The body is composed of trillion of cells that all need a constant supply of nutrients like sugars and oxygen to survive. Cells also produce waste matter that must be eliminated. Heart and blood vessels bring blood to every cell. The blood vessels bring blood to every cell. Without circulating blood, the body would quickly die. There are about five to six quarts of blood in the human body. About half the volume of blood is a saltwater liquid called plasma; the other half is composed of blood cells that perform critical functions. There are more red blood cells called erythrocytes in the body than any other type of cell. Each lives about one hundred and twenty days and is constantly replaced by younger red blood cells. Red
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