176 



ANIMAL BIOLOGY 



Aorta 



passes through a series of vessels that, if they could be arranged 

 continuously, would, it has been estimated, encircle the Earth! 



The rate of flow is greatest when the blood leaves the heart and 

 gradually diminishes until, in the capillaries of both the pulmonary 



and systemic systems, it is 

 reduced to a minimum. On 

 the return trip from the cap- 

 illaries through the veins the 

 rate of flow gradually in- 

 creases, though it reenters 

 the heart at a slower rate 

 than it departed. Thus of 

 the 23 seconds which it takes 

 a unit of blood to make the 

 complete circuit in Man, 

 about two seconds are spent 

 in the capillaries - - a rela- 

 tively long time when it is 



Superior, 

 vena cava 



Pulmonary' 

 veins 



Right 

 auricle 



Inferior 

 vena cava 



Pulmonary 

 artery 



Pulmonary 

 veins 



Left 

 auricle 



, Left 

 ventricle 



Right ventricle 



Aorta' 



Fig. 123. - - Diagram of human heart realized that the 



average 



and associated blood vessels. The direc- | ength of the cap jH arv pa th 

 tion oi blood flow is indicated by arrows. , „„ . , „ 



(From Peabody and Hunt.) f about one-fiftieth of an 



inch. The chief factor un- 

 derlying the change in rate is simple. The blood, driven through- 

 out its course bv the same force — the heart beat — varies in rate 

 with the width of the bed 



Lymph tube 

 Tissue cells 

 Tissue fluid 



Artery 

 Capillaries 



Capillaries 



,.\ Tissue cells 



•Vein 

 Tissue fluid 



Lymph tube 



through which it is flowing. 



Although the area afforded 



individually by the arteries 



and veins is greater than 



that by any single capillary, 



nevertheless the total area 



afforded by the capillary sys- „ Lymph tube ' 



. . Fig. 124. — Diagram of the intimate 



tem IS enormously greater re i at ; ons between capillaries, lymphatics, 



than that by either the arte- and tissue cells. (From Peabody and 

 rial or venous system. The Hunt.) 



total surface of the capillaries of a man has been stated to be 

 about equal to the area of a city block. 



Moreover, since a liquid in a closed system of tubes must flow 

 from a region of high to one of low pressure, the blood pressure 

 continuously diminishes from heart back to heart again. But it 



