52 ORGANISM AND ENVIRONMENT 



results of Miss Fitz Gerald's observations on the 

 haemoglobin percentages in persons permanently living 

 at different altitudes. These observations were all 

 made by the colorimetric method of determination 

 which I introduced a few years ago, and with a care- 

 fully standardised instrument. It will be seen that 

 just as the alveolar CO 2 rises with fall in the baro- 

 metric pressure, so the haemoglobin percentage rises. 

 It appears also that in an atmosphere with a higher 

 oxygen pressure than air at sea level a decrease in the 

 haemoglobin percentage below what is termed "nor- 

 mal" would occur. Here also, then, the setting of the 

 regulation of haemoglobin percentage is altered by 

 change in environment. 



Using the carbon monoxide method of Lorrain 

 Smith and myself, we found that on going to a high 

 altitude not only the percentage amount, but also the 

 total amount of haemoglobin in the blood is increased. 

 The* total volume of the blood seems to diminish 

 at first, thus raising the concentration of haemo- 

 globin; but after a few days the volume of the blood 

 increases above normal. The regulation of total 

 haemoglobin, concentration of haemoglobin, and blood 

 volume are thus all dependent on the oxygen pressure 

 of the air breathed. 



I now come to what was the most striking result of 

 the expedition. In the lungs the blood is separated 

 from the alveolar air by an extremely thin membrane 

 consisting of the "protoplasm" of flattened epithelial 

 cells. Do these cells play any active part in the gaseous 

 exchange between the air and the blood? Or does 



