356 



LOADING UP 



If we consider the amounts taken up by the various tissues we 

 may arrive at some conclusion as to the mechanics of the processes 

 of saturation and desaturation. The average working man 

 weighs 70 kg. ; of this amount 15 per cent., or 10-5 kg., is fat or 

 fatty material ; 5 per cent., or 3-5 kg., represents the amount of 

 blood ; while the earthy constituents of bone (about 3 per cent.) 

 may be neglected. 



Distribution of Nitrogen in the Tissues of Men Weighing 70 Kg. 



Blood, as we have seen, can take up in simple solution about 

 0-87 c.c, of nitrogen for every 100 c.c. Taking the specific gravity 

 of blood as 1-06, we may consider that about 30 c.c. of nitrogen 

 are constantly in solution in the blood. Fat is capable of absorbing 

 six times as much nitrogen as an equal weight of blood, i.e. we 

 may write down 500 c.c. as the volume of the gas held by the fatty 

 matter of the body. Leaving out the earthy part of bone, the 

 remaining tissues account for about 435 c.c. 



Taking round figures, we see that the average man has, dissolved 

 in his blood, about a litre of nitrogen. The weight of this litre 

 is a function of the pressure under which it has been absorbed. 

 Looked at from another point of view, the weight of nitrogen held 

 in solution by the tissues is 32 times as great as that present in the 

 blood. If, therefore, the blood is, for the purpose of this calcula- 

 tion, considered as spread uniformly and at a uniform rate through- 

 out the body, the tissues would receive at the end of one complete 

 circuit of the blood after exposure to a sudden increase in air 

 pressure, 1/32 of the excess of nitrogen corresponding to complete 

 saturation at the new pressure. The second round of the circula- 

 tion would add 1/32 of the remaining deficit in saturation, and so 

 on. Haldane finds that it takes 23 rounds of the circulation 

 to half-saturate the tissues at the new partial pressure of nitrogen. 

 The progress of the saturation of the body with nitrogen may be 

 represented by a logarithmic curve (Fig. 84). As about 3-5 litres 

 of blood pass through the lungs every minute, and as the total 



