The Role of Blood

Blood Composition

  • Liquid connective tissue.
  • Composed of:
    • Plasma: 90% water, proteins (e.g., albumin).
    • Formed elements: Erythrocytes, leukocytes, platelets.
  • Haematocrit: Percentage of blood volume made of RBCs (47% ± 5% for males, 42% ± 5% for females).

Erythrocytes (Red Blood Cells)

  • Biconcave disc shape.
  • Filled with haemoglobin (Hb) for gas transport.
  • Major factor in blood viscosity.
  • Transports oxygen from lungs to tissues and carbon dioxide back to lungs.
  • Lifespan: 100-120 days.
  • Old RBCs are engulfed and broken down by WBCs in the spleen.

Leukocytes (White Blood Cells)

  • Less than 1% of total blood volume.
  • Primary function: defence against infection.
  • Increased numbers during infection (Leukocytosis): WBC count over 11,000/µl.
  • Types:
    • Neutrophils & monocytes: Phagocytose damaged cells and invaders.
    • Lymphocytes: Crucial to immunity, found mostly in lymphoid tissue.
    • Eosinophils: Attack antibody-coated objects, sensitive to allergens, reduce inflammation.
    • Basophils: Contain histamine (dilates blood vessels) and heparin (prevents blood clotting).

Hemostasis (Blood Clotting)

  • Platelets and plasma proteins initiate clot formation to prevent blood loss.
  • Stages:
    1. Vascular spasm
    2. Platelet plug formation
    3. Coagulation (blood clotting)