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Blood is a liquid with suspended cells 30-50% by volume. These cells are of three basic types, erythrocytes or rbcs, leucocytes or wbcs and thrombocytes or platelets. Two of these three cell types are odd, in being anucleate. All originate in the bone marrow, although the number of primary stem cell types is unresolved.
The circulatory system is the route by which the cells in your body get the oxygen and nutrients they need, but the blood is the actual carrier of the oxygen and nutrients. Blood is made mostly of plasma, which is a yellowish liquid that is 90% water. But in addition to the water, plasma contains salts, sugar (glucose), and other substances. And, most important, plasma contains proteins that carry important nutrients to the body’s cells and strengthen the body’s immune system so it can fight off infection.
The Red Cross and a number of other organizations run programs, known as blood banks, to collect, store, and distribute blood and blood products for transfusions. When blood is donated, its blood type is determined so that only appropriately matched blood is given to patients needing a transfusion. Before using the blood, the blood bank also tests it for the presence of disease-causing organisms, such as hepatitis viruses and human immunodeficiency virus (HIV), the cause of acquired immunodeficiency syndrome (AIDS). This blood screening dramatically reduces, but does not fully eliminate, the risk to the recipient of acquiring a disease through a blood transfusion. Blood donation, which is extremely safe, generally involves giving about 400 to 500 ml (about 1 pt) of blood, which is only about 7 percent of a person’s total blood.
