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). 



This "double circulation" makes it possible for the carbon 

 dioxid in the body to be completely exchanged for oxygen in a 

 comparatively 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, called hormoties. For example, 

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

 front of the larnyx, the blood absorbs a substance that has an impor- 

 tant influence on the development and working of the brain. Extracts 

 of the thyroid from calves can be used to correct deficiencies in the 

 production of this substance in human beings, with striking changes 

 in the further development of the patient. 



From the pancreas the blood absorbs a substance that has an 

 important influence on the oxidation of carbohydrates in the cells, 

 fnsi/Iin has been recently produced from the tiny lumps of tissue or 

 " islands " in pancreas ; and this has been used successfully in the 



