Composition and Functions of Blood

Complete study notes covering the definition, general characteristics, detailed composition of plasma and formed elements, hematopoiesis, and clinical correlations.

1. Composition and Functions of Blood

1.1 Definition and General Characteristics

Blood is a specialized connective tissue consisting of cells suspended in a liquid extracellular matrix called plasma. It circulates through the heart, arteries, capillaries, and veins, maintaining homeostasis throughout the body.

Key Parameters:

• Total blood volume: ~7-8% of body weight (5-6 L in a 70 kg adult male).
• pH: 7.35-7.45 (slightly alkaline).
• Temperature: ~38°C (slightly higher than core body temperature).
• Viscosity: 3-5 times greater than water.
• Color: Bright red (oxygenated) to dark red (deoxygenated).
• Specific gravity: 1.055-1.065.

1.2 Composition of Blood

Blood consists of two principal components:

1.2.1 Plasma

Plasma is the straw-colored liquid portion of blood.

• Water (92%): Serves as solvent for transport.
• Plasma Proteins (7%):
  • Albumin: Liver origin; maintains colloid osmotic pressure; transports fatty acids, bilirubin, and drugs.
  • Globulins: Alpha/beta transport lipids, iron, copper; gamma = immunoglobulins.
  • Fibrinogen: Liver origin; essential for blood clotting (converted to fibrin).
• Other Solutes (1%):
  • Electrolytes (Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, HPO4^2-)
  • Nutrients (glucose, amino acids, fatty acids, vitamins)
  • Waste products (urea, uric acid, creatinine, bilirubin)
  • Respiratory gases (O2, CO2) and hormones

1.2.2 Erythrocytes (Red Blood Cells)

Structure:

• Biconcave disc shape; 7-8 um in diameter.
• Anucleate (no nucleus) and lack organelles in mature form.
• Cell membrane contains spectrin and actin for flexibility through capillaries.
• Cytoplasm filled with hemoglobin (~280 million molecules per cell).

Hemoglobin:

• Molecular weight: 64,458 Da.
• Four polypeptide chains (2 alpha, 2 beta in adults = HbA) + four heme groups.
• Each heme contains Fe2+ binding one O2 molecule.
• Normal types: HbA (95-98%), HbA2 (2-3%), HbF (<1% in adults).

Function: Transport of O2 from lungs to tissues and CO2 from tissues to lungs.

Normal Values:

Lifespan: 120 days. Senescent RBCs removed by macrophages in spleen, liver, and bone marrow.

1.2.3 Leukocytes (White Blood Cells)

Leukocytes are nucleated cells critical for immune defense.

Normal total WBC count: 4,000-11,000 cells/uL (4.0-11.0 x 10^9/L).

1.2.4 Platelets (Thrombocytes)

• Small cell fragments (2-4 um) derived from megakaryocytes in bone marrow.
• Anucleate but contain mitochondria, lysosomes, and granules.
• Normal count: 150,000-400,000/uL (150-400 x 10^9/L).
• Lifespan: 8-10 days.

Functions:

1. Formation of platelet plugs during hemostasis.
2. Clot retraction through actin and myosin filaments.
3. Release of growth factors (PDGF) promoting tissue repair.

1.3 Hematopoiesis

Hematopoiesis is the process of blood cell formation. In adults, it occurs primarily in red bone marrow (sternum, ribs, vertebrae, pelvis, proximal femurs, and humeri).

Stages:

1. Pluripotent hematopoietic stem cells — self-renewal and differentiation.
2. Multipotent progenitor cells — myeloid and lymphoid lineages.
3. Committed progenitor cells — specific cell lines.
4. Precursor cells (blasts) — immature forms with specific morphology.
5. Mature cells — released into circulation.

Key Regulatory Factors:

• Erythropoietin (EPO): Kidney production; stimulates erythropoiesis.
• Colony-stimulating factors (CSFs): Stimulate granulocyte/monocyte production.
• Thrombopoietin (TPO): Stimulates megakaryocyte and platelet production.
• Interleukins: Regulate lymphocyte development and immune responses.

1.4 Functions of Blood

1.5 Clinical Correlation: Complete Blood Count (CBC)

The CBC is the most commonly ordered hematological investigation. It provides:

• RBC count, hemoglobin, hematocrit.
• RBC indices (MCV, MCH, MCHC, RDW).
• WBC count with differential.
• Platelet count.
• Peripheral blood smear morphology.

Summary — Composition and Functions of Blood