THE CHEMISTRY OF RESPIRATION 1073 



resting man. In the same way it is easy to account for the passage of 

 carbon dioxide in the reverse direction. This gas diffuses through a wet 

 membrane about twenty-five times as rapidly as oxygen, so that a differ- 

 ence of pressure between the blood and the alveolar air amounting to only 

 03 mm. Hg. would suffice to cause a passage outwards of the 250 c.c. 

 normally expired per minute. 



It is evident that the only limitation for the absorption of oxygen is given 

 by the power of the haemoglobin to combine with the oxygen which passes 

 through the alveolar wall into the blood-plasma. 



If we look at the dissociation curve of the oxyhsemoglobin in mammalian 

 blood given on p. 1061 we see that the amount of oxygen which can be taken 

 up by haemoglobin in the presence of the normal tension of carbon dioxide, 

 i.e. 40 mm. Hg., begins to diminish very rapidly when the pressure of the 

 oxygen falls below 50 mm. Hg. Thus at 40 mm. oxygen pressure and a 

 carbon dioxide tension of 40 mm., oxyhsemoglobin is about 65 per cent, 

 saturated, and at 30 mm. it is only 50 per cent, saturated. Under normal 

 circumstances the blood leaves the lungs over 90 per cent, saturated with 

 oxygen. If the saturation falls to 60 per cent, we should expect to obtain 

 evidence of failure of oxygen supply to the tissues. According to Loewy 

 the oxygen tension in the alveoli can sink to between 30 and 35 mm. Hg. 

 before any signs of oxygen lack make their appearance. These results were 

 obtained by exposing a man in a state of complete rest to reduced pressure 

 in an air-chamber. Under these conditions the slightest muscular exertion 

 would at once tend to cause distress fr.om deficient oxygen supply. The 

 exact percentage of oxygen in the inspired air which would give an alveolar 

 oxygen tension of 30 to 35 mm. varies with the depth of respiration. Thus 

 with shallow respiratory movements the pressure may sink to 35 mm. Hg, 

 when the inspired air contains as much as 12 per cent, oxygen. If the move- 

 ments be deeper the oxygen content of inspired air may be reduced to 9 or 

 10 per cent, before respiratory distress is observed. 



The view that in the interchange of gases in the lungs the membrane between the 

 blood and the alveolar air plays simply a passive part was till recently by no means 

 universally accepted. In Bohr's experiments on the tension of oxygen and carbon 

 dioxide in the blood as determined with his aerotonometer, oxygen tensions were often 

 found considerably higher in the blood than in the air of the alveoli, and in the same way 

 the carbon dioxide tension of the blood leaving the lungs was found to be less than the 

 carbon dioxide tension of the alveolar air. Krogh's experiments show conclusively, 

 however, that these results are not reliable, and that the difference between the tensions 

 in the alveoli and in the blood respectively is always such as to allow of the passage by 

 diffusion of oxygen inwards and carbon dioxide outwards from the blood. Moreover, 

 as Krogh points out, the structure of the pulmonary epithelium lends no support to the 

 view that it acts as a secreting membrane. In mammals the cells are of two kinds, viz. 

 small granular nucleated cells lying in the interstices of the capillaries, and larger, 

 extremely thin structureless plates, without nuclei, covering the capillaries. In birds, 

 where the gaseous exchange is of all animals the most rapid and efficient, the existence 

 of a lung epithelium has never been demonstrated, and the capillaries appear to be 

 almost completely free and to be surrounded with air on both sides. 



Bohr's view as to the secretory function of the pulmonary epithelium was supported 

 as concerns the intake of oxygen, by Haldane. This observer has devised a method of 



