EXCHANGE OF GASES IN THE AIR-VESICLES. I9I 



129. EXCHANGE OF GASES IN THE AIR-VESICLES. -The exchange of 

 gases between the gases of the blood and those in the air-vesicles occurs almost 

 exclusively through the agency of chemical processes, and therefore independently 

 of the diffusion of gases. 



Method. It is important to ascertain the tension of the and C0. 2 in the venous blood of 

 the pulmonary capillaries. Pfluger and Wolf berg estimated the tension by ' ' catheterising the 

 lungs." An elastic catheter was introduced through an opening in the trachea of a dog into 

 the bronchus leading to the lowest lobe of the left lung. An elastic sac was placed round the 

 catheter, and when the latter was introduced into the bronchus, the sac around the catheter was 

 distended so as to plug the bronchus. No air could escape between the catheter and the wall of 

 the bronchus. The outer end of the catheter was closed at first, and the dog was allowed to 

 respire quietly. After four minutes the air in the air-vesicles was completely in equilibrium 

 with the blood-gases. The air of the lung was sucked out of the catheter by means of an air- 

 pump, and afterwards analysed. 



Thus we may measure indirectly the tension of the O and C0 2 in the venous 

 blood of the pulmonary capillaries. The direct estimation of the gases in different 

 kinds of blood is made by shaking up the blood with another gas. The gases so 

 removed indicate directly the proportion of blood-gases. 



The following statement shows the tension and percentage of O and C0 2 in 

 arterial and venous blood, in the atmosphere, and in the air of the alveoli : 



V. 



O-Tension in the air of the alveoli of the 

 catheterised lung = 27'44 mm. Hg (cor- 

 responding to 3 "6 vol. per cent. ). 

 VI. 

 C0 2 -Tension in the air of the alveoli of the 

 catheterised lung =-27 mm. Hg (correspond- 

 ing to 3*56 vol. per cent.). 

 VII. 



I. 



O-Tension in arterial blood = 29 '6 mm. Hg 

 (corresponding to a mixture containing 3 *9 

 vol. per cent, of 0). 



O-Tension in the atmosphere = 158 mm. Hg 

 (corresponding to 20 - 8 vol. per cent.). 

 VIII. 



C0. 2 -Tension in the atmosphere = "38 mm. 

 Hg (corresponding to 0*03-0 '05 vol. per 

 cent.). 



II. 



C0 2 -Tension in arterial blood = 21 mm. Hg 

 (corresponding to 2 '8 vol. per cent. ). 



III. 

 O-Tension in venous blood = 22 mm. Hg 

 (corresponding to 2*9 vol. per eent.). 



IV. 



C0 2 -Tension in venous blood=41 mm. Hg 

 (corresponding to 5 "4 vol. per cent. ). 



When we compare the tension of the O in the air (VII. = 158 mm. Hg) with 

 the tension of the O in venous blood (III. = 22 mm. Hg, or V. = 27-44 mm. Hg), 

 we might be inclined to assume that the passage of the O from the air of the air- 

 vesicles into the blood was due solely to diffusion of the gases ; and similarly, we 

 might assume that the C0 2 of the venous blood (IV. or VI.) diffused into the air- 

 vesicles, because the tension of the C0 2 in the air is much less (VIII.). There are 

 a number of facts, however, which prove that the exchange of the gases in the lungs 

 is chiefly due to chemical forces. 



[V. Fleischl finds that fluids yield up their gases very much more easily when they receive a 

 shock, and he regards the shock communicated to the blood, by the contraction of the heart, as 

 an important factor in preparing the blood for the diffusion of C0 2 from the blood-plasma 

 into the lungs. ] 



[Changes produced in the Blood by Respiration. The blood of the pul- 

 monary artery is changed from venous into arterial blood ( 39), the most obvious 

 alterations being (1) the change in colour from dark crimson to bright scarlet. (2) 

 It loses C0 2 . (3) It gains O. (4) The reduced Hb of the venous blood is con- 

 verted into Hb0 2 . (5) As to a supposed difference of temperature, see 209, 3. 

 (6) Pawlow finds that blood which passes several times through the lungs loses its 

 power of coagulation. Are we to assume that the pulmonary tissues have the 

 property of destroying the fibrin-ferment f] 



1. Absorption of 0. Concerning the absorption of O from the air in the alveoli 

 into the venous blood of the lung-capillaries, whereby the blood is arterialised, it is 



