THE CHEMISTRY OF RESPIRATION. 409 



exposed to the air would take up one fifth only of that amount 

 at ordinary temperatures, and still less at the temperature 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 conse- 

 quently 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 estimated that the oxygen only exists in such quantity 

 that its partial pressure is equal to 130 millimeters of mer- 

 cury, 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 mountaineers can breath 

 freely and get all the oxygen they want; the distress felt for 

 a time by persons unused to living in high altitudes is due 

 mainly to circulatory disturbances resulting from the low 

 atmospheric pressure. So long as the partial pressure of that 

 gas in the lung air-cells is above 25 millimeters of mercury, 

 the amount of it taken up by the blood depends 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 

 one half or a fourth its normal bulk, does not increase the 

 quantity of oxygen absorbed by the blood, apart from the small 

 extra quantity dissolved by the plasma. The widespread state- 

 ments as to the exhilaration caused by breathing pure oxygen 

 are erroneous, being founded on experiments made with im- 

 pure gas. 



The General Oxygen Interchanges in the Blood. Sup- 

 pose we have a quantity of arterial blood in the aorta. This, 

 fresh from the lungs, will have its haemoglobin almost fully 

 combined with oxygen and in the state of oxyhsemoglobin. 

 In the blood plasma some more oxygen will be dissolved, viz., 

 so much as answers to a pressure of that gas equal to 130 

 mm. (5.2 inches) of mercury, which is the partial pressure of 



