THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 223 



the venules the blood passes to larger and larger veins with 

 thicker and stronger walls until the main trunks are reached 

 which pour the blood accumulated from all parts of the 

 body into the right chamber of the heart. 



Many of the veins He just beneath the skin where they 

 may be so prominent at times that they are clearly seen. 

 On the back of the hand, for example, they are usually 

 evident as a coarse network. By pressing on one with the 

 tip of a finger and sweeping out the blood toward the wrist 

 with the thumb, a valve (a cup-shaped sac attached at the 

 side of the vein) can be demonstrated which does not 

 permit a back flow. Wherever the veins are rhythmically 

 pressed upon, as by the muscles in walking, the valves 

 require the blood within them to be forced towards the 

 heart. This pumping action on the veins may aid greatly 

 in hastening the circulation. 



A similar system of arteries and veins connects the 

 capillaries of the lungs with the heart. It is important to 

 note that in the lungs, as in all other parts, the flow of blood 

 through the capillaries is the essential process. Only in the 

 capillary region do the necessary exchanges occur. AH 

 of the rest of the circulatory system exists to maintain the 

 flow in that region where it is serviceable. 



Tissue fluid is produced by the filtering of a portion of 

 the plasma through the capillary wafl. In some parts of 

 the body, e.g. in the hver, the capillaries are so "permeable" 

 that the process of filtration occurs continuously; in other 

 parts, e.g. in the limbs, it occurs only when the parts are 

 active. Under such conditions the fluid may be formed more 

 rapidly than it can be carried away, and therefore the part 

 may become perceptibly larger. 



The tissue fluid is returned to the blood in two ways. 

 It may pass back in part through the capillary wall when 

 the activity of the part ceases, or it may enter a definite 

 system of thin-walled tubes, the so-called "lymphatics," 

 and be conducted through them to a large vein near the 

 heart where the lymph is delivered as a stream into the 

 blood. The larger lymphatic vessels, like the veins, are 

 provided with valves and in consequence every little pres- 

 sure exerted on them pushes the lymph onward to the exit. 



