Blood is commonly described as a connective tissue, which plays a very important part in the maintenance of life. It is a thick, opaque fluid made up of plasma, in which floats formed elements the blood cells or corpuscles. The blood gets bright red in colour in the arteries where it is oxygenated. It is slightly alkaline in reaction and forms about 1/12th of the body weight, so that the average volume is about 5-6 liters.
Although it appears as a faint yellow fluid, actually it consists of two parts- a liquid part, the plasma and a solid part- the formed elements or different type of cells, which remain suspended in the plasma. The cells are called as the blood corpuscles- R.B.C. W.BC. and blood platelets. The cells form 45% and the plasma 55% of the total volume of blood.
The plasma is the fluid part of the blood. It is a clear, straw-colored watery liquid. It consists of:
1. Water 90 to 92%
2. Solid 8-10%. The solids include:
(a) Proteins: 7% of albumins, globulins, fibrinogen, prothrombin and heparin.
(b) Inorganic constituents: 0.9% of Sodium, Calcium, Potassium, Magnesium etc.
(c) Organic Constituents: glucose, amino acids, fatty acids, glycerol, vitamins and non-nitrogenous substances like urea, uric acid, creatinine, ammonia etc.
3. Respiratory gases: Oxygen, carbon dioxide and nitrogen.
4. Anti-bodies and anti toxins which protect the body against bacterial infection.
5. Certain hormones and enzymes.
6. Coloring matter: Yellow colour of plasma is due to the presence of small amount of bilirubin, carotene etc.
Functions of Blood Plasma
1. Essential for blood clotting: The presence of fibrinogen and prothrombin in the plasma takes part in blood clotting, which are produced in the liver.
2. Plasma maintains colloidal osmotic pressure of blood and help in regulating the distribution of fluid between the blood and tissues.
3. The protein which plasma contains gives the blood the sticky consistency called viscosity, which is necessary to prevent too much fluid passing through the capillary wall into the tissues. This viscosity of blood assists in the maintenance of blood pressure.
4. Plasma is concerned with erythrocytes (R.B.C.) sedimentation rate. An increase in fibrinogen raises the sedimentation rate of red blood cells.
5. Acts as a buffer: The salts in the plasma are necessary or the building of protoplasm and they act as buffer substances neutralizing acids or alkalis in the body and maintaining the correct reaction of blood.
6. Acts as a protein reserve: During starvation the tissue can draw protein from the plasma store.
7. Protects against infection: The anti-bodies and antitoxins provide protection against infection and neutralize the poisonous bacterial toxins.
8. Help in transportation: Plasma protein combines with certain substances can help to carry them in the blood stream.