OXYGEN. 417 



to dissociate the oxy hemoglobin. This has, in fact, been found to 

 be the case. 1 For the cells themselves, therefore, at any given 

 moment, only the uncombined oxygen is available. The oxygen con- 

 tained in the oxyhemoglobin serves, as it were, as a reserve supply. It 

 is only the merely mechanically absorbed oxygen which determines the 

 pressure of the oxygen in the blood, and determines thereby the gas 

 exchange. Naturally the extent of the giving up of oxygen to the tissues 

 depends solely upon the magnitude of this pressure exerted by the absorbed 

 oxygen in the blood. The fact that the oxygen in the blood is not merely 

 absorbed, but for the most part held in a state of loose combination, is of 

 great significance for the entire metabolism. The animal organism is, 

 within fairly wide limits, independent of the partial pressure of the oxygen 

 in the surrounding atmosphere. From rarefied air, the blood removes 

 the oxygen and combines it with hemoglobin. We can imagine that the 

 process takes place in somewhat the following manner: First of all the 

 oxygen is absorbed by the blood-plasma from the alveolar air in accord- 

 ance with the gas laws; but inasmuch as this dissolved oxygen constantly 

 tends to combine with the hemoglobin, more and more oxygen is taken up 

 from the air. In this way a considerable store of oxygen is laid away in 

 the organism, by means of which it is able to satisfy, at any time, any 

 unusual and unexpected demands for this important gas. 



The question next arises as to how the amount of oxygen that is taken 

 up by arterial blood corresponds to the quantity absorbed when a sample 

 of blood is thoroughly shaken with air. Experiment has shown that 

 normally the blood is nearly saturated with oxygen, for but little more is 

 absorbed when it is shaken with air. The taking up of oxygen by the 

 blood is dependent upon certain definite conditions. This is evidently true 

 of a small amount that is mechanically absorbed by the plasma. The 

 amount of oxygen which unites with the hemoglobin, on the other hand, 

 is entirely independent of the laws which govern the absorption of gases 

 in cases where no chemical combination takes place. Paul Bert 2 studied 

 the influence of the temperature. At higher pressures he was not able to 

 detect any definite influence, but with lower gas pressures there was less 

 absorption at the temperature of the body than at, say, the room tempera- 

 ture. The fact that the absorption of the oxygen depends upon the pres- 

 sure of the gas is clearly shown by the following table prepared by Krogh. 3 

 Krogh examined the blood from horses at 38 C., and determined the 

 amount of chemically combined oxygen; i.e., from the total amount of 



1 Christian Bohr: Blutgase und respiratorischer Gaswechsel, Handbuch der Physio- 

 logie des Menschen. Vol. I, pp. 221, 222 (1905). 



2 La pression barometrique, Paris, 687 (1878). Cf. also A. Lowy: Zentr. Physiol. 13, 

 449 (1899); Arch. Physiol. Anat. 1904, 231 and 565. 



3 Skand. Arch. Physiol. 16, 390 (1894). 



