PHYSIOLOGY OF ORGANS _ 151 



diffusion of gases. The tension of carbon dioxide in the tissue cells is 

 greater than in the blood of the capillaries. This tension has been found 

 by experiment in man to be about 50 to 70 mm. of mercury in the tissues 

 and about 35 mm. in the arterial blood. Under these conditions the 

 carbon dioxide diffuses from the cells into the lymph and from the lymph 

 through the capillary walls into the blood stream. Here some of the 

 carbon dioxide goes into solution in the plasma, some enters into chemical 

 combination with sodium and potassium salts in the plasma and in the 

 corpuscles, and some into chemical combination with the hemoglobin. 

 The tension of carbon dioxide in the alveolar air of the lungs of man has 

 been determined to be equal to 35 mm. of mercury while that in the ven- 

 ous blood is 42.6 mm. As a result of this difference in tension the carbon 

 dioxide diffuses through the membrane into the alveoli of the lungs. 

 From the alveoli it is carried to the exterior with the exhaled air. The 

 method of elimination of carbon dioxide from gills does not differ in its 

 essential features from the method outlined above. The quantities, 

 however, are proportionately smaller. 



Elimination of Water. — Water is eliminated from the lungs, skin and 

 kidneys, but the relative importance of these organs varies widely in 

 different species. In the frog the elimination of water through the skin 

 may be very great especially if the frog is in dry air. Under such cir- 

 cumstances the loss of water from the skin may be sufficient to kill the 

 frog in a few hours. In the dog very little water is eliminated through the 

 skin, while in other mammals which perspire the elimination of water 

 through the skin may be considerable. In man the quantity of sweat 

 may amount to 2 or 3 liters per day, but the quantity is very variable. 

 Sweat is secreted by the sweat glands, of which there are about two mil- 

 lions in the human skin. The lungs of man eliminate an appreciable quan- 

 tity of water as water vapor but this quantity varies within narrow limits. 

 In the dog, which eliminates very little water through the skin, the quantity 

 of water eliminated by the lungs varies considerably being much greater 

 when, because of heat or exertion, the dog pants. The kidneys are the 

 most important organ for elimination of water. The quantitj^ of water 

 eliminated through them in man varies inversely with the quantity elim- 

 inated through the skin. 



Nitrogen Elimination. — Normal digestion of proteins in the alimentary 

 canal gives rise to amino-acids and some ammonia. Probably the hydroly- 

 sis of proteins in the protoplasm yields the same nitrogenous compounds. 

 Some of the ammonia is eliminated in the form of ammonium salts and 

 some of the amino-acids in the free or combined form. The larger 

 proportion of each substance, however, is first converted into urea. 

 It has been established that urea is not formed in the kidneys, which 

 eliminate it, but is formed in the Hver and perhaps also in the tissues 

 whence it is carried to the kidneys by the blood. Other sources of urea 

 may be ignored in this account. 



