Blood
Plasma, red and white blood cells, platelets, hemostasis and blood typing
By the end of this unit you can…
- ✓Describe the composition and functions of blood and its normal values
- ✓Describe erythrocyte structure, hemoglobin, erythropoiesis and the fate of old RBCs
- ✓Classify leukocytes and describe the function of each
- ✓Explain the three steps of hemostasis and disorders of clotting
- ✓Explain ABO and Rh blood groups, transfusion reactions and hemolytic disease of the newborn
Key terms
Practice →1Composition & functions
Blood is the only fluid connective tissue: living formed elements suspended in a nonliving fluid matrix, plasma. Spun in a centrifuge, blood separates into three layers:
| Layer | % of volume | Contents |
|---|---|---|
| Plasma (top) | ~55% | ~90% water + proteins, nutrients, gases, hormones, wastes, electrolytes |
| Buffy coat (middle) | < 1% | Leukocytes and platelets |
| Erythrocytes (bottom) | ~45% = hematocrit | Red blood cells (normal ~47% ± 5 in males, 42% ± 5 in females) |
Blood is ~8% of body weight; average volume is 5–6 L in males and 4–5 L in females. Normal pH 7.35–7.45; temperature ~38 °C; more viscous than water.
| Function category | Examples |
|---|---|
| Distribution | Delivers O₂ and nutrients; carries wastes to elimination sites; transports hormones |
| Regulation | Maintains body temperature, normal pH (buffers), and fluid volume |
| Protection | Prevents blood loss (clotting); prevents infection (antibodies, complement, WBCs) |
| Plasma protein | Made by | Role |
|---|---|---|
| Albumin (~60%) | Liver | Main contributor to osmotic pressure; carrier; buffer |
| Globulins (~36%) | Liver (α, β) & plasma cells (γ = antibodies) | Transport; immunity |
| Fibrinogen (~4%) | Liver | Forms fibrin threads in clotting |
2Erythrocytes (red blood cells)
Erythrocytes are small biconcave discs with no nucleus and no organelles — essentially bags of hemoglobin. The biconcave shape gives a huge surface area for gas exchange; flexible spectrin lets them squeeze through capillaries. They have no mitochondria, so they make ATP anaerobically and don't consume the O₂ they carry. ~4.3–5.2 million/µL (females) and 5.1–5.8 million/µL (males).
Hemoglobin (Hb): four polypeptide chains (2 α, 2 β) each bound to a heme group with an iron atom that binds one O₂ → each Hb carries 4 O₂. Normal Hb ~12–16 g/100 mL (females), 13–18 g (males). Oxyhemoglobin is bright red; deoxyhemoglobin dark red; carbaminohemoglobin carries ~20% of CO₂.
Hematopoiesis occurs in red bone marrow; all formed elements arise from the hematopoietic stem cell (hemocytoblast). Erythropoiesis (RBC production) takes ~15 days: stem cell → myeloid stem cell → proerythroblast → erythroblasts (Hb synthesis, nucleus ejected) → reticulocyte → mature erythrocyte. Reticulocytes are ~1–2% of RBCs — a count reflects the rate of production.
- 1Stimulus: hypoxia (low blood O₂) — from too few RBCs, low Hb, or low O₂ availability (high altitude).
- 2The kidneys release erythropoietin (EPO).
- 3EPO stimulates red marrow to speed erythropoiesis.
- 4More RBCs → O₂-carrying ability rises → hypoxia removed → EPO falls.
Dietary needs: iron, vitamin B₁₂ and folic acid (for DNA synthesis in dividing cells).
