1124 PHYSIOLOGY 



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 from 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 nim. Hg. 

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

 movements 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 

 determining the oxygen tension of the blood in the lungs founded on the use of carbon 

 monoxide. It has already been mentioned that carbon monoxide has the power of dis- 

 placing oxygen from oxyhaemoglobin to form a much more stable compound, carboxy- 

 haemoglobin. If blood be shaken up with a mixture of oxygen and carbon monoxide, 

 the haemoglobin distributes itself between the two gases. In order however to get an 

 equal distribution, it is necessary to take a very small percentage of carbon monoxide, 

 owing to its greater avidity for haemoglobin. Thus, if haemoglobin solution be shaken 

 up with air containing '07 per cent, of CO, the result is a mixture of equal parts of oxy 

 and carboxyhaemoglobin. The affinity of CO for haemoglobin would thus appear to be 



21 

 about = 300 times the affinity of oxygen for haemoglobin. 



Carbon monoxide is not destroyed in the body, so that if a mixture containing a 

 small proportion of CO be breathed, this gas will be taken up until a certain percentage 

 of haemoglobin is converted into CO -haemoglobin and the tension of CO in tlrc tissues 

 and fluids of the body is equal to that of the inspired air. The amount of haemoglobin 

 which is converted into carboxyhseinoglobin will serve as a measure of the relative 

 tensions of CO and oxygen in the lungs. If the oxygen tension of arterial blood were 

 the same as that of the alveolar air, we should expect that, with a given percentage of 

 CO in the air breathed, the final saturation with CO of the blood within the body would 

 be the same as the saturation of blood when shaken outside the body with air con- 

 taining the same percentage of CO as in the air breathed. It was found by Haldane 

 however that in all cases the percentage of CO haemoglobin formed was much less in the 

 body than outside the body. Thus in blood shaken up with air containing 20*9 per cent, 

 oxygen and *045 per cent. CO, the amount of carbon monoxide ha-mu^lobm formed \\a- 

 31 per cent, of the whole haemoglobin. When the same mixture was inhaled for three 

 or four hours by a man, the percentage of CO haemoglobin in his blood rose only to 



