176 The Vertebrate Organ Systems 



THE BLOOD 



In most many-celled animals, the outer surface of the body is sub- 

 jected to a continuously changing environment. Those cells that form 

 the bulk of the organism, however, are never exposed to these hazards ; 

 instead they exist in a remarkably constant environment. Particularly 

 is this true among the homoiothermic vertebrates. 



All of the internal organs contribute to the maintenance of this 

 constant internal environment. But it is probably the blood that plays 

 the most important role. The blood, through the medium of its liquid 

 portion, bathes every cell of the organism and brings in food, hormones, 

 oxygen, and protective materials. Further, it removes any cellular 

 wastes and secretions. In short, the blood is the agent which maintains 

 the constant conditions about the cell. The maintenance of this con- 

 stant internal environment or homeostasis is the fundamental problem 

 of all the bodily organs. 



The amount of blood circulating remains quite constant at all times. 

 The measurement of this volume is difficult but can be approximated 

 by the use of dyes which are injected into the blood stream. By this 

 method, it is estimated that in mammals the blood composes about one- 

 thirteenth or 6 to 8 per cent of the total body weight. 



Fresh blood when examined under a microscope is seen to be com- 

 posed of small bodies floating in a liquid. The most numerous and 

 conspicuous of these bodies or formed el emeu fs are the red blood cor- 

 puscles or erythrocytes. Less easily seen are the many zvhite blood cells 

 or leucocytes, and the platelets. The liquid portion of the blood, the 

 plasma, is a straw-colored fluid which contains many dissolved sub- 

 stances such as salts, foods, hormones, etc. The formed elements com- 

 pose about 40 to 50 per cent of the total blood volume ; the plasma, from 

 50 to 60 per cent. The exact proportions vary within narrow limits. 



The Formed Elements of the Blood. — The red blood cells or 

 erythrocytes (Fig. 52) are the most conspicuous and numerous of the 

 formed elements of the blood. In human beings, they average about 

 5,000,000 per cubic millimeter in men and about 4,500,000 in w^omen. 

 The individual erythrocyte of human beings is a biconcave disc about 

 0.008 mm. in diameter and 0.002 mm. in width at the edge. A nucleus 

 is present only in the immature erythrocytes, but is lost before the cells 

 are released into the circulation. Most mammals do not have nucleated 

 red blood cells ; however, many other vertebrates, such as the frog, 

 retain the nuclei. Each red blood cell has a structural network in 



