CHAPTER 27 



Blood and Circulation 



All animals, from the simplest protozoa to the most complex verte- 

 brates, must have some arrangement for transporting a wide variety of 

 materials throughout their bodies. As we pointed out in Chapter 5, the 

 simple diffusion of molecules always plays an important part in trans- 

 portation and this is adequate in itself in the smaller and less active 

 organisms. But the vertebrates and many of the higher invertebrates are 

 so large and active that diffusion alone cannot suffice. Complex cir- 

 culatory systems are necessary for the rapid transport of digested food 

 from the alimentary tract, and of oxygen from the lungs, to all the 

 tissues, and for carrying carbon dioxide and other metabolic wastes to 

 the sites where they are discharged from the body. 



The vertebrate circulatory system not only transports gases, foods 

 and waste products, but has other important functions as well. By con- 

 veying hormones it supplements the nervous system in the integration 

 of body activities. It plays an important role in maintaining the con- 

 stancy of the internal environment. The blood carries away excess water 

 from the tissues and su})plies water when necessary. It helps to regulate 

 the pH of the body fluids. The rate of its circulation through the skin 

 is a factor in the control of body temperature in birds and mammals. 

 Special cells in the blood function in wound healing and in protecting 

 the body from the invasion of viruses and bacteria. 



The circulatory system includes not only the complex system of 

 vessels but also the fluids within them. There are about 15 liters of 

 extracellular fluid in the body of an adult man, and about one-third 

 of this is blood. The remainder includes the tissue fluid that lies between 

 and bathes the cells of the body, the lymph that moves slowly in the 

 lymph vessels, the cerebrospinal fluid in the cavities of the central 

 nervous system, the aqueous and vitreous humors of the eye, and the 

 fluids in the coelom. The chief difference between blood and tissue fluid 

 or lymph is the presence of red blood cells and abundant soluble pro- 

 teins in the blood. 



The fundamental pattern of the vessels in a mammal is shown in 

 Figure 27.1. A muscular heart propels blood through arteries to capil- 

 laries in the tissues. Exchanges between the circulatory system and the 

 cells of the body can occur only through the walls of the capillaries. 

 Molecules of nutrients, wastes, oxygen, carbon dioxide and water, but 

 not the large protein molecules or the red blood cells, pass readily 



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