674 



PHYSIOLOGY OF RESPIRATION. 



which the oxygen pressure is very low. An additional fact of im- 

 portance in this connection is shown by the curves produced 

 in Fig. 275. In the tissues where the oxyhemoglobin undergoes 

 dissociation with the liberation of oxygen, and in the lungs where 

 the hemoglobin takes up oxygen from the alveolar air, the blood 



Fig. 275. — Curves of dissociation of tlie oxyhemoglobin at different pressures of 

 oxygen. Five curves are shown to indicate that the dissociation of the oxyhemoglofjin 

 is greatly influenced by the presence of CO2. The figures along the ordinates (10 to 100) 

 indicate percentages of saturation of the hemoglobin with oxygen, while the figures along 

 the abscissa (0 to 160) indicate different pressures of oxygen. The curve marked 5 mm. 

 CO2 shows the amount of combination of oxygen and hemoglobin when the CO2 is absent 

 or present only in traces. In this curve at a pressure of 30 mms. of oxygen it will be seen 

 that the hemoglobin is 80 per cent, saturated with oxygen, while with a pressure of 40 mms. 

 of CO2, which approximates that in the body, the hemoglobin at the same pressure of 

 oxygen is only 50 per cent, saturated. (After Bohr.) 



is surrounded by a medium containing both oxygen and carbon 

 dioxid. Laboratory experiments show that the presence of the 

 carbon dioxid influences the dissociation curve in the direction 

 indicated in the figure. At a carbon dioxid pressure equivalent to 

 that of the alveolar air (40 mms.) it will be noticed that the curve 

 is lower, that is, at any given pressure of oxygen, 30 mms., for 

 example, less oxygen will be held by the hemoglobin, or the per- 

 centage saturation of the hemoglobin with oxygen will be lower. 

 So also changes in the reaction of the blood toward the acid side, 

 the condition designated in medicine as acidosis, will be accom- 

 panied by a similar lowering of the curve of dissociation, that is, 

 by a diminished capacity of the hemoglobin for oxygen at these 

 pressures below the saturation point. In determining the actual 

 dissociation curve of the blood in man it is desirable, therefore, 

 to obtain the figures expressing the percentage saturation of the 

 hemoglobin in atmospheres containing carbon dioxid in amounts 

 equal to that found in the alveolar air of the individual examined. 



