346 RESPIRATION 



such as muscles, may or may not have a great blood supply, ac- 

 cording to the amount of work a man is doing. We can easily 

 see, therefore, that the time taken for different parts of the body 

 to become saturated with nitrogen will vary greatly. 



Taking into consideration the amount of fatty material in the 

 body, we estimated that the whole body of a man weighing 70 

 kilos will take up about i liter of nitrogen for each atmosphere 

 of excess pressure — about 70 per cent more nitrogen than an 

 equal weight of blood would take up. Now the weight of blood in a 

 man is about 6.5 per cent of the body weight; hence the amount 

 of nitrogen held in solution in the body, when it is completely 



1 70 

 saturated with nitrogen, will be about — — or 26 times as great 



as the amount held in the blood alone. If, therefore, the composi- 

 tion of the body were the same at all parts, and the blood dis- 

 tributed itself evenly to all parts, the body would have received at 

 one complete round of the blood after sudden exposure to a high 

 pressure of air one twenty-sixth of the excess of nitrogen cor- 

 responding to complete saturation. The second round would 

 add one twenty-sixth of the remaining deficit in circulation, i.e., 

 1/26 X 25/26 of the total excess. The third round would add 

 1/26 X (25/26 X 25/26), and so on. On following out this calcu- 

 lation, it will be seen that the body would be half saturated in 

 less than 20 rounds of the circulation, or about ten minutes, and 

 that saturation would be practically complete in an hour. The 

 progress of the saturation would follow the logarithmic curve 

 shown in Figure 89. Actually the rate of saturation will vary 

 widely in different parts of the body ; but for any particular part 

 the rate of saturation will follow a curve of this form, assuming 

 that the circulation rate is constant. 



There is abundant evidence, both from human experience and 

 from experiments on animals, that liability to compressed-air 

 illness increases with duration of exposure. We found that in 

 goats the liability increased up to about 3 hours' exposure, but 

 did not increase further even with far longer exposure. In man, 

 on the other hand, limitation of exposure to 3 hours has been 

 found to diminish the liability distinctly, and we calculated from 

 the goat experiments, taking into account the greater rate of 

 circulation in the goat on account of its much smaller weight (see 

 Chapter X), that in man the liability would increase up to about 

 5 hours' exposure. We had therefore to allow for parts of the 

 body which would only become half saturated in about i J4 hours, 

 but for nothing slower than this. 



