CIRCULATION AND RESPIRATION IN ANIMALS 173 



rate of flow gradually increases 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 

 relatively long time when it is realized that the average length 

 of the capillary path is about one fiftieth of an inch. The 

 principle underlying the change in rate is simple. The blood, 

 driven throughout its course by the same force the heart 

 beat varies in rate with the width of the bed through 

 which it is flowing. Although the area afforded individually 

 by the arteries and veins is greater than that by the single 

 capillaries, nevertheless the total area afforded by the capillary 

 system is enormously greater than that by either the arterial 

 or venous system. 



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 should be noted, that although the pressure in 

 the capillaries of any region as a whole is greater than in the 

 veins which they supply, nevertheless the pressure in a single 

 capillary is very low, as is demanded by its delicate wall. 



Thus it is apparent that in the capillaries the blood moves 

 very slowly under low pressure for it is here that the blood 

 does its work. All the rest of the vascular system heart, 

 arteries, and veins is arranged to give the blood just this 

 opportunity in the capillaries. 



It is in the capillaries that the blood vascular system turns 

 over the work of distribution to the lymphatic system. As 

 has been said, lymph to all intents and purposes consists of 

 plasma and white corpuscles from the blood which have 

 passed through the thin capillary walls, carrying along food 

 materials, oxygen, etc., to exchange for the various waste 

 products of metabolism of the cells which it bathes. Thus 



