Blood is a fluid which circulates in a closed system of blood vessels, in multicellular and highly complex vertebrate animals. Blood consists of a pale yellow fluid called plasma and solid elements such as red blood cells (erythrocytes) while blood cells (leucocytes) and platelets (thrombocytes).

Blood contains 55% of the plasma and 45% of the cellular fraction. The water constitutes about 80% of blood by weight. The cellular fractions composed of erythrocytes (5 x 106 cells/μl) leucocytes (4.5- 11 x 103 cells/μl) and thrombocytes (3 x 105 cells/μl). The plasma contains a large number of organic and inorganic substances in solution.[userpro_private]
These subtances may be diffusible substances including various electrolytes, anabolic and catabolic substances formed during metabolism, vitamins and certain hormones. The non-diffusible constituents of plasma are albumin, globulin, fibrinogen, enzymes, lipids etc.

Blood perform the following functions:
1. Blood transport oxygen from lungs to the tissues and carbon dioxide from the tissues to the lungs. Thus it is responsible for the important process of respiration.
2. It transports absorbed dietary nutrients from the digestive tract to all the body tissues.
3. It transports metabolic wastes to the kidneys, lungs, skin and intestine for removal.
4. It transports various hormones and minerals.
5. It maintains normal acid – base balance in the body.
6. It regulates water balance.
7. It regulates body temperature by the distribution of body heat.
8. White blood cells and antibodies in blood provide, defense against various type of infections.

Formation of blood
Blood biosynthesis is known as haematopoiesis. The process consist of production, development and maturation of cellular elements of blood. In adults the production of blood cells is carried out by the bone marrow while in the developing fetus haematopoisis takes place in extra medullary regions till the bone marrow is matured and functional.


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