Blood Composition and Functions

Blood - Introduction

• Blood is a vital fluid in humans and other animals
• Nourishes body organs and tissues
• Carries away waste materials

Composition of Blood

• Plasma (55%):
  • Mostly water (90%)
  • Proteins (8%)
    • Albumin (4-4.5 gm%)
    • Globulins (2.5-2.7 gm%)
      • Alpha 1 and 2, beta 1 and 2, and gamma globulins
    • Fibrinogen (200-400 mg%)
    • Prothrombin (10 mg%)
  • Organic substances (1.1%)
    • Nutrients (as digested food)
    • Waste products (as urea and cellular waste products)
    • Other constituents (e.g., hormones, vitamins, lipids, and enzymes)
  • Inorganic substances (0.9%)
    • Ions like Na+, Cl-, and HCO3-
• Cells (45%):
  • White blood cells (WBCs, or leukocytes)
  • Red blood cells (RBCs, or erythrocytes)
  • Platelets (thrombocytes)

Functions of Plasma Proteins

• Nutrition: Plasma proteins are used in protein metabolism.
• Hemostasis (Blood Clotting): Fibrinogen and prothrombin are essential for the coagulation process.
• Blood Viscosity: Fibrinogen and globulins affect viscosity, important for arterial blood pressure maintenance.
• Suspension of RBCs: Important for ESR (erythrocyte sedimentation rate) which is dependent on fibrinogen and globulin levels.
• Carrier: Plasma proteins transport important elements like hormones, vitamins, and minerals, preventing rapid elimination in urine.
• Immunity: Gamma globulins are used to exert passive immunity.
• Regulation of capillary permeability: Plasma proteins maintain normal capillary permeability.
• Osmotic function: Plasma protein osmotic pressure is important for water reabsorption in capillaries, counteracting capillary filtration pressure. Albumin is the most contributing protein to plasma osmotic pressure.

Hemostasis (Blood Clotting)

• Vascular spasm: Injured vessel walls contract to reduce blood loss. This involves local myogenic contraction, local nervous reflexes, and vasoconstrictor substances.
• Platelet plug formation: Platelets adhere to injured sites, release substances (e.g., serotonin and thromboxane A2), and aggregate to form a plug.
• Clot formation: Fibrinogen is converted to insoluble fibrin, forming a network to trap blood cells .
• Clot retraction: Shrinkage of the clot to decrease the damaged vessel lumen, caused by substances like retractozyme and contractile proteins in platelets.
• Repair of injured blood vessel: Platelet-derived growth factor (PDGF) stimulates fibroblasts and smooth muscle, leading to the formation of fibrous tissue.