The body fluid pumped by the heart through the vessels of those animals, all vertebrates and many invertebrates in which diffusion alone is not adequate for transport of materials, and which therefore require blood circulation system. Blood plays a part in every major bodily activity. As the body’s main transport medium it carries a variety of materials like oxygen and nutrients such as glucose to the tissues for growth and repair, carbon dioxide and wastes from the tissues for excretion, hormones to various tissues and organs for chemical signaling, digested food from the gut to the liver, immune bodies fro preparation of infection and clotting factors to help stop bleeding to all parts of the body.

Blood also plays a major role in homeostasis, as it contains buffers which keep the acidity (ph) of the body fluids constant and by carrying heat from one part of the body to another, it tends to equalize body temperature. The adult human has about 6 quarts of blood, half plasma and half blood cells (erythrocytes or red cells and leukocytes or white cells), the thrombocytes or platelets. The formation of blood cells or hemopoiesis occurs in bone marrow, lymphoid tissue and the reticuloendothelial system. Red cells about 5 million per cubic mm are produced at a rate of over 100 million per cubic mm and live only about 120 days. They have no nucleus, but contain a large amount of the red pigment hemoglobin, responsible for oxygen transfer from lungs to tissues and carbon dioxide transfer from tissues to lungs. White cells, about 6,000 per cubic mm, are concerned with defense against infection and poisons. There are 3 types of white cells, granulocytes (about 70%), which digest bacteria and greatly increase in number during acute infection, lymphocytes (20-25%), which participate in immune reactions and monocytes (3-8%), which digest non bacterial particles, which live for about 8 days and which are much smaller than white cells and about 40 times as numerous, assist in the initial stages of blood clotting together with at least 12 plasma clotting factors and fibrogen. This occurs when blood vessels are damaged, causing thrombosis and when hemorrhages occur. Blood from different individuals may differ in type of antigen on the surface of its red cells and the type of antibody in its plasma. Consequently, in a blood transfusion, if the blood groups of the donor and recipient are incompatible with respect to antigens and antibodies present, a dangerous reaction occurs, involving aggregation or clumping of the red cells of the donor in the recipient’s circulation. Many blood group systems have been discovered, the first and most important being the ABO system by Karl Landsteiner in 1900. In this system, the blood is classified by whether the red cells have antigens A (blood group A), B (group B), A and B (group AB) or neither A nor B antigens (group O). Another important antigen is the Rhesus antigen (or Rh factor). People who have the Ph factor (84%) are designated Rh+, those who do not, Rh-. Rhesus antibodies do not occur naturally but may develop in unusual circumstances. In a few cases, where Rh+ women are pregnant with Rh+ babies, blood leakage from baby to mother which may progressively destroy the blood of any subsequent baby.