THE RESPIRATORY CHANGES IN THE TISSUES. 365 



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 breath- 

 ing, 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 con- 

 tinue 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 ought to be found to 

 be equal to, and not more than equal to, the carbonic acid pressure of the 

 venous blood of the pulmonary artery. And this is the result which has 

 been arrived at : it has been found that the pressures of the carbonic acid 

 of the occluded air and of the venous blood of the right side of the heart 

 are just about equal. Hence the evidence, so far as it goes, is distinctly in 

 favor of the view that the escape of carbonic acid from the blood into the 

 pulmonary alveoli is simply due to diffusion, and that there is no need to 

 seek for any further explanation. 



As far then as can be seen at present, both the entrance of oxygen and 

 the exit of carbonic acid by which venous blood is converted into arterial 

 are the simple physical results of the exposure of the blood in the pulmon- 

 ary capillary to the air of the pulmonary alveoli. 



THE RESPIRATORY CHANGES IN THE TISSUES. 



301. In passing through the several tissues the arterial blood becomes 

 once more venous. The oxy-hsemoglobin becomes considerably reduced, and 

 a quantity of carbonic acid passes from the tissues into the blood. The 

 amount of change varies in the various tissues, and in the same tissue may 

 vary at different times. Thus, in a gland at rest, as we have seen, the venous 

 blood is dark, showing that the haemoglobin is to a large extent in the reduced 

 condition ; when the gland is active, the venous blood in its color, and in 

 the extent to which the haemoglobin is in the condition of oxy-hsemoglobin, 

 resembles closely arterial blood. The blood therefore which issues from a 

 gland at rest is more " venous " than that from an active gland ; though 

 owing to the more rapid flow of blood which, as we saw in an earlier sec- 

 tion, accompanies the activity of the gland, the total quantity of oxygen 

 taken up from and of carbonic acid discharged into the blood from the 

 gland in a given time may be greater than the latter. The blood, on the 

 other hand, which comes from an active, i. e., a contracting muscle, is, in 

 spite of the more rapid flow, not only richer in carbonic acid, but also, 

 though not to a corresponding amount, poorer in oxygen than the blood 

 which flows from a muscle at rest. 



