Red Blood Cells
Red blood cells, when mature, are shaped like a saucer and are thinner in the middle than around the edges. They have no nucleus, as do most cells, and therefore, cannot be considered living cells. The red cells are also called erythrocytes. There are about 25 trillion (25,000,000,000,000) red cells in the body of an adult. The cell walls of this number of erythrocytes, if spread out flat, would provide enough cell walls to cover the skin of 2,000 men.
Because they contain no nuclei, red cells are continually being worn out and replaced by new ones. In a healthy man, seven to ten million red cells are removed from the blood every second and replaced by an equal number of new ones. Old cells are removed and destroyed by the liver and spleen. New ones are produced by and released from the marrow of the bones.
The redness of the erythrocytes is due to the hemoglobin they contain. Hemoglobin (a complex compound of protein and iron) gives the blood its capacity to carry 60 times as much oxygen as could be carried by an equal volume of water. Hemoglobin is bright crimson when carrying a rich supply of oxygen. It becomes purplish when its supply of oxygen is replaced by carbon dioxide from the cells of the body. This is why blood coming from an artery looks so much redder than blood from a vein.
The volume of oxygen that the blood can carry at one time totals about 1200 cubic centimeters (cc). The body uses this much oxygen in about five minutes while resting, and in a few seconds during exercise. Red cells give up their carbon dioxide and get fresh oxygen whenever they pass through the capillaries of the lungs, usually doing this about three to five times per minute.
White Blood Cells
In an amount of blood containing four and one-half to five million red cells, there are also about five to nine thousand white cells. These are of several kinds. Cells with a single nucleus and clear cytoplasm containing no granules are the lymphocytes and monocytes. Other white blood cells have a sectioned nucleus and contain granules within the cytoplasm. These are called eosinophils, basophils andneutrophils according to the way in which the granules take up dyes.
Sixty-five to seventy percent of the white blood cells are neutrophils and circulate in the blood stream. Lymphocytes are present in the lymphoid tissues of the lymphatic system. They account for 20-25% of the white blood cells. White blood cells are formed in the bone marrow and lymphoid tissue.
White blood cells are called leucocytes. They are important in defending the body against bacteria. Like the amoeba, leucocytes can move about by forming pseudopodia. They leave the blood stream and pass through the vessel wall into the tissues where they engulf and remove bacteria. In a healthy person, a leucocyte measuring only 10 microns (10 millionth of a meter) in diameter can move toward bacteria at a rate of 16 microns per minute. Leucocytes congregate in large numbers wherever there is inflammation. They are normally present in larger numbers in infants and children than in adults.
Platelets are fragments of protoplasm without nuclei. When they disintegrate, they free thromboplastin, an enzyme which starts the clotting of blood when there is a cut in the blood vessel. Thromboplastin in the presence of calcium produces active thrombin. The thrombin reacts with the protein fibrinogen dissolved in the blood plasma to form a solid clot. The fluid part of the plasma that remains in called serum.
Blood clots are very important in the healing of wounds. They plug the opening so that blood cannot continue to drain from the vessels. They also provide a solid support over which new cells can glide to close the wound and regenerate lost tissue.
Other Plasma Proteins
Many proteins circulate within the plasma that are responsible for protecting the body against invading substances from other living bodies such as viruses, pollens, bacteria. The globulin proteins are connected with the formation of antibodies and the process of immunization. Albumins also circulate in the plasma and are largely responsible for keeping the pressure of the blood at the proper level. All of the plasma proteins act as a reserve supply of protein nourishment when the body is in a condition of malnutrition.