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1. BLOOD

2. Blood is a fluid connective tissue which transports substances from one part of the body to another. It provides nutrients and hormones to the tissues and removes their waste products. Blood , confined in the cardiovascular system, constitutes a major part of the extracellular fluid of the body

3. 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

4. Some of the important physical characteristics of blood are: • Colour of blood is opaque red due to the pigment haemoglobin in the RBCs . The arterial blood is bright red and venous blood is dark red in colour . • Volume of blood in an average adult is about 5–6 L (8% of the body weight or 80 ml/kg body weight ). • Viscosity of blood is five times more than that of water . • Specific gravity of blood is 1.050–1.060. Specific gravity of RBC is greater (1.090) than that of plasma (1.030 ). • pH of blood is about 7.4 (ranges from 7.38 to 7.42), i.e. it is alkaline in nature . In acidosis, pH of blood falls below 7.38 and in alkalosis pH is more than 7.42.

5. Blood is composed of two main components, plasma and cellular elements . Plasma constitutes about 55% of the blood volume. It is a clear straw coloured fluid portion of blood. Plasma proteins, an important constituent of plasma, form about 7% of its volume. Cellular elements of blood are about 45% of the total blood volume and constitute the so-called packed cell volume (PCV). Blood cells are: • Erythrocytes or red blood cells (5 million/mm3), • Leucocytes or white blood cells (4000–11,000/mm3) and • Platelets or thrombocytes (1.5–4 lakh/mm3).

6. Functions of blood 1 . Nutritive function. Blood carries the nutritive substances like glucose, amino acids, fatty acids, vitamins, electrolytes and others, from the gut to the tissues where they are utilized 2. Respiratory functions. Blood picks up oxygen from the lungs and delivers it to the various tissues. The most important function of blood is the uninterrupted delivery of O2 to the heart and brain. It also carries away CO2 from the tissues to the lungs, from where it is expelled out in the expired air 3. Excretory function. Blood transports various metabolic waste products , such as urea, uric acid and creatinine, to the excretory organs (kidney, skin, intestine and lungs) for their disposal.

7. 4. Transport function The various hormones produced by the endocrine glands, the biological enzymes and antibodies are transported by the blood to the target tissue, to modulate metabolic process. 5. Protective function Blood plays an important role in the defence mechanism of the body: • Neutrophils and monocytes engulf the microorganisms entering the body by phagocytosis. • Lymphocytes and gamma globulins initiate an immune response . • Eosinophils accomplish detoxification, disintegration and removal of foreign proteins.

8. 6. Homeostatic function Blood plays an important role in maintaining the internal environment of the body (homeostatic function): • The water content of blood is freely interchangeable with the interstitial fluid and helps in maintaining the water and electrolyte balance of the body. • Plasma proteins and haemoglobin act as buffers, and help in maintaining the acid–base balance and pH of the body fluids.

9. 7. Maintenance of body temperature Blood plays an important role in regulation of the body temperature, as described: • Specific heat of blood is high, which is useful in buffering the sudden changes in the body temperature. • High heat conductivity of blood renders it possible for the distribution of heat from deep organs to the skin and lungs for dissipation. • Due to high latent heat of evaporation of blood, a large amount of heat is lost from the body by evaporation of water from the lungs and skin.

10. 8. Storage function. Blood serves as a ready-made source of substances stored in it (such as glucose, water, proteins and electrolytes for use in emergency conditions like starvation, fluid loss and electrolyte loss).

11. PLASMA AND PLASMA PROTEINS

12. PLASMA Plasma is the clear straw-coloured fluid (with dissolved solid substances) portion of the blood minus its cellular elements. It constitutes about 55%of the blood volume. Composition Water . Water is the main constituent of plasma, forming 91% of it. Solids. The solids dissolved in the plasma constitute a total of 9% of the plasma. The solid constituents of the plasma are given below. • Plasma proteins form 7% of the solids in plasma. Their normal value ranges from 6.4 to 8.3 g/ dL . They include albumin, globulins,fibrinogen and others.

