KESPIEATION. 597 



as a rule, amount to about 3 per cent. This difference in pressure is, 

 therefore, sufficient to explain the entrance of oxygen from the air in 

 the pulmonary alveoli to the blood in the pulmonary capillaries. The 

 laws of absorption of oxygen by the blood-corpuscles are, while partially 

 dependent upon the differences in oxygen tension in the blood and in the 

 capillaries, not solely dependent upon them ; otherwise an increase in the 

 oxygen tension in the atmosphere would cause a proportionate increase 

 in the amount of oxygen absorbed, a decrease in the oxygen tension a 

 diminution of the amount of this gas which enters in the blood. This is 

 not, however, the fact, for it has "been determined, as already stated, that 

 hut a limited quantity of oxygen is capable of combining with haemo- 

 globin. If the oxygen of the atmosphere be decreased the tension of 

 this gas in the lungs will also be decreased, and, therefore, to a certain 

 extent the blood will be insufficiently^ aerated ; this will perhaps explain 

 the dyspnoea which is so often noted in ascending to great heights, as to 

 the tops of mountains, and in balloons. 



As regards the escape of carbon dioxide from the blood in the capil- 

 laries of the pulmonary artery, we are perhaps warranted in assuming 

 that they are governed purely by the laws of gaseous tensions ; since 

 we may assume that even although we do not know exactly what the 

 tension of carbon dioxide in the pulmonary alveoli is, and although it 

 must be greater than that in the expired air, it is, nevertheless, less than 

 that of the venous blood. 



The tension of carbon dioxide in the air of the alveoli has been 

 placed at twenty-seven millimeters of mercury. 



The tension of carbon dioxide in the blood in tlie right side of the 

 heart has been placed at forty-one millimeters of mercury, or, in other 

 words, fourteen millimeters higher than the tension in the alveoli. This 

 difference will, in all probability, be sufficient to inaugurate phenomena 

 of diffusion. 



Thus, in passing through the pulmonary capillaries the venous blood 

 yields up carbon dioxide and absorbs oxygen, and becomes converted 

 from a dark-red venous blood into the bright red arterial blood. 



The characteristics of arterial blood are preserved throughout the 

 entire arterial system until the capillaries are reached, and when the 

 capillaries have been traversed we find that again the arterial blood loses 

 its characteristic properties and now becomes venous. In other words, in 

 the capillaries a part of the oxygen disappears and is replaced by carbon 

 dioxide. In the tissues, therefore, a process takes place which is exactly 

 the reverse of that which has been described as occurring in the lungs. 

 It may be assumed that the oxygen leaves the blood in the same manner 

 as it enters in the pulmonary capillaries ; in other words, in the tissues it 

 becomes exposed to a surrounding medium whose gaseous tension in 



