ARTERIAL AND VENOtTS BLOOD 203 



through the thin walls of the capillaries in the manner 

 already described (see p. 175). The plasma composition 

 indicates that it is the part of the blood which carries the 

 food to the tissue cells. 



Unloading of wastes by the blood. — The carbon dioxide 

 given off by the tissues is removed from the blood almost 

 entirely in the lungs. It is noteworthy that the lungs do 

 not free the blood of wastes in general, and on that account 

 should not be said to purify the blood. In other words, 

 their great function is to exchange oxygen for carbon 

 dioxide or aerate the blood. 



Most of the urea and true wastes of the body are 

 removed by the kidneys and the skin, this waste being 

 collected from all parts of the body by the veins and 

 conveyed to these organs by special arteries, such as the 

 renal artery to the kidneys. 



Arterial and venous blood. — From the description given 

 above it is readily seen that the blood in the arteries and 

 veins varies in composition in different parts of the body. 

 In general, however, we may say that the veins contain 

 less oxygen and more carbon dioxide than the arteries. 

 The redder color, due to the larger proportion of oxygen 

 present, has led to the custom of representing arteries 

 by red, and veins by blue lines in a diagram of circulatory 

 systems. The notable exceptions to this rule are the 

 pulmonary veins and arteries. The pulmonary vein con- 

 tains very little carbon dioxide, while the pulmonary 

 artery contains very little oxygen. 



In general, then, the terms arterial and venous are used 

 to indicate relative amounts of oxygen and carbon dioxide 

 in the blood. In that sense the pulmonary vein contains 

 arterial blood, and the pulmonary artery, venous blood. 



