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A TEXTBOOK OF PHYSIOLOGY 



of oxygen chemically combined increases inappreciably. Therefore, 

 when blood is shaken with air at ordinary atmospheric pressure, it 

 becomes practically saturated with oxygen, and the degree of satura- 

 tion in the above table is obtained by comparing the amount of oxygen 

 combined with the blood at a given pressure of oxygen with the 

 amount combined when shaken with air at atmospheric pressure. 

 By taking the amount of oxygen combined to 100 c.c. of blood as 

 ordinates, and the oxygen tensions as abscissae, the curve of dissocia- 

 tion of oxy haemoglobin is obtained. Thus, by joining the points 

 between the shaded portions of Fig. 137 a curve is obtained which 

 shows the percentage of oxyhaemoglobin of the total haemoglobin to 

 the concentrations of oxygen dissolved in the fluid at all pressures 

 up to 100 millimetres. 



100 



10 20 30 40 00 fO 70 80 90 ICO 



FIG. 138. OXYGEN DISSOCIATION CURVES OF HUMAN BLOOD EXPOSED TO 0, 3, 20, 

 40, AND 90 MILLIMETRES CO 2 . (Barcroft.) 



Ordinate=percentage saturation; abscissa = oxygen pressure. 



Various factors are found to modify this dissociation curve. Such 

 factors are 



1. The H ion concentration present in the blood. The greater 

 the H ion concentration, the less the amount of oxygen which com- 

 bines with the blood. This is well seen in conditions when there is 

 increased concentration of carbon dioxide and of lactic acid (Fig. 138). 



2. Solutions of haemoglobin give quite different values to those of 

 haemoglobin undischarged from the corpuscles. The haemoglobin forms 

 90 per cent, of the dried weight of the corpuscles, and much more is 

 present than can be in solution. It must be held in some peculiar 

 colloidal state. 



3. The proportion of salts present in the blood (Fig. 139). 



4. The temperature (Fig. 140). 



