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collect lymph from the tissue spaces and, by means of a system in which small vessels unite to form larger ones, carry it toward the heart, As the lymph vessels increase in size, the walls become stronger until they are composed of three layers, like blood vessels. Along the path of the larger lymphatic are valves that prevent backflow of lymph.

Lymphatic channels from the upper half of the right side of the body converge to form the right lymphatic duct, which empties into the right subclavian vein. Drainage from the remainder of the body is by way of the thoracic duct, which empties into the left subclavian vein.

LYMPH NODES

Lymph nodes, which are frequently called glands but are not true glands, are small beanshaped bodies of lymphatic tissue found in groups of two to fifteen along the course of the lymph vessels. They usually occur singly just beneath the skin. Nodes vary in size and act as filters to remove bacteria and particles from the lymph stream. Lymph nodes also participate in the manufacture of white blood cells and thus in the immunity functions of the body.

Commonalities of the upper respiratory and digestive systems

Figure 3-34.—Commonalities of the upper respiratory and digestive systems.

THE RESPIRATORY SYSTEM

Respiration (breathing) is the exchange of oxygen and carbon dioxide between the atmosphere and the cells of the body. There are two phases of respiration:

Physical, or mechanical, respiration involves the motion of the diaphragm and rib cage. The musculoskeletal action, which resembles that of a bellows, causes air to be inhaled or exhaled.

Physiological respiration involves an exchange of gases, oxygen and carbon dioxide, at two points in the body. The first is the transfer that occurs in the lungs between the incoming oxygen and the carbon dioxide present in the capillaries of the lungs (external respiration). The second transfer occurs when the oxygen brought into the body replaces the carbon dioxide build up in the cellular tissue (internal respiration).

Normally, oxygen and carbon dioxide exchange in equal volumes; however, certain physiological conditions may throw this balance off. For example, heavy smokers will find that the ability of their lungs to exchange gases is impaired, leading to shortness of breath and fatigue during even slight physical exertion. This is the direct result of their inability to draw a sufficient amount of oxygen into the body to replace the carbon dioxide build-up and sustain further muscular exertion. On the other side, hyperventilation brings too much oxygen into the body,