RBCs live 100–120 days. Old cells are destroyed by macrophages in the spleen (and liver): heme → iron (stored as ferritin/hemosiderin, recycled) + bilirubin (yellow pigment, excreted by the liver in bile); globin → amino acids.
| Disorder | Cause / features |
|---|---|
| Anemia | Blood's O₂-carrying capacity too low → fatigue, pallor, shortness of breath, chills |
| Hemorrhagic anemia | Blood loss |
| Iron-deficiency anemia | Low iron → small, pale RBCs (microcytic) |
| Pernicious anemia | Lack of intrinsic factor (needed to absorb B₁₂) → large, pale cells; treated with B₁₂ injections |
| Aplastic anemia | Destruction or inhibition of red marrow (drugs, radiation) |
| Sickle-cell anemia | Abnormal Hb (HbS — one amino acid changed) makes RBCs crescent-shaped when O₂ is low → they block vessels and rupture |
| Polycythemia | Abnormal excess of RBCs → thick blood; can be from bone marrow cancer or high altitude (secondary) |
3Leukocytes (white blood cells)
Leukocytes are the only complete cells in blood (nucleus + organelles), making up < 1% of blood volume (4,800–10,800/µL). They defend against disease, using diapedesis to leave capillaries and chemotaxis to follow chemical trails to damaged tissue. Leukocytosis = WBC count > 11,000/µL (normal response to infection); leukopenia = abnormally low count.
| Leukocyte | % of WBCs | Look | Function |
|---|---|---|---|
| Neutrophil (granulocyte) | 50–70% (most) | Multilobed nucleus (3–6 lobes); pale lilac granules | Phagocytize bacteria — first responders; numbers rise in acute bacterial infection |
| Lymphocyte (agranulocyte) | 25–45% | Large dark round nucleus, thin rim of cytoplasm | Immunity — T cells attack virus-infected and tumor cells; B cells → plasma cells → antibodies |
| Monocyte (agranulocyte) | 3–8% | Largest; kidney/U-shaped nucleus | Leave blood and become macrophages — chronic infections; activate lymphocytes |
| Eosinophil (granulocyte) | 2–4% | Bilobed nucleus; red granules | Kill parasitic worms; role in allergies and asthma |
| Basophil (granulocyte) | 0.5–1% (rarest) | Bilobed nucleus; large dark purple granules | Release histamine (inflammation, vasodilation) — like mast cells |
4Platelets & hemostasis
Platelets are cytoplasmic fragments of huge cells called megakaryocytes; they have no nucleus, live ~10 days and are regulated by thrombopoietin. Normal count 150,000–400,000/µL.
- 1Vascular spasm — smooth muscle in the damaged vessel wall contracts, immediately reducing blood loss.
- 2Platelet plug formation — platelets stick to exposed collagen (with von Willebrand factor), become activated and release chemicals (ADP, serotonin, thromboxane A₂) that attract more platelets — a positive feedback cycle.
- 3Coagulation — a cascade of clotting factors (intrinsic and extrinsic pathways converge) forms prothrombin activator → converts prothrombin → thrombin → thrombin converts soluble fibrinogen → fibrin threads, which mesh into a clot (with Ca²⁺ required at many steps).
Then clot retraction (platelets pull the edges together; serum is squeezed out) and repair (PDGF stimulates healing); finally fibrinolysis — plasmin dissolves the clot. Normal clotting is prevented in intact vessels by smooth endothelium, nitric oxide and prostacyclin, and anticoagulants like heparin and antithrombin III.
| Disorder | Problem |
|---|---|
| Thrombus / embolus | Clot in an unbroken vessel / a clot that breaks free and travels (pulmonary embolism, stroke) |
| Thrombocytopenia | Too few platelets → widespread small bleeds (petechiae) |
| Hemophilia | Hereditary lack of a clotting factor (hemophilia A = factor VIII — X-linked) |
| Impaired liver function | Less synthesis of clotting factors; vitamin K deficiency |
5Blood groups & transfusions
RBC membranes carry glycoprotein antigens (agglutinogens). The plasma contains preformed antibodies (agglutinins) against the antigens not present on your own cells. Mismatched transfusion → donor RBCs agglutinate (clump) and are destroyed (hemolysis) → kidney failure, possibly fatal.
| Blood type | RBC antigens | Plasma antibodies | Can receive |
|---|---|---|---|
| A | A | Anti-B | A, O |
| B | B | Anti-A | B, O |
| AB | A and B | None | A, B, AB, O — universal recipient |
| O | None | Anti-A and anti-B | O only — universal donor |
Rh factor: Rh⁺ people (~85% of Americans) have the Rh (D) antigen. Unlike ABO, anti-Rh antibodies are not preformed — an Rh⁻ person makes them only after exposure to Rh⁺ blood.