THE CHEMISTRY OF RESPIRATION 1197 



dioxide, *.e.40mm.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., oxyhaemoglobin 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, satu- 

 rated 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 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 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 demon- 

 strated,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 displacing oxygen from oxyhaemo- 

 globin to form a much more stable compound, carboxyhsemoglobin. 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, 



