THE RESPIRATORY SYSTEM. 



259 



lactic acid. The greater ease with which the oxyhsemoglobin dis- 

 sociates in these circumstances increases the supply of oxygen available 

 for the tissues ; and this effect will be especially marked and beneficial 

 when the tissues are functionally active and are giving out large 

 amounts of carbonic acid and require a larger supply of oxygen. 



Although these conditions profoundly influence the rate of trans- 

 ference of oxygen from the blood to the tissues, they do not appreciably 

 affect the amount of oxygen taken up by the blood as it passes through 

 the lungs. The partial pressure of oxygen in the lungs is normally 



(00 



10 20 30 40 50 60 70 90 90 IOQ 



Oxygen pressure in mm. mercury. 



FIG. 100. A, B, and C show the effect of varying tensions of C0 2 in 

 blood on the dissociation curve of oxyhsemoglobin. The dotted 

 line D is the dissociation curve of oxyhsemoglobin in blood to 

 which 0'025 per cent, lactic acid was added. 



just over 13 per cent, (about 105 mm. Hg), and a consideration of 

 fig. 100 shows that at this pressure of oxygen the saturation of the 

 blood will be practically the same, whether the blood is normal or 

 whether it contains an excess of carbonic acid or a trace of lactic acid. 



In addition to the oxygen thus combined with haemoglobin, the 

 blood contains oxygen in simple solution in the plasma, amounting at 

 the body temperature to about 0'36 c.c. oxygen per 100 c.c. of blood. 



Carbonic Acid in Blood. The tension of carbonic acid in blood, 

 when determined by means of an aero tonometer, is equal to a partial 

 pressure of about 5 per cent, of carbonic acid. If the carbonic acid 

 were simply dissolved in the blood, the latter would* contain at this 



