Anatomy and Physiology

The Heart

The heart, a muscular double pump enclosed in a fibrous sac called the pericardium, supplies the force that moves the blood. As shown schematically in figure 28.1, the heart is divided into a right and a left side, separated by a septum, a wall of tissue, through which blood normally does not pass after birth. The right and left sides of the heart are both divided into two chambers, the atrium, which receives blood, and the ventricle, which discharges it. Blood from the right ventricle flows through the lungs and into the atrium on the left side of the heart. From the atrium, the blood passes into the left ventricle and is then pumped through the aorta to the arteries and capillaries that supply the tissues of the body. Blood that exits the capillaries is returned to the right atrium by the veins. The heart valves are situated at the entrance and exit of each ventricle and ensure that the blood flows in only one direction. Although not common, infections of the heart valves, muscle, and pericardium may be disastrous because they affect the vital function of blood circulation.

Arteries

Arteries have thick muscular walls to withstand the high pressure of the arterial system. The blood in arteries is bright red, the color of oxygenated hemoglobin. Infection of the arteries is unusual, although the aorta can be dangerously weakened by infection in syphilis. Arteries are subject to arteriosclerosis, "hardening of the arteries," a process that begins in teenage years or before and progresses through life, ultimately putting us at risk for heart attacks and strokes. A possible role of bacteria and viruses in arteriosclerosis has been considered for some years.

Veins

Blood becomes depleted of oxygen in the capillaries, causing it to assume the dark color characteristic of venous blood. Since pressure in the veins is low, one-way valves help keep the blood

Capillaries of head, upper body, and other tissues

Capillaries of head, upper body, and other tissues

Head Capillaries

Capillaries of body organs and other tissues

Figure 28.1 The Blood and Lymphatic Systems For simplicity, the spleen is not shown. It is a fist-sized, blood-filled lymphoid organ located high in the left side of the abdomen, behind the stomach. Disease conditions are indicated in red type.

Capillaries of body organs and other tissues

Figure 28.1 The Blood and Lymphatic Systems For simplicity, the spleen is not shown. It is a fist-sized, blood-filled lymphoid organ located high in the left side of the abdomen, behind the stomach. Disease conditions are indicated in red type.

flowing in the right direction. Veins are easily compressed, so the action of muscles aids the flow of venous blood.

Thus, as the blood flows around the circuit, it alternately passes through the lungs and through the tissue capillaries. During each circuit, a portion of the blood passes through organs such as the spleen, liver, and lymph nodes, all of which, like the lung, contain phagocytic cells of the mononuclear phagocyte system. These phagocytes help cleanse the blood of foreign material, including infectious agents, as the blood passes through these tissues.

Lymphatics

The system of lymphatic vessels begins in tissues as tiny tubes that resemble blood capillaries but differ from them in having closed ("blind") ends and in being somewhat larger. Lymph, an almost colorless fluid, is conveyed within these lymphatic vessels. It originates from plasma, the non-cellular portion of the blood, that has oozed through the walls of the blood capillaries to become the interstitial fluid that surrounds tissue cells. This fluid bathes and nourishes the tissue cells and then enters the lymphatics. Unlike the blood capillaries, the readily permeable lymphatic ves-

28.1 Anatomy and Physiology 717

PERSPECTIVE 28.1 Arteriosclerosis: The Infection Hypothesis

Arteriosclerosis, the main cause of heart attacks and strokes, is characterized by lipid-rich deposits that develop in arteries and can impair or stop the flow of blood.

In 1988, researchers in Helsinki, Finland, reported that patients with coronary artery disease, meaning arteriosclerosis of the arteries that supply the heart muscle, commonly had antibodies against Chlamydia pneumoniae.This organism is a tiny obligate intracellular bacterium that is responsible for a variety of upper and lower respiratory infections including sinusitis and pneumonia.The infections are common, and most people have been infected by C. pneumoniae by the time they reach adulthood.What was intriguing about the antibody studies was that the higher the titer of antibody, the greater the risk of coronary disease.Then, in 1996, viable C. pneumoniae were shown to be present in arteriosclerotic lesions, and other studies conclusively established that there was an association between the bacterium and arteriosclerotic lesions of both heart and brain arteries.

Does C. pneumoniae cause arteriosclerosis, does it worsen the effects of arteriosclerosis, or does it merely exist harmlessly in the lesions? How can the hypothesis that C. pneumoniae contributes to heart attacks and strokes be tested? Some studies of coronary patients treated with antibiotics effective against the bacterium have suggested, but not proven, a beneficial effect, and so we will have to await further studies for answers to these questions. It would truly be a revolutionary finding if C. pneumoniae proves to play an important role in arteriosclerotic heart and brain disease, currently numbers one and two killers of human beings!

sels take up foreign material such as invading microbes and their products, including toxins and other antigens. The tiny lymphatic capillaries join progressively larger lymphatic vessels. Many oneway valves in the lymphatic vessels keep the flow of lymph moving away from the lymphatic capillaries. Both contraction of the vessel walls and compression by the movements of the body's muscles force the lymph fluid along.

At many points in the system, lymphatic vessels drain into small, bean-shaped bodies called lymph nodes. These nodes are constructed so that foreign materials such as bacteria are trapped in them; the nodes also contain phagocytic cells and antibody-producing cells. Lymph flows out of the nodes through vessels that eventually unite into one or more large tubes that then discharge into a large vein, usually behind the left collarbone, and thus back into the main blood circulation. ■ lymphoid system, p. 396 ■ phagocytic cells, p. 384

When a hand or a foot is infected, a visible red streak may spread up the limb from the infection site (figure 28.2). This streak represents the course of lymphatic vessels that have become inflamed in response to the infectious agent. This condition is called lymphangitis. It may stop abruptly at a swollen and tender lymph node, only to continue later to yet another lymph node. This pause in the progression of lymphangitis

Figure 28.2 Lymphangitis Notice the red streak extending up the arm.The streak represents an inflamed lymphatic vessel.

demonstrates the ability, even though sometimes temporary, of the lymph nodes to clear the lymph of an infectious agent.

Blood and lymph carry infection-fighting leukocytes and such antimicrobial proteins as antibodies, complement, lysozyme, /i-lysin, and interferon. An inflammatory response may cause lymph and blood to clot in vessels that are close to areas of infection or antibody-antigen reactions, one of the ways the body has to localize an infection from the rest of the body. As with the nervous system, the blood and lymphatic systems lie deep within the body and are normally sterile. ■ antimicrobial substances, p. 375 ■ inflammatory response, p. 385

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