What is blood?

Blood is the fluid that circulates in a  person's heart, arteries, capillaries and veins, carrying nourishment and oxygen to and taking waste products from all parts of the body.

How much blood do I have in my body?
About 8 percent of a person's weight is blood. The amount of blood varies according to height and weight, but an average man has about 12 pints of blood, and the average woman has about 9 pints.

What is the composition of blood?
Blood is made up of several components, each with its own distinct function:

Red  Blood Cells carry oxygen from the lungs to the body's cells and bring carbon dioxide back to the lungs to be exhaled. Red cells are the most commonly  transfused component, because they are used by trauma patients and those undergoing surgical procedures. White Blood Cells protect the body against infection and disease, fight bacteria  and help the body develop immunity to disease. Platelets help the blood clotting process when a person  bleeds. Platelets are used most commonly to help patients receiving treatment  for cancer or leukemia.   Plasma, the liquid portion of the blood, transports the components, water and  nutrients to all cells of the body.  Plasma makes up about 55 percent of the  total volume of blood.

Who uses blood?

Women are transfused with slightly more than  half of the blood used in the United States, and people 65 and older use approximately 45 percent of all blood products. If you live to the age of 72,  there is a 95 percent chance that you will have a need for blood products during  your lifetime.