TRANSPORT OF OXYGEN 



19 



proportional to the length of the tube. The diffusion coeffi- 

 cient for O2 in air is the amount (expressed in ml) diffusing 

 per minute through 1 cm 2 area and 1 cm length at a pressure 

 difference of 1 atm (= 760 mm Hg pressure). Correspond- 

 ing determinations can be made on membranes as illustrated 

 in Fig. 2 (right) and a few results are here reproduced 

 (Krogh, 1919). 



Table 6 

 Diffusion coefficients at 20° 



The rate of diffusion in connective tissue is just one millionth 

 of the rate in air, and generally the rates in watery fluids or 

 animal membranes are 300,000 to several million times slower 

 than in air. 



It has been calculated for the oxygen supply to muscles 

 (Krogh, 1919) that thanks to the presence of an enormous 

 number of blood capillaries evenly distributed along the 

 muscle fibres the supply of oxygen by diffusion will be ample 

 even during the heaviest 'work — provided the blood is nor- 

 mally supplied with oxygen in the lungs. 



No actual determinations of diffusion rates for oxygen from 

 the alveolar air into the blood are available, and both the 

 membrane thickness and its area are largely unknown. 



The actual rate at which carbon monoxide is taken up in the 

 lungs of man, when supplied for a short time in low concen- 

 tration (so as to be harmless), has been measured (Marie 

 Krogh, 1915, Boje, 1933), and making certain probable 

 assumptions it can be calculated that diffusion will also pro- 

 vide the actual absorptions of oxygen observed during heavy 

 work and at high altitudes. 



