354 LOADING UP 



simple solution in the plasma. («) 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. 82). {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 

 s()lul)ility 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 

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

 haemoglobin and only 3 c.c. in simple solution (measured at 

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

 result would be that as there are about 3h 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 oxvffen in its 

 passage through the pulmonary capillaries, we may take for 

 granted that saturation with nitrogen under the same conditions 



