606 THE EXIT OF CARBONIC ACID. [BOOK n. 



remark that at ordinary altitudes, even taking into account the 

 diminution the oxygen undergoes before it reaches the pulmonary 

 alveoli, the partial pressure of the oxygen in the atmosphere 

 leaves a wide margin of safety. But at an altitude of 5500 

 metres (17000 feet) at which the pressure of the whole atmo- 

 sphere stands at about the limit given above of 300 mm., the 

 partial pressure of the oxygen will be such that the venous 

 blood cannot take up the quantity of oxygen proper to convert 

 it into arterial blood, since at this limit arterial blood begins 

 to give off oxygen. We may add that it is at this altitude 

 that breathing becomes especially difficult, but to this we shall 

 return. 



356. 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-pressure of ordinary venous blood. 

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

 instead of being moderately, it is largely and increasingly reduced; 

 instead of containing a comparatively small amount, it contains a 

 large and gradually increasing amount, of reduced haemoglobin. 

 And as the reduction continues to increase, the oxygen -pressure of 

 the venous blood also continues to decrease ; it thus keeps below 

 that of the air in the lungs. Hence apparently even the last 

 traces of oxygen in the lungs may be taken up by the blood, and 

 carried away to the tissues. 



The Exit of Carbonic Acid. 



357. 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 pressure of the air in the 

 pulmonary alveoli must always be less than that of the venous 

 blood 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., 



