Composition and Functions of Blood

COMPOSITION OF BLOOD

Total blood volume : 5-6 litres (8% of body weight or 80 ml/kg body weight)

Specific gravity : 1050-1060

Viscosity : 4-5 times that of water 

PH : 7.4 ± 0.05; alkaline

If anticoagulated sample of blood is allowed to stand in a narrow tube, on settling it separates out into cells and plasma.

CELLS

The cellular elements of blood represents 45% of the total blood volume, called Packed Cell Volume (PCV) or Haematocrit. It includes:

1. Erythrocytes or Red Blood Corpuscles (RBC's)

Normal count: 5 million/cumm (5 x 10 per cumm).

2. Leucocytes or White Blood Corpuscles (WBCs)

Normal count: 4,000-11,000/cumm (4-11 x10³ per cumm).

3. Platelets or Thrombocytes

Normal count: 1.5-4 lacs/cumm (0.15-0.4 x 10° per cumm).

PLASMA

1. Plasma is a clear, straw coloured portion of the blood and represents 55% of the total blood volume. It contains:

(i) 91% water; and

(ii) 9% solids. The solids comprises of: 

     (a) 1% inorganic molecules.

     (b) 8% organic molecules.

FUNCTIONS OF BLOOD

1. Respiratory: Transport of oxygen from lungs to the tissues and of carbon-dioxide from the tissues to the lungs.

2. Nutritive: Conveys absorbed food mate rials, glucose, amino acids, fatty acids, vitamins, electrolytes and trace metals from the alimentary canal to the tissues for utilization and storage.

3. Excretory: Transports the metabolic wastes e.g. urea, uric acid, creatinine etc. to the kidney, skin and intestine for their removal.

4. 'Homeostatic' for water, pH and electro lyte concentration: Blood forms internal environment of the cell i.e. 'MILLIEU INTERIEUR' in terms of volume, composition, concentration, pH and temperature, which is regulated to normal physiological limits with respect to minor changes in the body. This mechanism is called Homeostasis (W.E. Canon). Buffering power of haemoglobin helps to maintain constancy of blood pH.

5. Regulation of body temperature: Blood preserves the very narrow range in body temperature.

Blood whose major constituent is water has: 

(i) High specific heat - This buffers sudden change (rise temperature. or fall) in body

(ii) High conductivity - This helps to take out heat from an organ for uniform distribution throughout the body.

(iii) High latent heat of evaporation.

6. Chemical for communication and protection

(i) Concentration of hormones and various substances in the blood is regulated through feedback mechan isms.

(ii) Within blood circulates the entire complex of humoral antibodies important in defence against infection, initiation of inflammation and regulation of Haemostasis (clotting mechanism).

7. Plasma proteins functions

(i) Exert the osmotic pressure which influences the exchange of fluid between blood and tissues.

(ii) Act as a reservoir of proteins. 

(iii) Combine with many substances e.g. iron, thyroxine and steroid hormones to form transportable complexes from which the active components are released at the appropriate sites.