Composition of Blood

A human adult has about 5 L (5.3 qt.) of blood, which makes up about 9 percent of the body's weight. Blood is liquid connective tissue. It consists of a liquid called plasma and three kinds of blood cells: red blood cells, white blood cells, and platelets. Approximately 55 percent of blood volume is plasma. About 44 percent is red blood cells. The remaining 1 percent is white blood cells and platelets.

Plasma

The straw-colored, nonliving part of blood, called plasma, has many functions. For example, it carries nutrients such as amino acids and glucose molecules absorbed in the small intestine to body cells. Plasma also takes waste products away from the cells and delivers these wastes to the kidneys and sweat glands so they can be safely removed from the body.

Plasma is more than 90 percent water. The remainder con­sists of minerals and thousands of other compounds, including many proteins. These proteins assist in blood clotting, help maintain the body's water balance, and influence the exchange of materials between the circulatory system and the body cells. Also present in plasma are nitrogenous waste products and res­piratory gases. Some plasma, with fewer proteins, seeps through blood vessel walls. It fills spaces between body tissues and bathes every body cell. This fluid is known as tissue fluid.

Red Blood Cells

The blood cells that transport respiratory gases are called the red blooded cells. Red blooded cells, also called erythrocytes or red corpuscles, carry oxygen from the lungs to body cells. They also transport carbon dioxide from the cells to the lungs. A red blood cell has a nucleus when it is formed in red bone marrow. However, the nucleus and other organelles disappear as the red blood cell matures. Each cell becomes a disc-shaped sac with a thick rim and thin center. Almost the en­tire cell fills with hemoglobin, an iron-con­taining protein molecule that is bright red when combined with oxygen. One molecule of hemoglobin carries four molecules of oxygen. Hemoglobin is therefore an effective oxygen carrier. Red blood cells are so small that hundreds of them would be needed to encircle one strand of hair. The human body has about 25 trillion red blood cells. They are produced at the rate of over 10 billion per hour and have a life span of about 120 days. The dead cells are dismantled, and the hemoglobin is stored to be reused in new blood cells.

White Blood Cells

The white blood cells, also known as leukocytes or white corpuscles, are the body's main defense against viruses, bacteria, and other foreign organisms. In fight­ing invaders, white cells pass through blood vessel walls and into tissue fluid. They move like amoebas, attracted to the site of an infection by chemicals. The chemicals may be products of blood clotting. They may also come from bacteria, other leuko­cytes, or from degeneration of infected tissue. The white blood cells engulf and digest the invading organisms by a process known as phagocytosis.

Several kinds of white blood cells are found in blood. Most white blood cells are manufactured and stored in red bone mar­row until they are needed by the body. They are colorless, irreg­ularly shaped cells with nuclei. Although white blood cells are larger than red cells, they are considerably less numerous— about 1 white cell for every 750 red cells. The normal life span of white blood cells is about three days unless they are fighting infection. In that case, they may live only a few hours.

Platelets

Cell fragments called platelets, or thrombocytes, aid in blood clotting. Within five seconds after an injury occurs. the process of clotting, or coagulation begins. Platelets begin to stick to the rough surfaces created by damaged tissue, such as the tissue around a cut or a broken blood vessel. Some platelets break and release chemicals that cause nearby blood vessels to constrict, thus reducing bleeding. They also release an enzyme called thromboplastin, which triggers a process involving proteins in the plasma including prothrombin and fibrinogen. In the presence of calcium, thromboplastin causes prothrombin to change into thrombin. Thrombin is an enzyme that promotes the conversion of fibrinogen into fibrin. Fibrin forms strong, elastic protein threads into a mesh that traps blood cells and platelets around the edges of the injury. The result is a blood clot. Within minutes the clot begins to shrink, pulling together the injured ends of skin and forming a scab.

Like red blood cells, platelets lack nuclei and are formed in red bone marrow. Platelets are about one-third the size of red cells and number about 1 to every 20 red cells. Their life span is about 7 to 11 days.

