68 RESPIRATION 



mere concentration of substances in the body, that are of physio- 

 logical importance. To illustrate this distinction, the concentra- 

 tion of water in blood is much less than in a two per cent solution 

 of sodium chloride ; but the diffusion pressure of water in the blood 

 is much greater than in the salt solution. Hence water will pass 

 from the blood into salt solution. Similarly carbonic acid probably 

 passes by diffusion from the muscular substance into the blood 

 although the concentration of free carbonic acid in the muscle is 

 less than in the blood. 



Paul Bert's conclusion that it is the partial pressure of a gas 

 which is of importance as regards its physiological action can thus 

 be extended to every other substance present in the living body, 

 not excepting water. The partial pressure of a dissolved gas is of 

 decisive importance because the gaseous partial pressure, or vapor 

 pressure, is an index of the diffusion pressure of a substance in 

 solution ; but where the gaseous partial pressure is so low that it 

 cannot be measured, we must have recourse to other indices of the 

 diffusion pressure. 



It has been shown how important are the gas pressures in al- 

 veolar air. But the gas pressures of the blood in the systemic 

 capillaries are of still more fundamental importance. It is clear 

 that in order to understand how the oxygen pressure of the blood 

 is regulated we must know the connection between dissociation of 

 the oxyhaemoglobin of blood and fall in oxygen pressure. In other 

 words we must know what is called the dissociation curve of oxy- 

 haemoglobin in blood. 



The history of the growth of knowledge on this subject is some- 

 what curious. Paul Bert 13 made some rough determinations with 

 the pump of the amounts of oxygen in dogs' blood saturated with 

 air in which the oxygen pressure was varied. His results indicated 

 that in presence of oxygen reduced to a pressure of about 20 mm. 

 the blood at body temperature had lost half its oxygen. In a living 

 animal breathing air with an oxygen pressure of about 55 mm. (the 

 alveolar oxygen pressure being unknown) the blood had also lost 

 half its oxygen. When the blood was at a temperature below that 

 of the body the oxygen was dissociated much less readily. 



The subject was taken up again by Hiifner, who used a solution 

 of oxyhaemoglobin crystals in dilute sodium carbonate solution. 

 As the result, partly of experiments, and partly of calculation, he 

 published in 1890 a very symmetrical curve, according to which 

 oxyhaemoglobin does not lose half its oxygen till the oxygen pres- 



18 Paul Bert, La Pression Barometrique, p. 694, 1878. 



