CIRCULATION 



177 



should be noted that although the pressure in the capillaries of 

 any region as a whole is greater than in the veins which they sup- 

 ply, nevertheless the pressure in a single capillary is very low, as 

 is demanded by its delicate wall. 



Thus in the capillaries the blood moves very slowly under low pres- 

 sure and here the blood does its work - - contributes to the tissue 

 fluid and interchanges materials with it. All the rest of the vascu- 

 lar system — heart, arteries, and 

 veins — is arranged to give the 

 blood just this opportunity in the 

 capillaries. 



The tissue fluid is essentially 

 some of the blood plasma, with 

 white cells, that has passed 

 through the walls of the capil- 

 laries, carrying along food mate- 

 rials, oxygen, etc., to be exchanged 

 for the various waste products of 

 metabolism of the cells which it 

 bathes. Thus there is a continuous 

 drainage of fluid from the capil- 

 laries into intercellular spaces. 

 Some of this tissue fluid, with 

 waste products, etc., passes imme- 

 diately into the capillaries again, 

 but the excess passes into small 



LYMPH VESSELS. Thus lymph is some of the deeper lymphatics of the 

 essentially excess tissue fluid with human hand. (From Hough and 

 white cells augmented from lymph ^edgwic •) 



glands, etc., which passes through larger and larger lymphatic 

 vessels until it is finally delivered to veins in the region of the 

 neck, and the materials are restored to the blood. (Figs. 115, 

 124-126.) 



With such a marvellously complex transportation system, clearly 

 some provision must exist for regulating the blood flow in order to 

 meet the varying local demands of the organs of the body under 

 different physiological conditions. This is attended to chiefly by 

 nerve impulses which are conducted by a system of vasomotor 

 nerves and bring about the dilation or contraction of the smaller 

 arteries leading to an organ, and also by chemical substances in the 



Fig. 125. — The superficial and 



