238 INTERCHANGE OF GASES. 



If the inspired air contains a diminished quantity of oxygen, the necessary 

 amount of oxygen can still be supplied to a certain extent by more rapid and 

 deeper respirations. 



INTERCHANGE OF GASES BETWEEN THE BLOOD IN THE PUL- 

 MONARY CAPILLARIES AND THE AIR IN THE ALVEOLI. 



This interchange of gases is accomplished almost exclusively by 

 chemical processes, independently of the diffusion of gases. 



For the determination of the gaseous interchange it is first necessary to ascer- 

 tain the tension of the oxygen and the carbon dioxid in the venous blood of the 

 pulmonary capillaries. Pfluger and Wolffberg have accomplished this by cathe- 

 terization of the lungs. An opening is made in the trachea of a dog, and an 

 elastic catheter (Fig. 90, a) is introduced into the bronchus leading to the lower 

 lobe of the left lung. In order to have the bronchus fit closely around the catheter, 

 the latter is made to pierce a rubber sac inflated by means of a communicating 

 rubber-ball pump c. In this way no air from that part of the lung can escape at 

 the side of the catheter. The tube is at first closed at its outlet, and the dog is 

 allowed to breathe independently and as quietly as possible. After four minutes 

 the alveolar air in the closed-off part of the lungs is in complete equilibrium with the 

 blood- gases. By means of a mercurial air-pump the air in the lungs is sucked 

 out of the catheter (at 6) and analyzed. The tension of the carbon dioxid and 

 the oxygen in this air will indicate in an indirect way the tension of these two 

 gases in the venous blood of the pulmonary capillaries. 



For the direct estimation of the gases in various specimens of blood, these 

 gases are removed by shaking the blood with another kind of gas. The composi- 

 tion of the mixture will indicate the proportions in which the blood-gases have 

 been mixed, and will thus serve to determine their tension. It is desirable to use 

 as much blood as possible with a small quantity of gas; the amount of the latter 

 should be about the same as that supposed to be present in the blood. 



In the following table are shown the tension and the percentage of 

 oxygen and carbon dioxid in arterial and venous blood, as well as in the 

 atmosphere and the air of the closed-off alveoli : 



I. V. 



Tension of O in arterial blood = Tension of O in the alveolar air of 



29.6 mm. of mercury; increased by the catheterized lung = 27.44 mm. of 



warming; corresponding to a gaseous mercury; corresponding to 3.6 vol. per 



mixture containing 3.9 per cent, of O. cent. 



II. VI. 



Tension of CO 2 in arterial blood = Tension of CO 2 in the alveolar air 



21 mm. of mercury; corresponding to of the catheterized lung = 27 mm. of 

 2.8 vol. per cent. mercury; corresponding to 3.56 vol. 



per cent. 

 III. 



Tension of O in venous blood = 22 VII. 



mm. of mercury; corresponding to 2.9 Tension of O in the atmosphere = 



vol. per cent. 158 mm. of mercury; corresponding to 



20. 8 vol. per cent. 

 IV. 



Tension of CO 2 in venous blood = VIII. 



41 mm. of mercury; corresponding to Tension of CO 2 in the atmosphere = 



5.4 vol. per cent. 0.38 mm. of mercury; corresponding to 



from 0.03 to 0.05 vol. per cent. 



If the tension of the oxygen in the atmosphere (VII) be compared 

 with that in venous blood (III) or in the alveoli (V) it will be seen that the 

 absorption of oxygen into the blood during respiration can occur in the 

 form of an equalization of tension. Likewise a comparison of the 

 tension of the carbon dioxid in the atmosphere (VIII) with that in 

 venous blood (IV) or with that in alveolar air (VI) might explain the 



