Erythrocytes & Hemostasis – Quick Review

Erythropoiesis

• Red blood cells (RBCs) formed in red bone marrow.
• Stimulated by erythropoietin (EPO) released mainly by kidneys in response to blood O_2 levels.

RBC Turnover & Recycling

• Lifespan: 100!–!120 days.
• No protein synthesis → loss of elasticity → cannot self-repair.
• Spleen screens and removes aged, rigid cells; macrophages destroy them.
• Iron & globin recycled; part of heme → bile → feces.
• Key lab indices: RBC count, hematocrit, hemoglobin.

RBC Disorders

• Balance of production vs. destruction critical.

Anemia (deficiency)

• Etiologies
  • Blood loss.
  • ↓ Production (hypoplastic/aplastic): nutrient lack, ↓EPO (renal failure), marrow damage (mutations, autoimmune, toxins, drugs, radiation, idiopathic).
  • ↑ Destruction (hemolytic):
  • Inherited: intrinsic RBC defects (e.g., abnormal hemoglobin).
  • Acquired: toxins, drugs, autoimmune, infections, overactive spleen, transfusion mismatch.
• Consequences: hypoxemia → weakness, confusion; severe → organ failure, life-threatening.

Polycythemia (excess)

• Primary: polycythemia vera (myeloproliferative cancer).
• Secondary: chronic hypoxia → ↑EPO (smoking, pollution, emphysema, high altitude, intense training).
• Effects: ↑ blood volume/viscosity & BP → thrombosis (MI, stroke, PE); ↑ cardiac workload → heart failure.

Hemostasis

Physiological stoppage of bleeding; three sequential steps.

1. Vasoconstriction

• Immediate smooth-muscle contraction at injury.
• Sustained by humoral vasoconstrictors (e.g., 5-hydroxytryptamine).

2. Temporary Hemostatic (Platelet) Plug

• Platelet adhesion to exposed collagen/endothelium → swell, pseudopodia, degranulation.
• Release ADP & thromboxane A_2 → activate & recruit more platelets → aggregation.
• Platelet Activating Factor (from neutrophils/monocytes) amplifies aggregation.
• Prostacyclin limits plug spread.
• Low-dose aspirin blocks thromboxane A_2 synthesis → inhibits aggregation.

3. Definitive Hemostatic Clot

• Platelet plug triggers intrinsic pathway → prothrombin activator → fibrin formation → stable clot.