Blood is a fluid substance that circulates in the arteries and veins of the body. Blood is bright red or scarlet when it has been oxygenated in the lungs and passes into the arteries; it becomes bluish red when it has given up its oxygen to nourish the tissues of the body and is returning to the lungs through the veins and the tiny vessels called capillaries. In the lungs, the blood gives up the carbon dioxide wastes it has taken from the tissues, receives a new supply of oxygen, and begins a new cycle. This movement of blood is brought about by the coordinate activity of the heart, lungs, and blood vessels. Blood is composed of a yellowish fluid, called plasma, in which are suspended the millions of cells that constitute about 45 percent by volume of whole blood. It has a characteristic odor and a specific gravity between 1.056 and 1.066. In an average healthy adult, the volume of blood is one-eleventh of the body weight, or between 4.5 and 6 liters (5 and 6 qt). A great portion of the plasma is composed of water, a medium that facilitates the circulation of the many indispensable factors of which blood is composed. A cubic millimeter of human blood contains about 5 million red corpuscles called erythrocytes; 5000 to 10,000 white corpuscles called leukocytes; and 200,000 to 300,000 platelets called thrombocytes. The blood also carries many salts and organic substances in solution. Blood type, in medicine, classification of red blood cells by the presence of specific substances on their surface. Typing of red blood cells is a prerequisite for blood transfusion. In the early part of the 20th century, physicians discovered that blood transfusions often failed because the blood type of the recipient was not compatible with that of the donor. In 1901 the Austrian pathologist Karl Landsteiner classified blood types and discovered that they were transmitted by Mendelian heredity . The four blood types are known as A, B, AB, and O. Bl…
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