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.

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