Volume of Blood 

The average adult has approximately 10 to 12 pints of blood in his or her body. During each blood donation a pint of blood is collected. The small loss is quickly replaced by the body. The fluid portion is replaced within 24 hours, and the cells are replaced by the body's bone marrow within weeks, so that by the end of the month, the body has the same amount of blood as before the blood donation.

Blood appears to be a red liquid, but approximately 45% of the blood consists of cells, red blood cells (RBCs), white blood cells (WBCs) and platelets. The remaining liquid portion is plasma. Plasma consists largely of water with salts and proteins dissolved in it.

Red Cells 

The red cells are made in the marrow cavities of bones, particularly in the spine, ribs, breastbone, and skull.

The red cells contain a protein called hemoglobin. Hemoglobin contains iron and gives the red cells their color and their ability to carry oxygen from the lungs. After picking up oxygen in the lungs, red cells deliver it to tissues, where it is used to create energy.

Iron is a key raw material required by the red cell factories. It is necessary to have enough iron in the body to keep the production of hemoglobin at the normal rate. If iron is lacking, the amount of hemoglobin and the number of red cells in the blood is reduced. This gives rise to the condition known as anemia.

White Cells 

White cells are made in the bone marrow and in certain tissues of the body including the spleen, the lymph nodes, and the lining of the intestines. There is about one white cell for every 100 red cells. The white cells defend the body against infection. They could be thought of as the defending army, with various corps having their own specialties. One group, the lymphocytes, recognize that invading infectious agents (germs) such as bacteria, viruses, or parasites are foreign. The lymphocytes direct the attack on these germs. One of the ways they direct this attack is by producing antibodies that label the invading germs for destruction. Some antibodies then remain in the bloodstream for a while to protect the body from further attack.

Other groups of white cells, the granulocytes and the monocytes, attack germs either by gobbling them up, a process called phagocytosis, or by attacking them with various types of poisons.  Both forms of attack are made more efficient when directed by antibodies.

Platelets 

Platelets are formed by fragmentation of giant cells called megakyocytes that live in the bone marrow. There are about half a trillion platelets in the normal bloodstream. After a short life of 10 days, they are removed from the blood by the spleen, lymph nodes and liver.

Platelets help control bleeding by sticking to injured surfaces of blood vessels and providing the surface for clotting factors to accumulate on. This plugs up breaks in the blood vessels.

Plasma 

Plasma is composed of water (90%), proteins (7%), and very small amounts of fats, sugars, and mineral salts. The sticky or gummy quality of the blood that is necessary for maintaining normal blood pressure is partly caused by the plasma proteins, particularly albumin. Plasma proteins include blood clotting factors and antibodies.

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