EFFECT OF COMPRESSED AIR 355 



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

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

 ordinary solution exposed to compressed air. 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 («) to the absorption coefficient of the gas in 

 blood and tissues and (6) to the partial pressure of the gas in 

 blood and tissues. («) 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 tissvie 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 3 atmospheres brought about 

 by a rapid descent through 60 ft. of Avater to the bed of the sea. 

 The volume of gas in solution in the body is at once reduced 

 to one-third, viz. 0-005 c.c. per gram. At the same time the 

 blood in the lungs has its content of nitrogen reduced from its 

 normal value of 0-87 c.c. per 100 c.c. of blood to 0-29, with almost 

 immediate restoration to the normal figure. The litre of blood 

 in the capillaries of the lungs would now have in solution three 

 times the weight of nitrogen as under normal pressure. When 

 the blood arrives at the tissues, partition of its load will take 

 place. Each gram of tissue has a deficit of 0-01 c.c. of nitrogen, 

 and nitrogen will pass from blood to tissue till each gram of tissue 

 contains the normal value of 0-015 c.c. per gram. This value 

 will not be reached at once, because the very acquisition of 

 nitrogen by the tissue implies the loss of nitrogen by the blood. 

 The blood then returns to the lungs for a fresh charge, which it 

 again shares with the tissues, and so on. Haldane calculates 

 that somewhere about five hours are required before the body is 

 completely saturated with nitrogen after any change of pressure, 

 i.e. till the partial pressure of nitrogen in the tissues corresponds 

 with its partial pressure in the blood and so to its partial pressure 

 in the alveolar air. 



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