6.9. Circulatory system

Introduction

Substances within a single cell can be trans­ported by movement of cytoplasm. Diffusion and active transport carry food, chemicals, gases, and waste products into and out of the cells of simple organisms. Large multicellular animals, however, require a more elabo­rate system for the transport of the nutrients, oxygen, and wastes that circulate throughout the body. Pickup and delivery within the human body are handled by an effective transport system: a pump, carriers, and thousands of kilometers of tubes that run throughout the body. The system is called the circulatory system. The circulatory system not only car­ries nutrients, oxygen, and bodily wastes but it also provides the body with a natural defense mechanism against disease.

The blood

Blood is the chief carrier of the body's transport system. It car­ries nutrients and oxygen to body cells and transports carbon dioxide and other waste products away from the cells. Blood also combats disease and helps maintain body temperature.

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.

Circulation Through the Heart

Blood enters the right atrium through two large veins. A vein is a blood vessel that carries blood to the heart. The superior vena cava brings blood from the upper regions of the body; the inferior vena cava brings blood from the lower body. The blood entering the heart through these veins is dark red because it is deoxygenated—that is, without oxygen.

About 70 percent of the blood in the right atrium flows directly into the right ventricle. The remaining blood is forced into the ventricle by a mild contraction of the atrium. When the right ventricle contracts, the tricuspid valve closes, and blood is forced into the pulmonary artery. An artery is a blood vessel that carries blood away from the heart. The semilunar valve closes. The blood travels from the pulmonary artery into its two branches, one to each lung. In the lungs, die exchange of carbon dioxide from the deoxygenated blood and oxygen from freshly inhaled air takes place. The blood, now bright red and saturated with oxygen, enters die left atrium via the pulmonary veins. The path of blood from heart to lungs and back is called the pulmonary circulation.

The path of blood through the left side of the heart is similar to that through the right side. The contraction of the left ventri­cle is very powerful because it must force blood to the farthest regions of the body. Blood rushes from the left ventricle into the aorta, the largest artery. From the aorta, blood flows to all parts of the body through a system of increasingly smaller arteries.

The Heartbeat

The heart is really two separate pumps that operate simultaneously at about 70 contractions — heartbeats — per minute. Blood flows into both atria at the same time, and the atria contract together. Similarly, the ventricles contract to­gether. A ventricular contraction is called systol. Relaxation is called diastole.

The activity within the heart causes the heartbeat, a sound usually described as "lubb dup." The "lubb" sound is related to the closing of the tricuspid and mitral valves. The shorter and higher pitched "dup" comes very shortly thereafter and is re­lated to the closing of the semilunar valves. Certain types of heart disorders can be detected through irregularity in one or both sounds.

What causes the heart to beat regularly without any con­scious control? The heart has its own automatic pacemaker. It is a small region of muscle called the sinoatrial, or SA, node in the back wall of the right atrium. The SA node triggers each heartbeat with an impulse that causes the atria to contract. Within a tenth of a second, the impulse reaches the atrioventricular, or AV, node at the base of the right atrium. Within milliseconds, the AV node triggers an impulse that causes the ventricles to con­tract. In a disorder called fibrillation, contractions become ir­regular and rapid. These uncoordinated contractions affect the ventricles, and therefore the pumping of blood to the body.

Blood Vessels

Blood is carried to all parts of the body through 112,000 km (70,000 mi.) of blood vessels. Different types of vessels vary in size and structure.

Arteries have especially elastic, muscular walls. These walls consist of three layers of tissue. Ar­teries branch into smaller and smaller arteries until they become tiny vessels called arterioles. Arterioles continue to decrease in diameter until they branch into capillaries — vessels so narrow that red blood cells must pass through them in single file.

Capillaries are the smallest and most numerous blood ves­sels in the body. Every body cell is within 0.13 mm (0.005 in.) of one or more capillaries. Although other blood vessels trans­port nutrients and waste products, the actual exchange of these products between blood cells and body cells takes place through capillary walls. Capillary walls are only one cell thick. As a result, diffusion of nutrient molecules, waste products, and gases can take place quickly. Capillary walls also allow plasma to filter out of the blood to become tissue fluid.

