124 life's beginning on the earth 



the thin walls of the capillaries will attract fluid into them, 

 and the blood contains such substances: the proteins. The 

 proteins draw fluid toward the capillaries while the pressure 

 exerted by the heart tends to push fluid out of them. As a 

 result, in the normal body these two opposing forces coun- 

 terbalance each other. 



However, if the dissolved proteins of the blood are lack- 

 ing, there is no counterbalancing effect. The blood pressure 

 pushes fluid into the loose spaces of the body, causing 

 swelling of the ankles, the eyelids, the abdomen or elsewhere. 



direction of blood flow 



terminal arteries capillaries beginning of veins 



Fig. 36. Diagram of Arteries, Capillaries, and Veins 



The contractions of the heart force blood through the capillaries which 

 lie between in the tissue everywhere. Their walls are so thin that fluid 

 readily passes through them. But the blood passing through the capillaries 

 contains more dissolved substances than the fluid which surrounds the 

 capillaries. Consequently fluid will be drawn into the capillaries, thus 

 counteracting the tendency of the blood pressure to drive out fluid. After 

 passing the capillaries, the blood is collected by the much wider veins which 

 run mostly on the surface of the body and carry the blood back to the heart. 



We now understand why a swelling may occur after star- 

 vation or wasting diseases (see page 110 and figure 31). 

 Our food is the source of those proteins in the blood which, 

 by their attraction of water through the capillaries, oppose 

 the effect of blood pressure. These proteins are continu- 

 ously used up in life processes, and must be replenished 

 from the food intake through the intestinal tract. If the 

 food supply fails, the blood proteins must soon run low 

 Consequently, not enough proteins are left to keep the blood 

 fluid from leaking through the capillaries anywhere in the 

 body, and hunger edema (swelling) sets in. 