Blood Types

Occasionally an injury or a disorder is so serious that a person must receive blood from another person. A blood transfer, or transfusion, can succeed only if Mood of the recipient and donor match. Among the factors that must be considered in matching blood is blood type. Blood type is determined by the presence of an antigen on red blood cells. An antigen is any molecule that stimulates an organism to produce antibodies. An antibody is a protein that attacks, or neutralizes, the antigen that triggered its production. Microorganisms, such as bacteria and viruses, are antigenic. The antigens that result in blood types, however, are inherited. The most familiar blood-typing system is the ABO system. Under this system, the primary blood types are A, B, AB, and O. Type A blood has antigen A, and type B has antigen B. Type AB has both antigen A and antigen B, while type O has neither of these antigens.

Types A, B, and O also contain antibodies. Type A blood contains anti-B antibodies. Type B blood contains anti-A anti­bodies. Type O has both anti-A and anti-B antibodies. Type AB has neither of the antibodies. If two blood types are mixed dur­ing transfusion, antibodies may cause agglutination, or clump­ing, of red cells. Agglutination results, for example, if type A blood is mixed with type B blood. In this case, the anti-B anti­bodies in the type A blood will attack the antigens in the type B blood.

When a patient needs blood, doctors must first determine what the patient's blood type is. Modern medical practice, how­ever, requires that more than just blood type be analyzed. Other factors in donated blood must also be compatible for a transfu­sion to be successful.

Rh Factor

Another type of antigen is the Rh factor, which is present in about 85 percent of all people in the United States. These people are said to be Rh-positive (Rh⁺). People whose blood does not contain the Rh factor are Rh-negative (Rh^-). The Rh factor can cause a problem to children of an Rh^- woman. If the father is Rh⁺, the child could have Rh⁺ blood. If some of the Rh⁺ blood antigens from the unborn child enter the mother's bloodstream, her body produces anti-Rh antibodies. During any succeeding pregnancy, the mother's anti-Rh antibodies may pass into the child's bloodstream. If the unborn child is Rh⁺, the antibodies can cause clumping and destruction of the child's red blood cells, a condition known as erythroblastosis fetalis, or Rh dis­ease. The Rh factor problem can be a critical one. The result may be anemia, brain damage, or even death.

Two procedures are used to overcome the problem. The Rh- mother may be given a serum containing anti-Rh antibodies within 72 hours after the binh of her first Rh⁺ baby. The serum destroys the child's Rh⁺ blood antigens that have entered her system before her body can develop anti-Rh antibodies. The second procedure treats the child. If the unborn child of a later pregnancy has already developed Rh disease, a blood transfu­sion can be given to the unborn child to remove the antibodies from its blood.

Blood could not meet the body's needs if it did not flow. The circulatory system, therefore, includes a pump that forces blood to move and tubes through which it flows smoothly.

The Heart

The heart is a muscular organ that pumps blood to all parts of the body. When a person is resting, the heart pumps about 5 L (5.3 qt.) of blood each minute. When a person is exercising strenuously, however, the heart may have to pump up to seven times that amount.

Structure of the Heart

The heart is a fist-sized organ com­posed chiefly of cardiac muscle, nervous tissue, and connective tissue. It lies between the lungs and behind the breastbone. An average adult human heart weighs about 350-450 g (0.5-1 lb.). A tough protective sac called the pericardium surrounds the heart. The pericardium secretes a slip­pery liquid that acts as a lubricant, allowing the heart to move smoothly within the sac.

The right and left sides of the heart function as two completely separate pumps. An interior wall called the septum separates the two sides of the heart. Each side has an upper section called the atrium and a lower section called the ventricle.

The atrium and ventricle on each side are separated by a one-way valve. The valve on the right side is called the tricuspid valve. The valve on the left side is the bicuspid, or mitral valve. Another set of one-way valves, called the semilunar valves, separate the ventricles from the large blood vessels into which blood is pumped out of the heart. AH die valves prevent blood from flowing backward.