346 EXIT OF CARBONIC ACID. [BOOK n. 



The greater relative absorption at the lower pressures has a 

 beneficial effect in as much as it still permits a considerable quantity 

 of oxygen to be absorbed even when the partial pressure of oxygen 

 in the air in the lungs is largely reduced, as in ascending to great 

 heights. 



The statements made so far refer to ordinary breathing, but 

 the question may be asked, What happens when the renewal of 

 the air in the pulmonary alveoli ceases, as when the trachea 

 is obstructed ? In such a case the oxygen in the alveoli is found 

 to diminish rapidly so that the partial pressure of oxygen in them 

 soon falls below the oxygen tension of ordinary venous blood. But 

 in such a case the blood is no longer ordinary venous blood ; instead 

 of containing a comparatively small amount, it contains a large and 

 gradually increasing amount, of reduced haemoglobin. And as the 

 reduced haemoglobin increases in amount, the oxygen tension of the 

 venous blood decreases ; it thus keeps below that of the air in the 

 lungs. Hence even the last traces of oxygen in the lungs are 

 taken up by the blood, and carried away to the tissues ; so that 

 with the last heart's beat, when the oxygen in the lungs has sunk 

 to a mere fraction, the bands of oxyhaemoglobin may still, it is said, 

 be detected for a moment in the blood of the left side of the heart 

 shewing that oxygen has even still been absorbed. 



The exit of Carbonic Acid. 



It seems natural to suppose that the carbonic acid would 

 escape by diffusion from the blood of the alveolar capillaries 

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

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

 pulmonary alveoli must always be less than that of the venous 

 blood of the pulmonary artery and indeed ought not to exceed 

 that of the blood of the pulmonary vein. There are however 

 many practical difficulties in the way of an exact determi- 

 nation of the carbonic acid tension 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 tension 

 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, 



