202 PHYSIOLOGY FOR DENTAL STUDENTS. 



at atmospheric pressure.- After shaking, the solutions are removed 

 and the amount of oxygen in each sample is measured. 



The haemoglobin solutions in the tubes containing a partial 

 pressure of oxygen which is within two-thirds of that present in 

 air (between 90 and 152 mm. of mercury) are all almost satu- 

 rated with oxygen. In other words, at these pressures the haemo- 

 globin exists entirely in the form of oxyhasmoglobin. In the tube 

 containing one-half the pressure of oxygen in air (i. e., almost 76 

 mm. Hg. ) , the haemoglobin solution is 90 per cent saturated. At 

 about one-fourth the normal oxygen pressure in air (i. e., 40 nun. 

 Hg.), it is about 84 per cent saturated. At lower partial pres- 

 sures of oxygen, the ability of haemoglobin to unite with oxygen 

 very rapidly decreases. 



From these observations we must conclude that, as the pres- 

 sure of oxygen in contact with the haemoglobin solution increases 

 above zero, by graded stages, the amount of oxygen, per unit of 

 increase of oxygen pressure, that combines with haemoglobin at 

 low pressures is large, but becomes relatively less at higher pres- 

 sures. Or, conversely, if the haemoglobin saturated with oxygen 

 be subjected to decreasing oxygen pressure, the combined oxygen 

 is set free at first slowly and then more rapidly. 



If the oxygen-combining power of blood be investigated in ex- 

 actly the same way as described above and the results compared 

 with those of a pure haemoglobin solution, a marked difference 

 will be observed. At low pressures the oxygen is more easily re- 

 leased from the haemoglobin of the blood than from pure solu- 

 tions of haemoglobin. An inquiry into the cause of this difference 

 has revealed the following facts. The rate at which oxyhaemo- 

 globin breaks down into oxygen and haemoglobin, depends on 

 other factors besides oxygen pressure. These are: (1) tempera- 

 ture, (2) the presence of inorganic salts, and (3) carbon dioxide 

 or other weak acids in the blood. If haemoglobin be dissolved in 

 a saline solution containing the same concentration of inorganic 

 salts as is found in blood, it will take up oxygen in a manner 

 somewhat similar to blood under like oxygen pressures. The 

 similarity will become perfect if the saline solutions of haemo- 

 globin be subjected to the same pressure of carbon dioxide as that 



