260 EXCHANGES OF GASES BETWEEN THE AIR AND THE BLOOD. 



rise to a diffusion of the gases within the air-passages ; the C0 2 must 

 diffuse from the air-vesicles outwards, and the from the atmosphere 

 and nostrils inwards (compare p. 52). This movement is aided by 

 the cardio-pneumatic movement (Landois, p. 109). In hybernating 

 animals and in persons apparently but not actually dead, the exchange of 

 gases within the lungs can only occur in the above-mentioned ways. 



For ordinary purposes this mechanism is insufficient, and there are 

 added the respiratory movements whereby atmospheric air is introduced 

 into the larger air-passages, from which and into which the diffusion 

 currents of and C0 pass, on account of the difference of tension of 

 the gases. 



129. Exchange of Gases between the Blood of the 



Pulmonary Capillaries and the Air in 



the Air- Vesicles. 



This exchange of gases occurs almost exclusively through the agency 

 of chemical processes (independent 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. Pfliiger and Wolfberg 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 can 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 and C0 2 in 

 the venous blood of the pulmonary capillaries. The direct estima- 

 tion 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 tabular arrangement indicates the tension and per- 

 centage of O and C0 2 in arterial and venous blood, in the atmosphere, 

 and in the air of the alveoli : 



0-Tension in arterial blood =29 '6 mm. 

 Hg. (corresponding to a mixture con- 

 taining 3 - 9 vol. per cent, of 0). 



II. 



C0 2 -Tension in arterial blood = 21 mm. 

 Hg. (corresponding to 2 '8 vol. per 

 cent. ) 



III. 



0-Tension in venous blood =22 mm. Hg, 

 (corresponding to 2 '9 vol. per cent.) 



IV. 



COa-Tension in venous blood =41 mm. 

 Hg. (corresponding to 5 '4 vol. per 

 cent.) 



