Blood Composition and Components

Blood Composition and Components

Blood has four main components: (1) Plasma — the liquid (mostly water) that transports dissolved substances including glucose, hormones, and urea; (2) Red blood cells — biconcave cells containing haemoglobin that carry oxygen around the body; (3) White blood cells — which defend against infection (phagocytes engulf pathogens; lymphocytes produce antibodies); and (4) Platelets — cell fragments that trigger blood clotting when a vessel is damaged.

1. Plasma (55% of blood volume)

Plasma is the liquid component of blood, consisting of 90% water plus dissolved substances:

• Proteins: Albumin (maintains osmotic pressure), fibrinogen (blood clotting), antibodies (immune defense)
• Nutrients: Glucose, amino acids, fatty acids, vitamins
• Waste products: Urea, carbon dioxide, lactic acid
• Hormones: Chemical messengers transported to target organs
• Ions: Sodium, chloride, calcium, potassium for cellular function

2. Red Blood Cells (Erythrocytes)

Red blood cells are specialized for oxygen transport with remarkable adaptations:

• Biconcave shape: Increases surface area for gas exchange by 20-30%
• No nucleus: More space for hemoglobin (95% of dry weight)
• Flexible membrane: Allows passage through tiny capillaries (3 micrometers)
• Hemoglobin protein: Contains iron that binds reversibly with oxygen
• Small size (7 micrometers): Maximizes surface area to volume ratio

3. White Blood Cells (Leukocytes)

White blood cells defend against infection and disease with two main types:

Phagocytes (neutrophils and macrophages):

• Engulf and digest pathogens by phagocytosis
• Form pus at infection sites
• First line of cellular defense (innate immunity)
• Can move out of blood vessels to infection sites

Lymphocytes (B cells and T cells):

• B cells: Produce specific antibodies against antigens
• B cells also produce antitoxins — chemicals that neutralise the poisonous toxins released by pathogens
• T cells: Coordinate immune response and kill infected cells
• Provide specific immunity and immunological memory
• Smaller than phagocytes but live much longer

4. Platelets (Thrombocytes)

Platelets are cell fragments essential for blood clotting:

• No nucleus, produced by bone marrow megakaryocytes
• Release clotting factors when activated by vessel damage
• Form temporary platelet plug at wound site
• Lifespan of 8-10 days in circulation