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The medical evidence is clear: We cannot live without blood. It sustains life by supplying body tissue with oxygen from the lungs and returning harmful carbon dioxide to the lungs to be exhaled. Blood helps us grow by transporting nourishment from digestion and hormones from glands throughout the body. And it keeps us healthy by transporting disease-fighting substances to the tissue and waste to the kidneys.

But how exactly does blood do these things?

According to the American Red Cross, the average adult has about 10 pints of blood coursing through a miles-long network of vessels called arteries and veins. Arteries transport oxygenated blood from the heart to the rest of the body. Veins return the blood back to the heart and lungs, where it receives more oxygen.

Blood is made up of three types of cells: red blood cells (erythrocytes), white blood cells (leukocytes) and platelets. Resembling flattened discs, red blood cells have an indented center. They are responsible for carrying oxygen from the lungs to all the cells in the body and taking carbon dioxide back to the lungs to be exhaled. The body contains more red blood cells than any other type of cell, and each has a life span of about four months. Each day, the body produces new red blood cells. White blood cells move in and out of the bloodstream, helping the body to fight off diseases and infections. There are fewer white blood cells than red cells, but the body increases production of white blood cells to fight infection. There are three main types of white blood cells, and their life spans vary from a few days to months.

Approximately 55 percent of blood is plasma, a yellowish-colored liquid that carries the solid cells and the platelets, which help blood clot. Without blood platelets, you would bleed to death from a cut. The reason you never run out of blood when you bleed or donate blood is the body is constantly producing new blood cells inside the marrow of your bones. In adults, the spine, ribs and pelvis are the largest producers of blood. As the blood cells develop, they seep into the blood that passes through the bones and on into the bloodstream.

Blood also helps the body regulate its temperature and deliver nutrients to all parts of the body. It takes waste products to the lungs, kidneys and liver for disposal. Most people never experience a problem with their blood. But some blood disorders or diseases can cause illnesses that impair the ability of blood to function properly.

A common condition affecting red blood cells is anemia, where the blood?s capacity for carrying oxygen is reduced. Anemia is most commonly caused by an iron deficiency or lead poisoning, though it can be a complication of cancer or human immunodeficiency virus infection, especially in children. Hemophilia, sickle cell anemia and leukemia also commonly affect kids.

Though it all looks the same to the human eye, a microscopic view of blood reveals several differences. The main red blood cell groups are A, B, AB and O. The letters stand for two antigens (chemical substances that can be targeted by one's immune system) labeled A and B. Group O blood has neither antigen. Your blood type ? there are eight ? is passed down to you by your parents.

Knowing your blood type is important because every two seconds someone in the United States will need a blood transfusion, according to the Red Cross. Blood transfusions are used for trauma victims ? due to accidents and burns ? heart surgery, organ transplants, women with complications during childbirth, newborns and premature babies, and patients receiving treatment for diseases of the blood. The problem is, not all blood types are compatible. People with O- blood are particularly in demand because people of all blood types can receive O- blood.

U.S. institutions collected more than 15 million units of whole blood and red cells in 2001, the most recent year for which data are available, according to the National Blood Data Resource Center. Hospitals collected 7 percent of the total.

To give blood, you must:
Be in generally good health
Be at least 17 years of age
Weigh no less than 110 pounds
Have not received a tattoo within the past year
Have not donated whole blood within the past 56 days
For more information on donor restrictions, visit

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