,000–10,000 per µL — but they are the immune system's mobile units, and only ~2% circulate in blood at any moment; the rest patrol tissues. They divide into two groups. Granulocytes (visible granules in the cytoplasm): neutrophils (most abundant, ~50–70% of white cells) are the first responders that phagocytose bacteria; eosinophils (~2–4%) defend against parasitic worms and drive allergic responses; basophils (<1%) release histamine and heparin, promoting inflammation. Agranulocytes: lymphocytes (~20–40%) are the adaptive immune cells — B cells make antibodies, T cells kill infected cells and coordinate immunity; monocytes (~2–8%) leave the blood, mature into macrophages, and become powerful phagocytes and antigen presenters. An elevated count with a shift in the differential is a clinical clue to infection or allergy — a diagnostic habit, not a diagnosis.

Platelets and hemostasis

Thrombocytes (platelets) are not cells but fragments of megakaryocytes, giant marrow cells that shed membrane-bound packets into the blood; commonly taught reference is ~150,000–450,000 per µL. Their job is hemostasis — stopping blood loss — in three overlapping steps. (1) Vascular spasm: damaged vessel smooth muscle contracts, narrowing the lumen. (2) Platelet plug: platelets adhere to exposed collagen, become activated, and aggregate; activated platelets release chemicals (ADP, thromboxane) that recruit more platelets — a positive-feedback loop. (3) Coagulation: a cascade of clotting factors (plasma proteins, numbered I–XIII) converts prothrombin to thrombin, which converts soluble fibrinogen into insoluble fibrin threads that reinforce the plug into a stable clot. The cascade is calcium-dependent and can be triggered by either the tissue factor (extrinsic) or contact (intrinsic) pathway, which converge on the common pathway. Hemophilia is a hereditary deficiency of clotting factors (commonly factor VIII), causing excessive bleeding; thrombocytopenia (too few platelets) causes pinpoint bleeding and easy bruising.

Blood types: ABO and Rh

Red cell surfaces carry antigens (agglutinogens) that determine blood type. The ABO system: type A has A antigens and anti-B antibodies in plasma; type B has B antigens and anti-A antibodies; type AB has both antigens and neither antibody ("universal recipient" for red cells); type O has neither antigen and both antibodies ("universal donor" of red cells). Transfusing mismatched blood makes the recipient's antibodies bind the donor's red cells and agglutinate (clump) them, triggering hemolysis and potentially fatal reactions — so compatibility testing is mandatory. The Rh system adds the D antigen: Rh-positive people have it, Rh-negative people lack it and can develop anti-Rh antibodies after exposure (relevant in pregnancy if an Rh-negative mother carries an Rh-positive fetus). Blood types are inherited (ABO is a multiple-allele system with A and B codominant over O), a classic genetics connection.

Common Confusions