380 THE HUMAN BODY 



most of its oxygen. The red corpuscles are so many little packages 

 in which oxygen is stowed away. 



The compound formed between oxygen and hemoglobin is, how- 

 ever, a very feeble one; the two easily separate, and always do 

 so completely when the oxygen pressure in the liquid or gas to 

 which the oxyhemoglobin is exposed falls below 25 mm. of mer- 

 cury. There is some slight dissociation at pressures of 70 mm. 

 of mercury. Hence, in an air-pump, the blood only gives off a 

 little of its oxygen, until the pressure falls to about of an at- 

 mosphere, that is to 1 | JL =125 mm. (5 inches) of mercury, of 

 which total pressure one-fifth (25 mm. or 1 inch) is due to the 

 oxygen present. As soon as this limit is- passed the hemoglobin 

 gives up its remaining oxygen with a rush. 



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 't 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 w r ould 

 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, instead of 152 as 

 in ordinary air. In the second place, on account of the way in 

 which hemoglobin 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 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 cir- 

 culatory disturbances resulting from the low atmospheric pressure 

 and in part to another condition to be described presently, but not 



