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 blood cells transport oxygen from the lungs to the cells throughout the body. A protein called hemoglobin inside each red blood cell carries the oxygen and then releases it. White blood cells called leukocytes defend the body against invading microorganisms and other foreign substances. Some white blood cells directly attack and engulf germs, killing them. At the same time, these white blood cells alert other types of white blood cells to initiate and attack on other similar germs. These reactions are a part of the immune system response. Platelets called thrombocytes are small pieces of blood cells that help the blood clot. When a cut occurs bleeding happens because the walls of the tiny blood vessels at the site of the cut allowed the blood to escape. Platelets move to the site of the injury; stick to each other and to the edges of the cut blood vessels and forms a clot that seals the leak and keeps more blood from escaping. Chemicals in plasma make the clot strong and permanent. While platelets are helpful they can also form clots and cause arteries to constrict contributing to heart attacks, strokes, and insufficient blood flow to the legs.
If a person blood is deficient in red blood cells, platelets or clotting factors, thethey might require a blood transfusion.
