576 THE EXIT OF CARBONIC ACID. [BOOK n. 



into the air of the alveoli. But in order that diffusion should 

 thus take place, the carbonic acid pressure of the air in the 

 pulmonary alveoli must always be less than that of the venous 

 mood of the pulmonary artery, and ought not to exceed that of the 

 blood of the pulmonary vein. There are however many practical 

 difficulties in the way of an exact determination of the carbonic 

 acid pressure of the pulmonary alveoli (for though it must be 

 greater than that of the expired air, it is difficult to say how much 

 greater), and of the carbonic acid pressure of the blood at the same 

 time, so as to be in a position to compare the one with the other. 

 In the case of oxygen, there is always present in the lungs a 

 surplus of the gas, a portion only being absorbed at each breath ; 

 in the case of carbonic acid, the whole quantity comes direct from 

 the blood, and any modifications in breathing seriously affect the 

 amount given out. Thus when the breath is held for some time 

 the percentage of carbonic acid in the expired air reaches 7 or 8 

 p.c., but we cannot take this as a measure of the normal percentage 

 of carbonic acid in the pulmonary alveoli, since by the mere 

 holding of the breath the carbonic acid in the blood and hence in 

 the pulmonary alveoli is increased beyond the normal. 



The difficulties of the problem seem however to have been 

 overcome by an ingenious experiment in which there is introduced 

 into the bronchus of the lung of a dog a catheter, round which is 

 arranged a small bag ; by the inflation of this bag the bronchus, 

 whenever desired, can be completely blocked up. Thus, without 

 any marked disturbance of the general breathing, and therefore 

 without any marked change in the normal proportions of the 

 gases of the blood, the experimenter is able to stop the ingress 

 of fresh air into a limited portion of the lung. At the same 

 time he is enabled by means of the catheter to withdraw a 

 sample of the air of the same limited portion, and by analysis 

 to determine the amount of carbonic acid which it contains, 

 or in other words, the partial pressure of the carbonic acid. 

 The blood passing through the alveolar capillaries of this limited 

 portion of the lung naturally possesses the same carbonic acid 

 pressure as the rest of the venous blood flowing through the 

 pulmonary artery, a pressure which, though varying slightly 

 from moment to moment, will maintain a normal average. On 

 the supposition that carbonic acid passes simply by diffusion 

 from the pulmonary blood into the air of the alveoli, because the 

 carbonic acid pressure of the latter is normally lower than that of 

 the former, one would expect to find that the air in the occluded 

 portion of the lung would continue to take up carbonic acid 

 until an equilibrium was established between it and the carbonic 

 acid pressure of the venous blood. Consequently, if after an 

 occlusion, say of some minutes (by which time the equilibrium 

 might fairly be assumed to have been established), the carbonic 

 acid pressure of the air of the occluded portion were determined, it 