13. • Other organic molecules which form 1% of the solids include the following: • Carbohydrates, mainly glucose (100–120 mg/ dL ). • Fats are neutral fats (30–150 mg/ dL ), phospholipids (150–300 mg/ dL ) and cholesterol (150–240 mg/ dL ). • Nonprotein nitrogenous (NPN) substances (28–40 mg/ dL ) are ammonia (traces), amino acids, creatine (1–2 mg/ dL ),creatinine (0.6–1.2 mg/ dL ), xanthine (traces),hypoxanthine (traces), urea (20–40 mg/ dL ) and uric acid(2–4 mg/ dL ). • Hormones, enzymes and antibodies. • Inorganic substances which constitute 1% of the solids in plasma include sodium, potassium, calcium, magnesium, chloride, iodide, iron, phosphates and copper. Gases. Gases present in the plasma are oxygen, carbon dioxide and nitrogen. Serum Plasma from which fibrinogen and clotting factors (II, V and VIII) have been removed is called serum. Serum is formed when the blood is allowed to clot in a test tube and clot is retracted. Serum has a higher serotonin (5HT) content because of the breakdown of platelets during clotting.

14. Plasma proteins Plasma proteins form the major solid constituent of the plasma. The total plasma protein concentration is 7.4 g/ dL (ranges from 6.4 to 8.3 g/ dL ). The major forms of plasma proteins are albumin, globulins and fibrinogen. Since serum is plasma minus clotting protein (fibrinogen ),therefore , the albumin and globulins are usually referred to as serum albumin and serum globulins. Presently , more than 100 types of plasma proteins have been identified.

15. Functions of plasma proteins 1 . Exert osmotic pressure Protein molecules are unable to pass across the capillary membrane and consequently exert colloid osmotic pressure of about 25 mmHg on the capillary membrane. About 70–80% of the osmotic pressure is contributed by the albumin fraction. The colloid osmotic pressure plays an important role in the exchange of water between the blood and tissue fluid as explained below: • At the arterial end of capillaries, due to higher hydrostatic pressure than the colloid osmotic pressure, filtration of fluid out from vessels to tissue spaces occurs. • At the venous end of capillaries, due to lower hydrostatic pressure than the colloid osmotic pressure, absorption of fluid from the tissue spaces into the vessels occurs

16. 2. Contribution to blood viscosity Plasma proteins, owing to their size and particularly their shape, greatly contribute to the blood viscosity. Fibrinogen and globulins are significant contributors to blood viscosity because of their asymmetrical shape. The blood viscosity plays an important role in the maintenance of blood pressure by providing resistance to the flow of blood in blood vessels.

17. 3. Role in coagulation of blood Fibrinogen, prothrombin and other coagulation proteins present in the plasma play an important role in the coagulation of blood. Whenever there is injury to the blood vessels, fibrinogen is converted into fibrin which forms a blood clot

18. 4. Role in defence mechanism of the body The gamma globulins are antibodies which play an important role in the immune system meant for defence of the body against microorganisms . 5. Role in maintaining acid–base balance of the body Plasma proteins act as buffers and contribute for about 15% of the buffering capacity of blood. Because of their amphoteric nature, plasma proteins can combine with acids and bases as explained below:

19. 6. Transport function Plasma proteins combine easily with many substances and play an essential role in their transport • Carbon dioxide is transported by plasma proteins in the form of carbamino compound. • Thyroxine is transported by an alpha globulin called thyroxine-binding protein (TBP). • • Vitamin A, D and E are transported by the high- and low-density lipoproteins (HDL and LDL). • Vitamin B12 is bound to transcobalamin for transport. • Drugs of various types are transported after combining with albumin.

20. 7. Role as reserve proteins Plasma proteins serve as reserve proteins and are utilized by the body tissues during conditions like : • Fasting, • Inadequate protein intake and • Excessive catabolism of body proteins.

21. 8. Role in the suspension stability of the red blood cells Suspension stability refers to the property of red blood cells by the virtue of which they remain uniformly suspended in the blood. Globulins and fibrinogen accelerate this property . 9. Fibrinolytic function The enzymes of the fibrinolytic system digest the intravascular clot (thrombus ) and thus save from the disastrous effects of thrombosis. 10. Role of nourishment of tissue cells Plasma proteins are utilized by the leucocytes to produce substances known as trephones or carrel, which are essential for the nourishment of tissue cells.