64 ORGANISM AND ENVIRONMENT 



afford complete protection, similar to that produced, 

 as already described, when the oxygen pressure of 

 the arterial blood is greatly raised by placing the ani- 

 mal in compressed oxygen. The pressure difference 

 against which oxygen can be secreted in the lungs is 

 also dependent on the pressure of oxygen in the alveo- 

 lar air. When this becomes very low the pressure dif- 

 ference is diminished; and the flow of oxygen may be 

 actually reversed if the alveolar oxygen pressure is 

 low enough. A similar reversal seems to occur in the 

 case of the swim bladder; and sometimes the air in 

 the swim bladder seems to be utilized as a store of 

 oxygen, drawn upon when the blood is insufficiently 

 oxygenated by the gills. Possibly the active secretion 

 current is reversed in direction. 



Let us now compare the secretion of oxygen with 

 that of other substances by other secreting glands. In 

 the case of the kidney, various salts and crystalloid 

 substances, particularly urea, are actively secreted by 

 the gland cells, so that their concentration in the 

 urine is far greater than in the blood. For instance 

 there is usually about ten or fifteen times as much 

 urea in a given volume of urine as in the same vol- 

 ume of blood, and when the kidneys secrete sugar 

 there may be twenty or thirty times as much sugar 

 in the urine as in the blood. Here then we have other 

 cases of the flow of one kind of molecules being accel- 

 erated in one direction. In the kidney secretion we 

 also see that the acceleration may be in either direc- 

 tion, and that it depends upon the molecular concen- 

 trations in the liquids on the two sides of the secreting 