Deoxygenated blood travels from capillaries into small veins called venules. Veins increase in size as they approach the superior vena cava and inferior vena cava. Like artery walls, vein walls consist of three layers of tissue. However, the middle layer is less muscular than that of arteries.

Blood in veins generally must flow against the force of gravity — for example, from the feet to the heart. Two features prevent blood from flowing backward, away from the heart. The first is location. Many veins run through skeletal muscles.

As the muscles contract, the veins are squeezed and blood is pushed along. The second is a series of valves that keeps the blood from moving backward.

Circulatory system

Within the circulatory system are several subsystems. The path­way of blood from the heart to the lungs and back to the heart is called the pulmonary circulation. The pathway of blood from the heart to other parts of the body and back to the heart is called the systemic circulation.

Systemic circulation also has subsystems. Coronary circu­lation, for example, supplies the heart itself with blood. The left and right coronary arteries branch off the aorta and provide the heart continuously with oxygen and nutrients. The blood returns to the right atrium by way of a large vein called the coronary sinus.

Heart tissue must be nourished continuously. When some­thing prevents blood from reaching the cardiac muscle, the lack of oxygen causes the muscle cells to die. This condition, known as a heart attack, is one of the leading causes of death in the United States. A heart attack may result from a blood clot that blocks a blood vessel or from a gradual buildup of cholesterol, fibrin, and other cellular material inside the blood vessels. This buildup, called atherosclerosis, narrows the openings inside blood vessels.

Another part of systemic circulation is renal circulation, which carries blood to and from the kidneys. The left and right renal arteries branch from the aorta and enter the kidneys. Ni­trogenous waste products filter out of the bloodstream into renal capillaries. The blood then travels through renal veins to the inferior vena cava.

Hepatic portal circulation, a third part of systemic circula­tion, involves the digestive tract and liver. Mesenteric arteries carry blood from the aorta to the intestines, where water and molecules from digested food enter the capillaries. The blood, which is now enriched with nutrients, travels via the hepatic portal vein to the liver, where some nutrients are stored as glycogen. The hepatic artery supplies the liver with oxygenated blood. Blood leaves the liver and reaches the inferior vena cava through hepatic veins.

The lymphatic system

The lymphatic system is part of the body's circulatory system. Body fluids are carried in vessels of the lymphatic system as well as in blood vessels. Together the blood vessels and lymph vessels form the bodies vascular, or vessel, system.

Lymph originates from blood plasma and tissue fluid that surrounds all body cells. It provides the medium through which diffusion of nutrients and gases occurs. Each day slightly more fluid filters out of the capillaries than is leabsorbed. Lymph and the valuable proteins it contains are collected in lymph capillar­ies, tiny vessels in almost every organ. The largest of the lymph vessels are lymph ducts, which empty into the two subclavian veins located in the neck. In this way, fluid and proteins are returned to the bloodstream.

The lymph system also helps protect the body against infec­tion. Tiny bean-shaped organs called lymph nodes concentrated in the armpits, neck, and groin filter out such foreign matter as bacteria and viruses from lymph. Lymph tissue is also located in the tonsils, adenoids, spleen, thymus gland, digestive tract, and bone marrow. Lymph tissue also produces a type of white blood cell that helps the body fight disease.

If the lymphatic system malfunctions, excessive amounts of fluid collect in the body. This condition is known as edema. Generally, edema is a symptom of a more serious physical dis­order.

Blood and tissue fluid carry nutrients to body cells. These substances are necessary for healthy cells. The blood also carries substances that defend the body against diseases.

Nonspecific Defenses

Some defenses are called nonspecific defenses because they operate in the same way against all disease-causing microorganisms. Among nonspecific defenses are the skin and mucous membranes. They provide a mechanical barrier against pathogens which are disease-causing agents such as viruses and bacteria. If pathogens do enter the body, a type of white blood cell called a phagocyte engulfs and digests them. This process is known as phagocytosis. The dead bacteria and white blood cells may become pus. The presence of pus indicates an infection.

Virus-infected cells may also release the protein interferon. Interferon inactivates attacking viruses by preventing them from reproducing. All viruses stimulate the production of the same type of interferon, and interferon attacks all types of viruses.