86 RESPIRATORY MECHANISMS 



1 atmosphere, but the blood and gradually also the tissues 

 become saturated with the inert "nitrogen" at the high pres- 

 sure, and when the pressure is reduced by the diver returning 

 to the surface, bubbles of nitrogen may be suddenly released, 

 just as in a soda-water bottle when it is opened. When suffi- 

 cient bubbles are released in the blood itself they may block 

 the passage through capillaries and kill the patient, but also 

 the liberation of nitrogen bubbles in nerves and other tissues 

 may give rise to very distressing symptoms. To guard against 

 this danger it had become customary for divers to ascend with 

 extreme slowness, taking hours to get up from, say, 40 meters. 

 Haldane (1907) introduced the ascent by stages, having found 

 that the pressure could always safely be reduced to about half 

 when equilibrium had been established at a certain pressure. 

 In the lung of a seal or whale making a deep dive the air 

 must become compressed, and it was a mystery why they did 

 not develop caisson disease when staying long at great depths 

 and returning directly to the surface. This mystery has been 

 cleared up, at least in principle, by Scholander. In a man 

 in a diving dress the lungs are normally inflated and the circu- 

 lation through them is normal. In the diving animals the 

 lungs become greatly reduced in volume by the pressure of the 

 water. The alveolar surface is therefore reduced and the 

 thickness increased, tending to reduce the rate of nitrogen 

 diffusion so that the animals can attain greater depths and 

 stay down longer without much danger. The slowing of the 

 blood-flow must further reduce the diffusion, and it is quite 

 possible that only a fraction of the total blood becomes super- 

 saturated and that the nitrogen concentration is rapidly 

 reduced when this is mixed with the rest of the blood. Scho- 

 lander points out that the presence of a large "dead space," 

 made up of the trachea and bronchi, further reduces the 

 effective volume and alveolar surface of the lungs, because 

 most and eventually all of the air is transferred to the dead 

 space from which very little diffusion will take place, and 

 he shows that in the diving mammals the lungs can be com- 

 pletely collapsed and all the air transferred to the air passages. 



