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 
