BY H. S. HALCRO WARDLAW. 799 



may be taken as exhibiting the exchange of carbon dioxide and 

 of oxygen between the blood and pulmonary tissues, and the 

 alveolar air under the conditions of the experiments. 



If diffusion play a part in this exchange of gases between the 

 alveolar air and the blood, the variations in the rates of exchange 

 are likely to be expressed by an equation of the form 



d(P-p);dt= -n(P-p) (1) 



where P is the effective, not necessarily the actual, tension of the 

 gas in the venous blood entering the lungs, p the tension in the 

 alveolar air at the moment, and n a constant. The work of Mosso 

 (1904), of Haldaneand his collaborators (/oc.cti!., and Christiansen 

 and Haldane, 1914),of Krogh and Krogh( 1910), and of others, has 

 shown that the tensions of the carbon dioxide in the arterial 

 blood leaviny the lungs must be very close to the alveolar tension 



With regard to the tension of oxygen in tlie arterial blood, 

 opinion is not so unanimous. Barcroft and Cooke (1913) found 

 arterial blood (human) to be 94% saturated with oxygen. Twort 

 and Hill (1915) showed, however, that, during rest and shallow 

 respiration, the degree of saturation may be considerably lower. 



According to the above equation, if the tensions of the gases 

 in the venous blood entering the lungs, after the stoppage of 

 the exchange with the air occurring in normal respiration, 

 remain constant for a period long enough, the alveolar tensions 

 will approach very closely to the venous, and the blood will pass 

 through the lungs practically unchanged. 



Equation (1) is converted by integration into the form 



log(P-p) = loga-nt (2) 



where a is another constant 



If the figures for p given in the above tables vary with the 

 times of stoppage of normal respiration in the manner described 

 by this equation, then, if instead of plotting the tensions against 

 times, the logarithms of the differences of these tensions from 

 certain constant tensions, P, be plotted, the curves obtained 

 will be straight lines. The values of the constant tensions, P, 

 towards which the tensions, p, approach, may be calculated by 

 converting equation (2) into the form 



P-p = a/10"* (3) 



