THE GASES OF THE BLOOD 263 



during rest and when normal air is breathed the oxygen tension in 

 arterial blood never exceeds the tension in the alveolar air. He still 

 believes, and brings forward evidence for his belief, that even the best 

 aerotonometric methods yield with arterial blood oxygen pressures 

 lower than the true oxygen pressure in the blood leaving trie pulmonary 

 capillaries. But he no longer doubts that a sufficient slope of pressure 

 exists between the alveoli and the blood to explain the absorption of 

 oxygen under ordinary conditions. 



To all intents and purposes, then, we may look upon the contro- 

 versy as closed, and with the happy result, too rare in physiological 

 disputes, that a practically unanimous conclusion has been arrived 

 at. While the oxygen tension in arterial blood may fall only slightly 

 below that in alveolar air, the difference perhaps being even less than 

 that found by Krogh, the slope of pressure is always, under ordinary 

 conditions at least, from the alveoli to the blood. 



Mechanism of the Gaseous Exchange in the Lungs. Granting 

 that this is so, it must still be asked whether the diffusion of oxygen 

 from the lungs into the blood can take place rapidly enough to 

 account for the quantities actually absorbed. 



Calculations made on the basis of such anatomical and physical data 

 as are available (total surface of the lungs, thickness of the membrane 

 which separates the air of the alveoli and the blood in the capillaries, 

 rate of ' invasion,' or entrance of the gases into water; velocity of 

 diffusion of oxygen and carbon dioxide), indicate that even with differ- 

 ences of oxygen tension between the blood and the alveolar air, which 

 would lie within the limits of error of our present methods of measure- 

 ment, enough oxygen could diffuse across the pulmonary membrane to 

 cover the whole normal intake. 



For example, it has been shown from determinations of the ' invasion- 

 coefficient ' of oxygen that 300 c.c. of oxygen, the amount ordinarily 

 absorbed in a minute, in an average man, can be carried from the alveo- 

 lar air into the wet surface of the pulmonary epithelium with a difference 

 of oxygen pressure of a little over 3 mm. of mercury. If during mus- 

 cular exercise six or seven times as much oxygen were absorbed, the 

 necessary difference of oxygen pressure would still be only about 20 mm. 

 of mercury, less than 3 per cent, of an atmosphere. 



It is one thing, however, to know that the necessary oxygen can be 

 taken up into the surface of the pulmonary membrane by a purely 

 physical process, but quite another to prove that it can also be trans- 

 ported with sufficient speed across the thickness of the alveolar wall 

 into the blood. A direct test of this question, made by Krogh, has 

 also yielded an affirmative answer. He determined the amount of 

 carbon monoxide actually taken up in a given time in a human subject. 

 It was about 20 c c. per mm. of partial pressure in. the alveolar air per 

 minute when the subject was at rest, and a little above 30 c.c. when 

 breathing was forced, as it would be during heavy work. Now the 

 speed of diffusion of oxygen is somewhat greater than that of carbon 

 monoxide, so that at rest the subject could have absorbed between 

 200 and 300 c.c. of oxygen per minute with a difference _of oxygen 

 tension of only 10 mm. of mercury between the alveolar air and the 

 arterial blood. Even during hard muscular work the observed differ- 

 ences of oxygen tension seem adequate to the transport of the necessary 

 amount of oxygen. 



