384 PHYSIOLOGY CHAP. 



which represents the curve of dissociation of oxygen and haemo- 

 globin, in proportion with the fall of partial pressure in the mixture 

 of gases. The curve shows that at the partial pressure of 150 mm. 

 Hg (which is a little lower than that of the oxygen of normal air) 

 almost the whole of the haemoglobin (about 98 per cent) combines 

 with the oxygen ; that the dissociation proceeds very slowly till a 

 partial pressure of 50 mm. (which corresponds to about a third of 

 the partial pressure of the oxygen of normal air) is reached ; and 

 that it only becomes rapid at a partial pressure of 25 - 10 - 5 mm. Hg. 

 These results show that the blood, in consequence of the chemical 

 affinity of haemoglobin for oxygen, is able to provide itself with an 

 abundant supply, even when the organism is breathing an atmo- 

 sphere very poor in this gas ; while, on the other hand, the absorption 

 of oxygen in the blood cannot rise far above the normal, even when 

 the organism is made to respire an atmosphere of pure oxygen. 



A proof of this great independence of the absorption of the 

 oxygen of the blood from its partial pressure in the atmosphere is 

 shown in the fact that mammals do not exhibit any visible dis- 

 turbance of respiratory function when they are made to breathe an 

 artificial atmosphere three times richer, or one-half poorer in oxygen 

 than the normal air ; and it is only when the partial pressure of 

 oxygen falls below this limit that the respiratory movements are 

 progressively accelerated, and death from lack of oxygen only 

 occurs when the partial pressure of 2 is lowered to 3*5 mm. Hg 

 (W. Miiller, P. Bert). 



On examining in dogs how the oxygen content of arterial blood 

 varies with the progressive rarefaction of the atmospheric air 

 respired, it was found that it remains normal up to a total 

 pressure of 410 mm. Hg; that it diminishes slightly at a 

 pressure 378-365 mm. ( = about half an atmosphere) ; and that it is 

 only at a total pressure of 300 mm. that any conspicuous diminu- 

 tion of oxygen can be observed in arterial blood (Frankel and 

 Geppert). These facts agree with the observations made during 

 aerostatic ascents, which show that respiratory disturbances only 

 begin at a height of 5000 meters ( = 400 mm. Hg). On the 

 other hand, it has been observed on the high plains of the Andes 

 that men and animals can live as well at 4000 metres altitude as at 

 the level of the sea. 



Not quite the whole of the oxygen is in chemical combination 

 with haemoglobin ; a small fraction of it (0'1-0'2 vols. per cent) 

 is normally held in solution in the plasma. This quantity is, 

 however, less, under normal conditions, than what can be absorbed 

 by an equal volume of distilled water at the same tempera- 

 ture. It may vary according to the Henry-Dalton law, i.e. the 

 volume of oxygen dissolved in the plasma is proportional to its 

 tension. In proportion as the tissue elements absorb the oxygen 

 of the plasma, and the tension lessens, there must necessarily 



