DESATURATION RATE 271 



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 calcu- 

 lation, considered as spread uniformly and at a uniform rate 

 throughout 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 circulation would add 1/32 of the remaining deficit in satura- 

 tion, 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. 57). 

 As about 3-5 litres of blood pass through the lungs every minute 

 and as the total blood volume is also 3-5 litres, we may substitute 

 minutes for rounds of the circulation and state that it requires 

 23 minutes to render the tissues half saturated to a new pressure 

 of nitrogen. 



The process of desaturation, provided physiological conditions 

 are kept constant, follows the same curve. If the tissues are 

 exposed to blood carrying nitrogen in excess of the normal amount, 

 for sufficiently long to be in gaseous equilibrium with that blood 

 i.e. to be saturated then in order to prevent the formation of 

 bubbles, the process of desaturation would need to be carried out 

 at the same rate as the saturation. If the desaturation rate were 

 too rapid, then gas \vould be released from the tissues more 

 rapidly than it was being passed from blood to alveolar air. 

 This would entail a very slow and uniform rate of decompression. 

 A diver's ascent from the sea bed might have to be spread over 

 hours. Paul Bert, from his experiments on animals, concluded 

 that the decompression period should be 30 minutes for under 

 3 atmospheres, and 60 minutes for 3 to 4 atmospheres. This 

 ruling of the famous French scientist has never been carried out 

 in industrial practice, the usual period for " leaking out " being 



