364 RESPIRATION. 



this accords with the observation that in man, when the oxygen of inspired 

 air is gradually diminished without any other change in the air, symptoms 

 of dyspnoaa do not make their appearance until the oxygen sinks to 10 per 

 cent, in the inspired air, and must therefore be less than this in the pul- 

 monary alveoli. We may remark that at ordinary altitudes, even taking 

 into account the diminution the oxygen undergoes before it reaches the pul- 

 monary alveoli, the partial pressure of the oxygen in the atmosphere leaves 

 a wide margin of safety. But at an altitude of 5500 metres (17,000 feet), 

 at which the pressure of the whole atmosphere 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 con- 

 vert 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. 



299. 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. Whether or not the pul- 

 monary tissue and the capillary walls, because of their being living structures, 

 have any influence upon the transmission of oxygen must still be considered 

 an unsettled question. 



The Exit of Carbonic Add. 



300. 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 prac- 

 tical 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 car- 

 bonic 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 per- 

 centage of carbonic acid in the expired air reaches 7 or 8 per cent., 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 



