CHANGES IN AIR AND BLOOD IN RESPIRATION. 677 



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- 

 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 this 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 re- 

 sults that have been obtained from experiments upon defibrinated 

 blood probably represent, therefore, more nearly the conditions 

 of dissociation in the body. The results obtained by Bohr are 

 indicated in the curve of dissociation shown in Fig. 275, obtained 

 from experiments on dog's blood. At a pressure of oxygen of 

 152 mms. that is, when exposed to ordinary air the hemoglobin 

 is nearly or completely saturated with oxygen. If the oxygen 

 pressure is increased, if, for instance, the blood is exposed to pure 

 oxygen (pressure, 760 mms.), no more oxygen is combined 

 chemically by the hemoglobin. Additional oxygen will be taken 

 up by the blood, but only in so far as it can pass into solution in the 

 blood-plasma. Oxygen thus dissolved in the blood-plasma obeys 

 the physical law of solution, and will be at once given off when the 

 oxygen pressure of the surrounding medium is lowered. If the 

 pressure of oxygen falls below that of the air (152 mms.) the chemi- 

 cally combined oxyhemoglobin begins to dissociate slowly at first, 

 but as the pressure falls below 70 mms, the dissociation becomes 

 much more rapid, and the oxygen thus liberated from chemical 

 combination is from a quantitative standpoint much more impor- 

 tant than that freed from solution in the plasma. This, in fact, 

 is the process that takes place as the blood circulates through the 

 tissues. The arterial blood enters the capillaries with its hemo- 

 globin nearly saturated with oxygen, about 19 c.c. to each 100 c.c. 

 of blood. After it leaves the capillaries the venous blood contains 

 only about 12 volumes of oxygen to each 100 c.c. of blood. In the 



* Hufner, " Archiv f. Physiologic," suppl. volume, 1901, p. 213. 

 t Loewy, "Archiv f. Physiologic," 1904, p. 245. 



