CHANGES IN AIR AND BLOOD IN RESPIRATION. 625 



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

 indicated in the curve of dissociation shown in Fig. 254, obtained 

 from experiments on human 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 rums.) 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 

 passage of the capillaries, which takes only about one second, the 

 blood loses, therefore, about 35 per cent, or more of its oxygen. 

 The physical theory of respiration furnishes data to show that this 

 loss is due to a dissociation of the oxyhemoglobin, owing to the fact 

 that in passing through the capillaries the blood is brought into 

 exchange with a surrounding medium lymph, cell liquid in 

 which the oxygen pressure is very low. 



Condition of the Carbon Dioxid in the Blood. Carbon 

 dioxid is evidently contained in the blood in chemical combination 

 as well as in solution. The amount present, 40 to 45 volumes per 

 cent., is entirely too great to be accounted for by solution alone. 

 Moreover, the content of the blood in CO 2 does not vary proportion- 

 ally to the pressure of CO 2 in the surrounding medium in accordance 

 with the law of Henry. Examination of the blood by means of 

 the gas pump shows that CO 2 is contained in both the corpuscles 

 and the plasma or serum. The results of various analyses would indi- 

 cate that about one-third of the total amount is held in the corpuscles 

 and about two-thirds in the liquid of the blood, and, since the cor- 

 puscles make up about one-third of the bulk of the blood, this fact 

 would indicate that the CO 2 is distributed uniformly throughout the 

 mass of the blood. The condition of the CO 2 in the blood may be 

 considered under three heads: (1) The part in solution; (2) the part in 



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