624 



PHYSIOLOGY OF RESPIRATION. 



pressure in the alveoli of the lungs and circulates in the blood in 

 small amounts without exerting any immediate influence upon the 

 tissues. 



Condition of Oxygen in the Blood. That the oxygen is not 

 held in the blood merely in solution is indicated, in the first place, 

 by the large quantity present and, in the second place, by the fact 

 that this quantity does not vary directly with the pressure in the 

 surrounding medium. It is definitely known that by far the largest 

 portion of the oxygen is held in chemical combination with the 

 hemoglobin of the red corpuscles, while a much smaller portion, 

 varying with the pressure, is held in solution in the plasma. The 

 compound oxyhemoglobin possesses the important property that 

 when the pressure of oxygen in the surrounding medium falls suffi- 

 ciently it begins to dissociate and free oxygen is given off. The proc- 



/// /e/. 



7% 



z% /f. 



Fig. 254. Curve, dotted line, to show the dissociation of oxyhemoglobin in blood 

 under different pressures of oxygen. (After Loewy.) The ordinates give the percentages 

 of saturation of the hemoglobin with oxygen, assuming complete saturation (100 per cent.) 

 v/hen blood is exposed to atmospheric air. The corresponding pressures of oxygen are 

 represented along the abscissa, at the top in mms. of mercury, at the bottom in percentages 

 of an atmosphere. 



ess of dissociation is facilitated also by increase of temperature, 

 provided, of course, that it does not rise to the point of coagulating 

 the hemoglobin. The amount of dissociation that takes place under 

 different pressures of oxygen in the surrounding medium has been 

 studied both for solutions of pure hemoglobin * and for defibrinated 

 blood. f It would seem from recent work that the compound 

 between oxygen and hemoglobin is more easily dissociated when the 

 hemoglobin is in its natural condition in the corpuscles than when it 

 has been crystallized out and obtained in pure solutions. The 

 results that have been obtained from experiments upon defibrinated 

 blood probably represent, therefore, more nearly the conditions 



* Hiifner, "Archiv f. Physiologie, " suppl. volume, 1901, p. 

 tLoewy, "Archiv f. Physiologie," 1904, p. 245. 



213. 



