6. HAEMOPOIETIC SYSTEM – Composition and functions of blood

BLOOD AND HAEMOPOIETIC SYSTEM

The system of organs and tissues, including the bone marrow, spleen, thymus and lymph nodes, involved in the production of cellular blood components is known as haematopoietic system. Blood is a connective tissue that plays a vital role to carry various life processes and protects the body against diseases. Haematology is the branch of medical science concerned with the study of blood, blood-forming tissues, and blood disorders.

COMPOSITION OF BLOOD

Blood is a connective tissue having a liquid matrix with cells and cell fragments. Around 8% of total body weight is blood (adult female 4-5 litres and adult male 5-6 litres). Blood comprises of: 1) Blood Plasma (clear extracellular fluid), and 2) Formed elements (RBCs, WBCs, and platelets).

BLOOD PLASMA

Around 55% of blood (2.7-3.0 litres in an average human) constitutes blood plasma (golden-yellow in colour). Blood plasma has 92% water, 8% blood plasma proteins, and other substances in trace amount.

Components of Plasma

1. Plasma Proteins:
   • Albumin: Formed in liver; most abundant plasma protein; maintains osmotic pressure.
   • Globulin: Primary immunological agent; three types: alpha (α), beta (β), and gamma (γ) globulins.
   • Fibrinogen: Key component of blood clotting; solidifies to fibrin fibres.
   • Blood clotting factors (e.g., prothrombin).
2. Other substances (hormones, carbon dioxide, electrolytes, vitamins, etc.)

Functions of Blood Plasma Proteins

1. Oncotic Pressure: Helps keep fluid within blood vessels.
2. Transport: Act as carriers for hormones, vitamins, metals, and drugs.
3. Immunity: Immunoglobulins recognize and neutralize pathogens.
4. Clotting: Fibrinogen and clotting factors are essential for blood clot formation.
5. Buffering: Help maintain pH balance of blood.
6. Acute Phase Response: Production of acute-phase proteins during inflammation.
7. Enzyme Activity: Some plasma proteins have enzymatic activity.

FORMED ELEMENTS

The formed elements are cells and cell fragments suspended in plasma. Three classes: 1) Erythrocytes (RBCs), 2) Leukocytes (WBCs), and 3) Thrombocytes (platelets).

Red Blood Cells (Erythrocytes)

RBCs are discotic-shaped cells constituting 99% of the blood and carrying haemoglobin molecules. Each cell has 65% water and 35% solids (33% haemoglobin + 2% stromal meshwork).

Functions of RBCs

1. Oxygen Transporter: Participate in oxygen transportation.
2. Release of ATP and Vessel Dilation: Shear stress causes ATP release, relaxing and dilating vessel walls.
3. Release of S-nitrosothiols: On deoxygenation, release S-nitrosothiols to dilate vessels.
4. Immunity: Release free radicals that rupture bacterial cell walls.

White Blood Cells (Leukocytes)

WBCs fight against external organisms. They are classified as granulocytes (with granules) and agranulocytes (without granules).

Types of Leukocytes

• Granulocytes:
  • Neutrophils (60-70%): 5-7 lobed nucleus; hydrolytic enzymes digest bacteria.
  • Eosinophils (1-5%): 2-3 lobed nucleus; stained with acid dyes (eosin).
  • Basophils: S-shaped 2-3 lobed nucleus; contain heparin, histamine, and serotonin.
• Agranulocytes:
  • Monocytes (1-6%): Ovoid or kidney-shaped nucleus; largest blood cells (diameter 12-20µ); transform into macrophages.
  • Lymphocytes (20-40%): Small and slightly spherical with large round nuclei; include B-lymphocytes, T-lymphocytes, and Natural Killer cells.

Functions of WBCs

1. Phagocytosis: Digestion of foreign particles and bacteria by Neutrophils.
2. Antibody Formation: Formation of B- and γ-serum globulin.
3. Fibroblasts Formation: Transform into fibroblasts and help in repair process.
4. Heparin Secretion: Secreted by Basophils; blocks intravascular clotting.
5. Anti-histamine Function: Defensive function against allergic conditions.

Platelets (Thrombocytes)

Platelets are small, non-nucleated discs (diameter 2-4µm), produced from cytoplasm of megakaryocytes in red bone marrow. Normal count: 2,00,000-3,50,000/mm³. Help in haemostasis (blood clotting).

Functions of Platelets

1. Vasoconstriction: Secrete serotonin to constrict damaged vessels.
2. Platelet Plug Formation: Adhere and aggregate to form temporary seal.
3. Coagulation (Blood Clotting): Prothrombin activator → thrombin → fibrin.
4. Fibrinolysis: Breakdown of clot; plasminogen converts to plasmin to dissolve fibrin.

FUNCTIONS OF BLOOD

1. Transportation: Transports O₂, CO₂, nutrients, wastes, hormones.
2. Thermoregulation: Transfers heat during blood circulation.
3. Hydraulic Functions: Blood flow restriction causes engorgement (e.g., erection).
4. Act as a Vehicle: Carries hormones, vitamins, and essential chemicals.
5. Maintenance of Ion Balance: Maintains ionic balance between cells and tissues.
6. Regulation of Water Balance: Freely interchangeable with interstitial fluid.
7. Regulation of Acid-Base Balance: Plasma proteins and haemoglobin act as buffers.
8. Regulation of Body Temperature (Homeostasis): Maintains balance between heat loss and gain.
9. Storage Function: Reservoir of water, proteins, glucose, sodium, and potassium.
10. Defensive Function: WBCs protect against bacteria and antigens; lymphocytes develop immunity.