Composition and Functions of Blood

Composition and Functions of Blood is a specialized body fluid essential to sustaining life. Its composition is a complex mixture of cells, proteins, ions, and other substances, each contributing to the vital functions that maintain homeostasis in the body. Below is a detailed examination of the composition and functions of blood.

Composition of Blood

Blood consists of two major components: plasma and formed elements. Each part has unique characteristics and functions.

1. Plasma (55% of Blood Volume)

Plasma is the liquid portion of blood, about 55% of its total volume. Plasma is the transport medium for the formed elements (cells) and dissolved substances.

1. Water (90% of Plasma):

• • Acts as a solvent for carrying other components.
  • Helps regulate body temperature by distributing heat.

2. Plasma Proteins (7-8% of Plasma):

Synthesized mainly in the liver, plasma proteins perform a range of functions:

a. Albumin:

• • The most abundant plasma protein.
  • Maintains oncotic pressure, preventing water from leaking from blood vessels into tissues.
  • Acts as a carrier for fatty acids, hormones, and drugs.

b. Globulins:

• • Immunoglobulins (antibodies) play a key role in immune defence.
  • Transport globulins bind to and transport ions, hormones, and lipids.

c. Fibrinogen:

• • A critical component in blood clotting (coagulation).
  • Converted into insoluble fibrin during clot formation, helping to seal wounds and stop bleeding.

d. Nutrients:

• • Glucose: Primary energy source for cells.
  • Amino acids: Building blocks for proteins.
  • Lipids: Transported in complexes with proteins (lipoproteins) for energy or storage in adipose tissue.

e. Electrolytes:

• • Include sodium, potassium, calcium, bicarbonate, chloride, and magnesium.
  • Helps maintain fluid balance and osmotic pressure and contributes to nerve transmission and muscle contraction.

f. Gases:

• • Oxygen (O₂): Dissolved in plasma and bound to haemoglobin in red blood cells.
  • Carbon dioxide (CO₂): Transported in dissolved form as bicarbonate and bound to haemoglobin for exhalation by the lungs.

g. Waste Products:

Include urea, creatinine, and uric acid, which are by-products of metabolism filtered out by the kidneys.

h. Hormones:

Transported in plasma to target tissues, they regulate physiological processes such as metabolism, growth, and reproduction.

2. Formed Elements or blood cells (45% of Blood Volume)

The formed elements of blood consist of cellular components that serve specific functions.

A. Red Blood Cells (RBCs) / Erythrocytes

• Structure:
  • RBCs are biconcave, disc-shaped cells.
  • They lack a nucleus and most organelles, maximizing space for haemoglobin.
  • The biconcave shape increases the surface area for gas exchange.
• Haemoglobin:
  • Haemoglobin is a protein in RBCs that binds to oxygen in the lungs and carries it to tissues.
  • It also binds carbon dioxide, a waste product, and returns it to the lungs for exhalation.
  • Each haemoglobin molecule contains iron, which is essential for oxygen binding.
• Function:
  • RBCs are responsible for transporting oxygen from the lungs to tissues and returning carbon dioxide to the lungs.
• Lifespan: Approximately 120 days. Macrophages remove old or damaged RBCs in the spleen and liver.

B. White Blood Cells (WBCs) / Leukocytes

WBCs are the body’s defence against infections, foreign invaders, and abnormal cells. They are classified into two main categories: granulocytes and agranulocytes.

1. Granulocytes:

Contains granules with enzymes that are involved in immune responses.

• Neutrophils:

Function: The most abundant type of WBC, they act as the first line of defence against bacterial infections. Neutrophils are highly phagocytic, engulfing and digesting bacteria and cellular debris.

Lifespan: Short, usually a few hours to a few days.

• Eosinophils:

Function: Combat parasitic infections and play a role in allergic reactions and asthma by releasing histamine and other mediators.

Lifespan: Several days.

• Basophils:

Function: Release histamine and heparin during allergic reactions, promoting inflammation and increasing blood flow to affected tissues.

Lifespan: Hours to a few days.

2. Agranulocytes:

Lack of visible granules in the cytoplasm.

• Lymphocytes:
  • Function: Major players in the adaptive immune response.
    • T-Cells: Directly attack virus-infected or cancerous cells and regulate other immune responses.
    • B-Cells: Produce antibodies that target specific antigens (foreign substances).
    • Natural Killer (NK) Cells: Detect and destroy abnormal cells without prior activation.
• Monocytes:
  • • Function: Differentiate into macrophages once they leave the bloodstream and enter tissues, where they phagocytize pathogens, dead cells, and other debris.
    • Lifespan: Several hours to days in circulation, months to years in tissues as macrophages.

C. Platelets (Thrombocytes)

• Structure: Small, disc-shaped cell fragments derived from megakaryocytes in the bone marrow.
• Function:
  • Platelets are essential for blood clotting. They adhere to damaged blood vessel walls and form a plug that prevents blood loss.
  • They release chemical signals that activate the clotting cascade, converting fibrinogen to fibrin, which forms a stable clot.
• Lifespan: Approximately 8-10 days.

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Functions of Blood

Blood has many vital functions of blood for life, categorized into transport, regulation, protection, and homeostasis.

1. Transportation

• Oxygen and Carbon Dioxide: RBCs transport oxygen from the lungs to tissues and return carbon dioxide from tissues to the lungs.
• Nutrients: Blood delivers nutrients absorbed from the digestive system (e.g., glucose, amino acids, fatty acids) to cells throughout the body.
• Waste Products: Blood transports metabolic wastes, such as urea and carbon dioxide, to excretory organs (kidneys and lungs) for removal.
• Hormones: Blood carries hormones secreted by endocrine glands to target organs, coordinating processes like growth, metabolism, and reproduction.

2. Regulation

• Body Temperature: Blood helps regulate body temperature by absorbing and distributing heat. It adjusts blood flow to the skin for heat loss or retention.
• pH Balance: Blood maintains a stable pH (around 7.35-7.45) through buffering systems like bicarbonate.
• Fluid and Electrolyte Balance: Plasma proteins and electrolytes help regulate water movement between blood and tissues, maintaining proper hydration and electrolyte levels.

3. Protection

• Immune Response: WBCs and antibodies in the blood detect and neutralize pathogens such as bacteria, viruses, and other harmful invaders. Leukocytes act as defenders against infection.
• Blood Clotting: Platelets and clotting factors in blood initiate hemostasis. In vessel injury, platelets form a plug, and fibrin stabilizes the clot, preventing excessive blood loss.

4. Homeostasis

• Osmotic Pressure: Proteins like albumin in plasma maintain osmotic pressure, preventing fluid leakage from blood vessels into tissues.
• Gaseous Exchange: Blood maintains oxygen and carbon dioxide levels in balance with respiratory activity, ensuring proper cellular respiration and metabolic function.