THE CIRCULATION 



305 



It is only here that the blood performs its indispensable functions. The 

 capillaries are the essential portion of the circulatory system the arteries 

 and veins are only subsidiary to them. 



At least five sorts of processes take place in the capillaries and through 

 its walls. These are : the out-flow through the walls of the nutrient blood 

 plasma on the physical principle of osmosis, determined by its saline 

 molecular density; the out-flow of oxygen by diffusion ; the inflow of carbon 

 dioxide also by diffusion ; diapedesis or the passing through of the leuko- 

 cytes; and a probable, but little known, vaso-motion. The first three of 

 these have already been explained after a fashion in the chapters on 

 nutrition and on respiration. 



FIG. 165 



FIG. 166 



Blood-capillary: a, one of the Lymphatics of the small intestine. (Poirier and Charpy.) 

 endothelial cells which entirely 

 compose the tube (unless a plexus 

 of T nerve-fibrils is also present). 

 (Bates.) 



The Lymphatic Portion of the Circulation. As the blood circulates 

 through the capillaries only part of it at each passage performs some 

 specific function, the remainder flowing on to make itself useful to the 

 tissues perhaps the next time it passes through capillaries. Meanwhile it 

 is carrying back to the heart and lungs its share of carbon dioxide and 

 bearing off for excretion into the kidneys its burden of dissolved tissue- 

 waste. Of the capillary-blood a part osmoses through the thin endothe- 

 lial plates forming the capillaries to serve the tissue-cells immediately 

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