320 - Multicellular Animals, Especially Man 



meated by blood capillaries to such an extent 

 that the total area of the capillaries in our 

 muscles alone measures more than 5000 square 

 meters. The very rapid flow of blood and the 

 almost instantaneous equilibrium that takes 

 place between the blood and lymph in the 

 capillaries all over the body enables the 

 vertebrate circulatory system to cope with 

 the distribution problems of the largest and 

 most active animals in existence. 



BLOOD 



If mammalian blood is prevented from 

 clotting, it will settle on standing, forming 

 two main layers. The upper straw-colored 

 layer is the fluid plasma, which accounts for 

 about 55 percent of the blood volume; and 

 the dark-red lower layer is a densely packed 

 mass of cells — the corpuscles. 



The Plasma. The plasma not only serves 

 as a vehicle for the blood corpuscles, but it 

 also transports a wide variety of substances 

 in solution (see Table 17-1). Some of these 

 substances — for example, foods and meta- 

 bolic wastes — enter and leave the blood 

 stream in the different parts of the system; 

 but others — such as proteins — remain in the 

 plasma quite indefinitely. Some of the pro- 

 teins, particularly fibrinogen, play an essen- 

 tial role in the clotting of blood (p. 32.'!); and 

 all the blood proteins probably help to main- 

 tain the osmotic properties of the plasma 

 (p. 334). 



Owing lo the presence of antibodies, blood 

 plasma is frequently able to neutralize speci- 

 fic poisons, such as the toxins produced by 

 infecting bacteria and other parasites. If a 

 small dose of rattlesnake venom is injected 

 into a horse, for example, the plasma of the 

 horse — after several days — is found to con- 

 tain a specific antibody capable of neutraliz- 

 ing a much larger dose of this venom. Con- 

 sequently serum from the treated horse can 

 be used to save the life of human snake-bite 

 victims. In general, the toxic product that 

 elicits the production of an antibody is called 

 an antigen; and virtually all protein com- 



pounds, provided they are foreign to the 

 organism producing the antibody, are capa- 

 ble of acting as antigens. Antibodies are 

 produced in various tissues of the body, but 

 they are carried mainly in the blood plasma. 

 A significant part of the organism's resistance 

 to infectious diseases depends upon the pro- 

 duction of specific antibodies, and a lasting 

 immunity to certain diseases indicates that 

 some antibodies, once they have been formed, 

 remain in the blood stream more or less in- 

 definitely. 



2-Z 



F-A 



Fig. 17-2. Showing the diameter and thickness of 

 a red blood cell. 



The Red Corpuscles. There are from four 

 to six million red cells per cubic millimeter 

 in the blood ol normal human adults, al- 

 though in males the count tends to run 

 about a million higher than in females. Each 

 individual erythrocyte is an extremely small 

 biconcave disc with a pale pink color (Figs. 

 17-2 and 17-3); but collectively they account 

 for the deep red color of the blood. 



Fig. 17-3. Normal human blood, showing relative 

 numbers of red and white cells. A field as large again 

 as that shown would probably contain no other white 

 cell. (From The Living Body, by Best and Taylor. Holt, 

 Rinehart and Winston, Inc.) 



