THE BLOOD AND CIRCULATION 323 



called lymph. The capillaries are also bathed by lymph. 



Composition and uses of lymph. — Lymph is composed 

 partly of water and partly of food material derived from the 

 capillaries through their walls. Lymph gives up food and 

 water to the tissue cells and receives from them, through 

 their walls, certain waste 'products. Oxygen also passes 

 from the capillaries through the lymph to the cells, and carbon 

 dioxide passes from the cells through the lymph to the blood. 



We see, therefore, that the important changes in the 

 blood take place in the capillaries and that the lymph sur- 

 rounding the cells functions as a medium of interchange 

 between the blood and the cells. This interchange is ac- 

 complished by the process of osmosis (see p. 298). 



The veins. — As the arteries break up into capillaries, so 

 'the capillaries unite, after leaving the tissues, to form veins. 

 The blood that flows to the tissues through the arteries is 

 laden with food and oxygen. We have seen that much of 

 this is exchanged in the capillaries for carbon dioxide and 

 waste products. Hence it is that the blood in the veins is 

 laden with these substances and needs to be purified before 

 it again becomes fit for use by the tissue cells. 



The blood is partly purified in the kidney, partly in the 

 liver, and partly in the lungs. The blood from the intestines, 

 stomach, spleen and pancreas, passes by way of the portal 

 vein to the liver. In the intestines it has gathered the di- 

 gested food, as already described on page 313. The portal 

 vein breaks up into capillaries in the liver and some of the 

 particularly deleterious waste products and poisonous sub- 

 stances that may have passed into the blood with the digested 

 food, are here removed. From the liver the hepatic vdn car- 

 ries the blood into the vena cava ascending and thus back to 

 the right auricle of the heart. Veins from other parts of the 

 body, as the arms, legs, and muscles, return the blood from 

 the capillaries of these parts, directly to the vena cava de- 

 scending and so to the rig'ht auricle (fig. 165). 



