268 LOADING UP 



an air pump. Paul Bert in a remarkable series of experiments 

 (1870-1880) proved that these bubbles were nitrogen and that 

 they might block up the capillaries in some part of the body and, 

 by cutting off that part from the blood supply, produce one or 

 other of the symptoms mentioned above. 



If merely the pressure of the surrounding air is increased, why 

 should nitrogen alone be set free on decompression ? When a 

 person is placed in compressed air, the blood passing through the 

 lungs dissolves the same volume of the atmospheric gases as it 

 does under normal conditions, but the weight of gas absorbed 

 will be increased above normal in proportion to the increase in 

 partial pressure of each gas in the alveolar air. Now we have seen 

 that the partial pressure of carbon dioxide in alveolar air is a 

 constant, hence there can be no increase in the amount of carbon 

 dioxide present in the blood during exposure to compressed air. 

 Oxygen is carried in two ways, (a) by haemoglobin, and (b) in 

 simple solution in the plasma, (a) At atmospheric pressure the 

 haemoglobin is almost saturated with oxygen, the little erythro- 

 cyte barges are comfortably filled. Increase of alveolar tension 

 may produce a slightly better oxygenation of the haemoglobin, 

 but it requires a very marked increase of pressure to make an 

 appreciable increase in the amount of oxygen carried by this 

 means (Fig. 56). (b) According to Dalton's Law, the amount of 

 gas dissolved is directly proportional to its partial pressure. 



At body temperature and normal pressure, arterial blood holds 

 3 c.c. of oxygen in solution in every litre of fluid. If the pressure 

 is increased x times, then each litre will still dissolve 3 c.c. of 

 oxygen, but this oxygen will weigh x times as much as normally. 

 On being carried to the tissues, the blood will share its dissolved 

 oxygen with them in proportion to its partial pressure and to its 

 solubility in the various tissues. These tissues will use up the 

 dissolved oxygen in preference to that carried by the corpuscles, 

 and as the amount in solution, except after exposure to enormous 

 pressures, is only a small percentage of the total available oxygen 

 in the arterial blood, it will soon be used up. We again draw 

 attention to the fact that increase in the available oxygen does not 

 cause increase in its utilisation by the cell. A candle burns more 

 brightly in oxygen and soon ends its light-giving career. The 

 cell " ca's canny " holds on the even tenor of its way, takes up the 

 oxygen it requires for its immediate needs and keeps no store but 

 the tiny quantity dissolved in its protoplasm. 



To take a concrete example. At 38 C. and atmospheric 



