494 



ORGAN SYSTEMS OF MAN 



,... I . i . ' . i j '■ ' ■■- ' ,. . .- . ' - ■ ' ' . ' . 'i . ' . i . .. ., 



.mu6cla loyar 

 -iendotbcliun^i 



lum 



Fig. 19-4. Arteries and veins differ only in thickness of their walls, whereas capillaries are composed only of the 

 layer of cells that lines the larger vessels. In this figure the arrows indicate the path of the blood. Some of the 

 various cellular components of the blood are also shown. 



a second auricle, together with a pidmonary 

 circulation, appeared. This was only a par- 

 tial solution, for the oxygenated and re- 

 duced blood became somewhat mixed. In 

 a further step forward, a complete two- 

 chambered heart appeared in the higher 

 reptiles. This proved so satisfactory that it 

 was retained in both mammals and birds. 

 Once the two hearts were formed, the 

 vertebrates were fully equipped for success 

 on land and evolution was extremely rapid, 

 as evidenced by the tremendous variety of 

 land forms. Let us examine the circulatory 

 system of man as an example of this system 

 in land vertebrates. 



THE MACHINERY OF HUMAN 

 CIRCULATION 



The over-all design of circulation in man 

 (Fig. 19-3) is similar to that of air-breath- 

 ing vertebrates in general, differing only in 

 such minor details as the number and loca- 

 tion of vessels. 



The functional part of circulation is the 

 circulating fluid, blood, the medium that 

 carries food and oxygen to the cells. The 

 elaborate mechanism of the heart, arteries, 

 veins, and capillaries is merely a mechani- 

 cal device to circulate the blood so that its 

 contents may reach every cell in the body. 



