BREATHING IN RAREFIED AND- COMPRESSED AIR 423 



atmospheric pressure and to liberate it again when the animal is decom- 

 pressed. The cause of delay is that the tissue fluids other than the blood 

 take much longer than would be expected to reach equilibrium with the 

 partial pressure of gas in the blood plasma. 



To understand why this delay should occur, let us suppose that the 

 only gas concerned is nitrogen. As the pressure rises, the blood in the 

 capillaries of the lungs must dissolve nitrogen in proportion to the pres- 

 sure of this gas in the alveoli; the blood carries the dissolved gas to the 

 tissues and these dissolve it until the pressure is again equalized between 

 them and the blood. The blood, after giving up its excess of dissolved 

 nitrogen, returns to the lungs and again becomes saturated and this goes 

 on until blood and tissue have become saturated with gas at the external 

 pressure. The tissues are two-thirds water and they contain (in man) 

 from 15 to 20 per cent of fat. Fat, however, dissolves five times more 

 nitrogen than water (Vernon) ; consequently, it takes longer for a given 

 volume of tissue than of blood to become saturated at a given pressure. 



The blood in man constitutes one-twentieth of the body weight; so 

 that if the tissues were all liquid they would dissolve 20 times as much 

 nitrogen as the blood. On account of the fat which they contain, however, 

 the tissues take up more than this proportion namely, in an average 

 man about 35 times more than the blood. All the blood in the body takes 

 about one minute to complete a round of the circulation, so that in this 

 time, after being suddenly subjected to an increased pressure assuming 

 that the blood circulates equally throughout the body the tissues will 

 be one-thirty-fifth saturated; in the next minute another thirty-fifth of 

 thirty-four thirty-fifths will be saturated, and so on. After five minutes 

 the body will be about 22 per cent, and in 25 minutes about one-half, 

 saturated; but it will take about two hours before saturation is complete. 

 These calculations assume that the blood is evenly distributed through- 

 out the body; but this is not the case, for its mass movement varies 

 considerably in different parts, being much greater in the active muscles 

 and in the glands than in passive structures, such as fat. These less vas- 

 cular parts will therefore lag behind the others in taking up their full 

 quota of gas, and therefore prolong the time necessary for complete 

 saturation of the body as a whole. 



We see therefore that, after some time in compressed air, the blood 

 and active tissues will be saturated and contain volumes of dissolved 

 gas in proportion to their relative bulks ; the fat, although not saturated, 

 will yet contain up to five times more gas than an equal volume of 

 blood, and the passive tissues will be incompletely saturated. 



These considerations regarding the saturation of the different parts 

 of the body apply also to its desaturation. Suppose, for example, that 



