1 88 ELEMENTARY BIOLOGY 



artery carries blood into the arm, and as the arteries divide, 

 becoming smaller and smaller, we at last reach the capillaries 

 in the hand, from which we started (Fig. 70). 



The double circulation makes it possible for the carbon dioxid 

 in the body to be completely exchanged for oxygen in a com- 

 paratively short time. In the human body all of the blood 

 passes through the heart (and therefore through the capillaries 

 of the lungs) once in from twenty-three to thirty seconds. 



The exchange of gases in the air sacs of the lungs has 

 already been described (see p. 148). 



218. Changes in the blood. When in the capillaries of the 

 various tissues of the body, the blood absorbs from the surround- 

 ing lymph (by osmosis) carbon dioxid, urea, and other sub- 

 stances that are present in relatively large proportions (that is, 

 compared to their concentration in the blood plasma) ; and it 

 loses by the same process food materials, salts, oxygen, and 

 ferments that are relatively more abundant in the blood than 

 in the surrounding liquids. 



In certain parts of the body additional changes take place 

 in the composition of the blood. In the intestines, for example, 

 much of the digested food is absorbed into the blood. In the 

 kidneys much of the urea, salts, and other waste substances 

 are taken from the blood. 



219. Ferments in the blood. In the capillaries of certain organs 

 the blood receives, in addition to the usual waste products, various 

 special ferments, or chemical substances. For example, from the 

 thyroid gland, which is a Y-shaped, spongy body lying in front of the 

 larynx, the blood absorbs a substance that has an important influence 

 on the development and working of the brain. 



From the pancreas the blood absorbs a substance that has an 

 important influence on the oxidation of carbohydrates in the cells. 



From little capsules that lie just above the kidneys the blood 

 absorbs a ferment that influences the muscles of the blood vessels, 

 and also has some effect on the nervous system. The amount of this 

 secretion is increased whenever strong feelings are aroused, such as 

 fear or anger. An increase in the amount in the blood acts upon 



