The Circulatory System
         The circulatory system is the body's transportation network; it conveys the necessary food and oxygen to the cells and carries away the cells' waste products. Materials entering and leaving the cells are transported in the blood through a system of blood vessels-the arteries, veins, and capillaries. The blood is pumped through the body in a continuous circuit by the heart.

The Heart
         The heart is located in the chest cavity between the lungs, slightly to the left of the center of the body. In an adult the heart is roughly the size of a closed fist and weighs about eleven ounces. The heart of an average adult beats about seventy-five times a minute. The period during which the heart muscle contract is called systole; the period during which it relaxes is called diastole. Heartbeat can be measured by counting the pulsations that can be felt at pressure points in the wrist or neck. These pulsations give us the pulse rate, which is identical to the rate of the beat of the heart. The normal heart is strong enough to withstand the demands of the most vigorous physical activities.

Blood Vessels
         Arteries. The system of blood vessels that carries blood from the heart to the body is the arterial system. The principal artery in the body, the aorta, is connected directly to the heart; the left side of the heart discharges blood into the aorta. Arteries lead continually into smaller arteries and arterioles, which are no larger than a human hair. The arterioles lead into the capillaries.

         Capillaries. Capillaries are the primary distributors of blood to the cells of the body. Because the walls of a capillary consist of a single layer of cells, nutrients, oxygen, and other substances carried in the blood can easily diffuse out of a capillary and into the body cells. Similarly, waste products from the cells, such as carbon dioxide, can easily enter the bloodstream through the capillary walls.

         Veins. Upon leaving the capillaries, blood drains into very tiny veins called venules. The venules join together to form veins, and the veins increase in size as they approach the heart. Veins return blood of reduced oxygen content to the heart.

Blood Pressures
         Blood pressure refers to the pressure of the blood against the inner walls of the arteries. Like body temperature, it can be determined easily and quickly and is one indication of a person's health.
         In a young adult male systolic pressure displaces about 120 millimeters of mercury, and diastolic pressure about 80 millimeters of mercury. This is usually written 120/80 (one-twenty over eighty). The reading for a young adult female is usually about 110/70. Systolic pressure gradually increases with advancing age, but diastolic pressure normally falls into the 70 to 90 range even in later years. Everyone's normal blood pressure varies, however, at different times during the day, from day to day, and from year to year. Blood pressure increases when a person is involved in strenuous physical activity or is in an aroused emotional state. Blood pressure is normally lowest when a person is sleeping.

Blood
         If a sample of blood is drawn from the body, treated to prevent clotting, and left to stand for a while, it will separate into two layers. The bottom layer (about 45 percent of the total quantity of blood) will consist mainly of red blood cells, plus a much smaller number of white blood cells and blood platelets. The upper layer (about 55 percent of the total quantity) will be a clear, slightly yellow liquid called plasma.

         Plasma. Plasma is about 92 percent water. The remaining 8 percent consists of chemical substances dissolved or suspended in the water: proteins, salts, amino acids, glucose, fats, hormones, antibodies, enzymes, vitamins, dissolved gases such as oxygen and nitrogen, and waste products. These substances help to keep the body alive and functioning normally.

         Red Blood Cells. Red blood cells are vital to the body because they transport oxygen to the cells and carry away carbon dioxide. Arterial blood is about 20 percent oxygen by volume, whereas blood returning to the heart via the veins is only about 14 percent oxygen by volume. A shortage of red blood cells or hemoglobin results in a condition called anemia. Anemic individuals are typically pale, weak, and lethargic, because their body cells are not receiving enough oxygen. Iron-deficiency anemia can be caused by an excessive loss of blood due to internal bleeding, as with ulcers, or by a deficiency of iron in the diet. In the latter case, the anemia can usually be alleviated by increasing the intake of iron, preferably through iron-rich foods. Pernicious anemia, on the other hand, results from failure of the bone marrow to produce a sufficient number of red blood cells. This disease is generally the result of a vitamin B12 deficiency (B12 is necessary for the production of red blood cells) or of exposure to excessive radiation. Sickle-cell anemia is a hereditary disease involving the hemoglobin in red blood cells; it affects mainly black people.

         White Blood Cells. The primary function of white blood cells, or leukocytes, is to protect the body against foreign microorganisms Leukocytes destroy microorganisms by surrounding them, absorbing them into their cell bodies, and digesting them. Because of their role in fighting bacteria, leukocytes increase in number enormously during periods of illness. In fact, a physician can often diagnose a specific disease by noting the number and kind of leukocytes in the body.

         Blood Platelets. Blood platelets play an essential part in blood clotting. When bleeding occurs, the platelets disintegrate, initiating a series of chemical reactions that check the flow of blood.

Blood Groups
         Nearly everyone at some time is given a blood test to determine which blood group he or she belongs to: type A, B, AB, or O. These blood groups have varying degrees of compatibility, and therefore it is essential to know the blood type of both the donor and the recipient before a transfusion is attempted. The mixing of incompatible blood groups produces clotting.
The compatible blood types are as follows:

Recipient            Donor
    A                      A, O
    B                       B, O
    AB                    AB, A, B, O
    O                      O