CAISSON DISEASE 269 



pressure 1 litre of blood contains 200 c.c. of oxygen carried by 

 haemoglobin and only 3 c.c. in simple solution (measuredat N.T.P.). 

 Sixfold increase of pressure makes no appreciable difference to the 

 value of the corpuscular oxygen but increases the dissolved 

 oxygen to about 18 c.c. That is, the ratio of dissolved to " bound v 

 oxygen is increased from about 1/70 to 6/70. The entire result 

 would be that as there are about 3| litres of blood in the average 

 man the venous blood would carry not more than 20 per cent, more 

 oxygen than normally. In other words, the desaturation of 

 haemoglobin would take place to quite the same extent as under 

 atmospheric pressure. This eliminates oxygen as the gas causing 

 ' compression illness " and leaves nitrogen alone to be dealt with. 

 Let us first consider how the nitrogen taken up by the blood 

 from the alveolar air is distributed to the various tissues of the 

 body. In view of what we have seen as to the ease and complete- 

 ness with which the blood becomes saturated with oxygen in its 

 passage through the pulmonary capillaries, we may take for 

 granted that saturation with nitrogen under the same conditions 

 is just as complete. It is also reasonable to suppose that Dalton's 

 Law of partial pressures is just as applicable to blood exposed to 

 compressed air as to any ordinary solution. This supposition is 

 supported by experimental facts. When blood is exposed to 

 compressed air it will absorb a volume of nitrogen commensurate 

 with the absorption coefficient of this gas in blood. During its 

 passage through the tissues it will share its load of nitrogen with 

 them in proportion (a) to the absorption coefficient of the gas in 

 blood and tissues and (b) to the partial pressure of the gas in 

 blood and tissues, (a) With regard to the first point, the solu- 

 bility of nitrogen per unit mass of tissue varies greatly. For 

 example, fat can absorb about six times as much nitrogen gas as 

 blood, while the earthy constituents of bone probably absorb only 

 an infinitesimal amount. With these two tissues excepted we 

 may consider that, as the others differ but slightly in chemical 

 and physical constitution from plasma, they also take up approxi- 

 mately the same quantities of gas. (b) Normally the tissues are 

 saturated with nitrogen at its partial pressure in the atmosphere 

 every gram of tissue contains approximately 0-0145 c.c. of 

 nitrogen. If the external pressure is increased, this volume will 

 immediately be diminished correspondingly, and the deficit will 

 be made good at the expense of the circulating blood. Take for 

 example the sudden increase in pressure to three atmospheres 

 brought about by a rapid descent through 60 ft. of water to the 



