EXCRETION OF CARBONIC ACID 109 



himself. Barcroft lived for six days in a chamber in which 

 the oxygen pressure was gradually reduced, the CO2 

 exhaled being absorbed. On the sixth day samples of 

 blood were taken from the radial artery during rest and 

 after a period of work on a bicycle ergometer. The results 

 show that at a reduced oxygen pressure corresponding to an 

 altitude of 18,000 feet, while work is being done, the arterial 

 blood is 834 per cent, saturated with oxygen, but when 

 the same blood was exposed in vitro to a sample of alveolar 

 air its oxygen content rose to 88*6 per cent. — a difierence 

 of 5-2 per cent. This corresponds to a difference of about 

 7*5 mm. between the pressure of oxygen in the alveolar air 

 and the tension of oxygen in the arterial blood. During 

 the exercise 750 c.c. of oxygen were used per minute. 

 In this experiment then the oxygen tension in the arterial 

 blood was lower than in the alveolar air — ^that is to say, the 

 passage of oxygen into the blood even under extreme 

 conditions could be explained by diffusion. 



The Excretion of Carbonic Acid 



The passage of carbonic acid out of the blood presents 

 no problem comparable with the entry of oxygen. The 

 pressure of COg in venous blood is always higher than in 

 the alveolar air, although the difference may sometimes 

 be very shght. But taking into account the rapidity of 

 diffusion of this gas, there is no difficulty in explaining 

 its exit from the blood by diffusion. 



THE INTERCHANGE OF GASES BETWEEN BLOOD 

 AND THE TISSUES 



Since there is no evidence of any storage of oxygen 

 within the cell, we may assume that the passage of oxygen 

 from the blood into the tissues is due to diffusion. In the 

 case of carbonic acid there is a tension of this gas within 

 the cell. We cannot estimate it directly, but we can arrive 

 at some idea of it from the tension of COg in the fluid 



