AIR-BREATHING IN FISHES 35 



the two sets of vessels and that this will be further aided by the 

 counter-current. It is believed that in this way a considerable 

 difference of oxygen tension can be maintained between the 

 efferent blood and the swimbladder gas (fig. 10c). Thus, oxygen 

 secreted into the swimbladder so as to maintain hydrostatic 

 equilibrium in a fish which swims to a greater depth does not 

 get lost via the circulation. For, although the blood may reach 

 equilibrium at the higher partial pressure of oxygen in the 

 swimbladder gas, during its passage away from the bladder this 

 blood will be close to blood of lower oxygen tension and the 

 extra oxygen will diffuse across the thin membranes of the rete 

 mirabile. In this way the counter-flow will ensure that there is 

 scarcely any loss of oxygen from the bladder. 



The gas secreted into the swimbladder is mainly oxygen but 

 there is also present some nitrogen and rare air gases such as 

 argon. All the oxygen comes from the molecular oxygen con- 

 tained in the water, which at depths of two to three thousand 

 metres remains at the same tension as the surface, i.e., one-fifth 

 of an atmosphere. Swimbladders of fishes at these depths may 

 contain 90 % nitrogen, and if constant volume is maintained the 

 oxygen in the bladder must be at a partial pressure of two to 

 three hundred atmospheres. It is evident, therefore, that the 

 blood in the hairpins of the rete mirabile must be in equilibrium 

 with this gas although when it passed through the gills it was in 

 equilibrium with the tension of oxygen in the water. The per- 

 formance of the rete in preventing the loss of oxygen at these 

 very high pressure gradients is quite fantastic, for the tension 

 must drop nearly 3,000 times during its passage along 1 cm. of 

 capillary. In terms of heat in a counter-current exchange system 

 it is as though boiling water passing in at one end and ice- water 

 at the opposite end were in contact for only 1 cm. yet the heat 

 transfer was complete to within 1 ten-thousandth of a degree! 



(ii) Functions of the Swimbladder 



The swimbladder wall is supplied with sensory endings which 

 probably detect the degree of stretch of the bladder wall pro- 

 duced by changes in volume of the bladder. This may result from 



