264 RESPIRATION 



So far, then, as the absorption of oxygen is concerned, there is every 

 reason to conclude that it is managed by physical processes alone. 

 Haldane, it is true, still contends that under exceptional conditions 

 when the call for oxygen is much increased, as during active mus- 

 cular exercise, or when an adequate intake is hampered by reduced 

 atmospheric pressure, as at high altitudes, the oxygen tension in 

 the arterial blood may materially exceed that in the alveolar air. 

 Those who have worked with other methods do not, however, grant 

 that even under these conditions there is any excess of oxygen 

 pressure in the blood. It may, of course, be formally admitted that 

 even if diffusion can account for the absorption of the whole of the 

 oxygen, this is not of itself a proof that it is by diffusion that the 

 thing is actually done ; it is only a reason for refusing to call in the 

 aid of a more recondite hypothesis, until the necessity for doing so 

 is clearly demonstrated. 



As regards the carbon dioxide the evidence is fully as clear. The 

 speed of diffusion of carbon dioxide across such a membrane as the 

 alveolar wall being much greater than that of oxygen, still smaller 

 differences of tension would suffice to permit the whole normal out- 

 put of that gas to be eliminated by diffusion. 



According to the observations of Bohr, nearly 500 c.c. of carbon 

 dioxide per minute could pass out of the blood by diffusion into the 

 alveolar air under a difference of partial pressure of i mm. of mercury. 

 This is with the ordinary breathing of a man at rest. With the in- 

 creased respiration associated with hard muscular work, this quantity 

 would be increased to between 700 and 800 c.c. The amount of carbon 

 dioxide eliminated during rest in an average adult (300 c.c.) can there- 

 fore be easily excreted by diffusion with a tension-difference of i mm. 

 of mercury. Even during very hard work the necessary tension differ- 

 ence need not be more than 3 mm. With a movement of carbon dioxide 

 so free as this, it is obvious that if the excretion of that gas is solely a 

 matter of diffusion its partial pressure in the arterial blood must be 

 nearly identical with its partial pressure in the alveolar air. 



A glance at the table on p. 261 shows that while the carbon di- 

 oxide tension of venous blood may sometimes, perhaps generally, 

 exceed that of the alveolar air, the difference is quite small. The 

 average for the observations on man with the pulmonary catheter 

 was 45 mm., which compares with an average alveolar tension of 

 42 mm. 



Experiments made by more modern methods have placed the 

 matter beyond doubt. Krogh proved, for example, that in rabbits 

 the carbon dioxide tension in the arterial blood was always slightly 

 (on the average 0-4 per cent, of an atmosphere, or 3 mm. of mercury) 

 higher than in air taken from the bifurcation of the trachea. Now 

 this air is known to contain slightly less carbon dioxide than the 

 alveolar air. Haldane 's results on the regulation of the respiration 

 (p. 281) would be unintelligible unless the carbon dioxide pressure 

 in the arterial blood adjusted itself with great rapidity, to changes 



