RESPIRATION. THE GASEOUS INTERCHANGES 421 



Consequences of the Peculiar Way in Which the Oxygen of the 

 Blood is Held. The first, and most important, is that the blood 

 can take up far more oxygen in the lungs than would otherwise be 

 possible. Blood-serum exposed to the air would take up only one- 

 half volume of oxygen per hundred of liquid at ordinary tempera- 

 tures, and still less at the temperature of the Body, were it not for 

 its hemoglobin. 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 dioxid will 

 be found; in the layers farthest from the exterior and only re- 

 newed by diffusion with the air of the large bronchi, it is estimated 

 that the oxygen only exists in such quantity that its partial pres- 

 sure is equal to about 100 mm. of mercury (j atmos.) instead of 

 152 (J atmos.) as in ordinary air. In the second place, on account 

 of the way in which hemoglobin combines with oxygen, the quan- 

 tity of that gas taken up by the blood is independent of such varia- 

 tions 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 breathe freely and get all the oxygen they want; the distress 

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

 in part to circulatory disturbances resulting from the low atmos- 

 pheric pressure and in part to another condition to be described 

 presently, but not at all to deficiency of oxygen. So long as the 

 partial pressure of that gas in the lung air-cells is well above 25 

 mm. of mercury, the amount of it taken up by the blood depends 

 on how much hemoglobin 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 quan- 

 tity dissolved by the plasma. 



The General Oxygen Interchanges in the Blood. Suppose we 

 have a quantity of arterial blood in the aorta. This, fresh from the 

 lungs, will have its hemoglobin practically saturated with oxygen 

 and in the state of oxyhemoglobin. In the blood-plasma some 



