GENERAL ZOOLOGY 



pressure in the veins is lower than it is in arteries. Movement of blood toward 

 the heart from the lower extremities of upright vertebrates such as man is 

 aided by valves in the veins of the legs. During movement of the legs, the 

 contraction of the skeletal muscles (p. 68) helps to push the blood along, 

 with the valves preventing backflow. 



The heart beats about 70 times per minute in a normal resting man. At 

 each contraction, a pulse wave moves along the arterial wall and can be felt 

 in any large peripheral artery. However, this pulse wave moves much faster 

 than the blood. By means of radioactive particles inserted in a peripheral 

 vessel, it can be shown that it takes 20 to 25 seconds for blood to repass the 

 point of insertion, that is, for a complete trip through the body. About 5 

 liters of blood leave the heart every minute during normally quiet living. 

 If you realize that the cardiac output is greatly increased during muscular 

 exercise, up to 30 liters per minute, and think of the years that the human 

 heart can function continuously, you will begin to appreciate what an amazing 

 organ it is. The mechanism and control of this remarkable pump will be 

 described later (p. 117). 



In spite of the fact that the pressure of the blood in the capillary is rela- 

 tively low, some filtration of the blood plasma occurs through the single- 

 celled wall, in addition to diffusion. Some white blood cells may also move 

 out between the cells of the lining. Thus, a fluid bridge is formed between 

 the contents of the capillary and the nearby cells; this fluid is called the 

 interstitial fluid or lymph. Some of it apparently re-enters the capillaries as the 

 pressure decreases in the bed, but much of it enters the closed terminal vessels 

 of the lymphatic system (Fig. 3.3). These lymphatics, as they are called, remain 

 thin-walled as they unite to form larger vessels which eventually empty into 

 large veins in the region of the neck in higher vertebrates. Lymph is returned 

 from the hind legs and trunk in a fairly large channel known as the thoracic 

 duct. In birds and mammals, the lymphatics pass through the lymph nodes 

 where lymphocytes are differentiated and enter the lymph (p. 66). The 

 lymph nodes are also important organs of body defense since foreign materials, 

 such as bacteria and venoms, are filtered out there and destroyed by macro- 

 phages (p. 64). Contraction of muscles in all parts of the body moves the 

 lymph through its thin-walled vessels, and valves control the direction of 

 movement. 



The general function of the circulating fluids as what we may call a common 

 carrier will become clear as the discussion progresses. The several phases of 

 this general function are as follows: (1) to carry necessary food materials 

 and oxygen to the cells; (2) to carry the waste products of metabolism away 

 from the cells; (3) to transfer hormones, or internal secretions (p. 95), from 

 one part of the body to another; and (4) in warm-blooded animals, like the 

 mammals, to transfer heat from regions of high oxidation and so aid in the 

 maintenance of a constant body temperature (p. 125). 



The Respiratory System. Respiratory organs are of two kinds: gills in 

 water-dwelling vertebrates and lungs in land dwellers. Gills are tufted or 



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