The Circulatory System
 

Transportation Of Blood  The Cardiac Muscle  Impluses  Transportation In Blood  PlasmaRed Blood Cell White Blood Cell  Platelets Blood Vessles  Arteries Capillaries  Vein Distribution Of Blood
 

Red Blood Cell

The oxygen-carrying red blood cells, also called erythrocytes, make up about 99% of the cells in the blood. They constitute about 40% of the total blood volume in females, and 45% in males. Each milliliter of blood contains about 5 billion erythrocytes. Proteins on the surface of red blood cells differ among individuals, thereby creating different blood types. The red blood cells resemble a ball of clay squeezed between thumb and forefinger. Its biconcave shape provides a larger surface area than would a spherical cell of the same volume and increases the cells ability to absorb and release oxygen through its membrane. The pigment hemoglobin causes the red colour of erythrocytes. This large, iron-containing protein accounts for about one-third the weight of the blood cell. About 97% of the oxygen carried by the blood are bound to haemoglobin. The haemoglobin binds loosely to oxygen, picking up oxygen in the capillaries of the lungs, where the concentration is high, and releasing it where the concentration is low, in other tissues of the body. After releasing its oxygen, some of the haemoglobin picks up carbon dioxide from the tissues for transport back to the lungs. The role of blood in gas exchange is discussed later in this chapter. Red blood cells are formed in the marrow, the soft interior portion of certain bones , including those of the chest, upper arms, upper legs, and hips. During their development, mammalian red blood cells lose their nucleic and their ability to divide. Without the ability to synthesize cellular materials, their lives are necessarily short; each cell lives about 120 days. Every second, over 2 million red blood cells die and are replaced by new ones from the bone marrow. Dead or damaged red blood cells are removed from circulation, primarily in the liver and spleen, and broken down to release their iron. The salvaged iron is carried in the blood to the bone marrow, where it is used to make more haemoglobin and packaged into new red blood cells.

Although the recycling process is efficient, small amounts of iron are excreted daily and must be replenished by the diet. Bleeding from injury or menstruation also tends to deplete iron stores.
The number of red blood cell in the blood is maintained at an adequate level through a negative feedback system that involved a hormone called erythropoietin. Erythropoietin is produce by the kidneys in response to oxygen deficiency. This lack of oxygen may be cause by a loss of blood, in sufficient production of hemoglobin, high altitude, or lung disease that interferes with gas exchange in the lungs. The hormones stimulate rapid production of new red blood cells by the bone marrow. When adequate oxygen levels are restored, erythropoietin production is inhibited, and the rate of red blood cell production returns to normal.

Blood is classified as type A, B, AB, or O depending on the presence or absence of specific proteins on the cell membranes of red blood cells. Type A blood has the A proteins on its red blood cells, type B has the B protein, type AB has both and type O has neither. Also, each blood type carries antibodies in the plasma to the proteins not present on its own red blood cells. Thus, a person with type A blood has antibodies to the B protein. If this person is transfused with type B blood, the antibodies to the B proteins attack the transfused red blood cells, causing them to clump together and block small; blood vessels, sometimes with fatal results.

Another type of proteins on the red blood cells is the Rh factor. If it is present, blood is described as Rh-positive, and if the protein is absent, the blood is Rh-negative. Un like A or B antibodies, antibodies to the Rh- protein form only after massive exposure to the protein. The first exposure- for example, transfusion of Rh-positive blood into an Rh-negative individual-generally causes no ill effects, but triggers the production of antibodies. Upon further exposure, the antibodies attack and destroy the Rh-positive red blood cells. If an Rh-negative woman marries an Rh-positive man, her children are likely to be Rh-positive, because Rh-positive blood is a dominant genetic trait. Her first Rh-positive child will trigger antibody production in her blood. Subsequent Rh-positive children will be in danger of being born with erythroblastosis fetalis, in which the mother's antibodies invade the fetus and attack its red blood cells, causing the child to be born severely anemic. Fortunately this condition can now be easily prevented by injections of a substance that prevents formation of Rh antibodies by the pregnant woman.