158 RESPIRATION 



about 700 to 800 a minute. A volume of blood equal to the whole 

 of that in the animal will pass round the circulation in one or two 

 seconds during exertion, so that any portion of blood will only be 

 present for an instant in the pulmonary capillaries in each round 

 of the circulation. Yet the time is sufficient for the chemical change 

 to occur in the blood, and doubtless far more than sufficient, since 

 we have to allow also for the time needed for the passage of oxy- 

 gen through the layer of living tissue separating the air from the 

 blood. In man the time available is much greater, so that the 

 absolute velocity of the chemical change does not come into con- 

 sideration at all, though of course the relative rates at which oxy- 

 gen is chemically associated with or dissociated from haemoglobin 

 at varying partial pressures of oxygen and varying temperatures, 

 determine the corresponding dissociation curves as experimentally 

 determined. 



A further group of causes of anoxaemia depends not on defec- 

 tive saturation in the lungs, but on defect in the charge of available 

 oxygen carried by the arterial blood, so that, with the existing 

 rate of circulation, the oxygen pressure in the systemic capillaries 

 falls too low. Of this group, carbon monoxide anoxaemia will be 

 considered first. 



The laws of combination of carbon monoxide with haemoglobin 

 have already been discussed in Chapter IV. My own interest in 

 carbon monoxide arose out of my connection with coal mining, as 

 it had become evident to me that carbon monoxide poisoning was a 

 common occurrence, and I wished to understand it as thoroughly 

 as possible. When Claude Bernard discovered the combination of 

 CO with haemoglobin he attributed death from CO poisoning to 

 the anoxaemia resulting from the fact that CO displaces the oxy- 

 gen of oxyhaemoglobin. CO was, however, very generally be- 

 lieved to have other physiological actions than those of anoxaemia, 

 and my first experiments were made with a view to clearing this 

 matter up. 



To put the matter to the test, I devised the following experi- 

 ment 19 (Figure 51). A mouse was dropped into a thick glass 

 measuring vessel filled with pure oxygen, and the pressure of 

 oxygen in this cylinder was then raised to two atmospheres by 

 connecting it with an oxygen cylinder in the manner shown. The 

 oxygen was then clamped off and another clamp opened, through 

 which the oxygen was directed into the top of another measuring 

 vessel full of water, and the water driven over into a third measur- 



"Haldane, Journ. of Physiol., XVII, p. 201, 1905. 



