THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 221 



also sugar and salts. And it is capable of clotting, though 

 the clot is less firm than that formed by the blood itself. 

 Since the tissue fluid Hes between the blood vessels and 



Lymph vessel 



T^lqhT heart 



ArTerles 

 Fig. 1. Diagram of circulation. 



Left heart chamber pumps blood out into arteries which distribute it to 

 capillaries. Venous blood is collected from capillaries and returned to right 

 heart chamber by veins. Thence it is pumped to lungs and onward to left 

 heart chamber. Tissue fluid, exuded through capillary walls, is collected in 

 lymph vessels and returned to veins near heart. 



(Modified from Paton.) 



the tissue cells, all the materials exchanged between the 

 cells and the flowing blood must pass through it. It is, 

 therefore, the direct intermediator for that exchange. 



The Circulation of Blood and Tissue Fluid. Because these 

 fluids are limited, the only way for them to serve con- 

 tinuously as carriers between the secluded cells and the 

 body surfaces is by being used over and over again. They 

 must circulate (see Fig. i). The blood is forced through the 

 vessels by. the contractions or "beats" of the heart — 

 essentially a powerful hollow muscle with two chambers, 

 right and left. Each chamber has inlet and outlet valves. 

 The nature of the muscle requires it to rest after each beat 

 before it can beat again. During the rest period blood 

 flows into the chambers of the muscle through the inlet 

 valves, into the right chamber from all the remote parts 

 of the body, into the left chamber from the lungs. When 

 the muscle contracts on its contents, these valves close. 



