384 TEE HUMAN BODY. 



amount at ordinary temperatures, and still less at the tem- 

 perature of the Body, were it not for its haemoglobin. In 

 the lungs even less would be taken up, since the air in the 

 air-cells of those organs is poorer in oxygen than the 

 external air; and consequently the partial pressure of 

 that gas in it is lower. The tidal air taken in at each 

 breath serves merely to renew directly the air in the big 

 bronchi ; the deeper we examine the pulmonary air the 

 less oxygen and more carbon dioxide would be found; 

 in the layers farthest from the exterior and only renewed 

 by diffusion with the air of the large bronchi, it is esti- 

 mated that the oxygen only exists in such quantity that 

 its partial pressure is equal to 130 millimeters of mercury, 

 instead of 152 as in ordinary air. In the second place, on 

 account of the way in which haemoglobin combines with 

 oxygen, the quantity of that gas taken up by the blood is 

 independent of such variations of its partial pressure in the 

 atmosphere as we are subjected to in daily life. At the top 

 of a high mountain, for example, the atmospheric pressure 

 is greatly diminished, but still we can breathe freely and get 

 all the oxygen we want. So long as the partial pressure of 

 that gas remains above 25 millimeters of mercury, the 

 amount of it taken up by the blood will mainly depend on 

 how much haemoglobin there is in that liquid and not on 

 how much oxygen there is in the air. So, too, breathing 

 pure oxygen under a pressure of one atmosphere, or air 

 compressed to ^ or i its normal bulk, does not increase the 

 quantity of oxygen absorbed by the blood, apart from the 

 small extra quantity dissolved by the plasma. All the 

 widespread statements as to the exhilaration and excite- 

 ment caused by breathing pure oxygen are, as a matter of 

 fact, erroneous, being founded on early experiments made 

 with impure gas, and since corrected by many competent 

 observers. 



The General Oxygen Interchanges in the Blood. We 

 may now try to depict what happens to the blood oxygen 

 in a complete circulation. Suppose we have a quantity of 

 arterial blood in the aorta. This, fresh from the lungs, will 

 have its haemoglobin almost fully combined with oxygen 



