498 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The results can be expressed in the form of a curve, the dissocia- 

 tion curve. 



The haemoglobin is almost completely associated with oxygen at 

 100 mm. ; at atmospheric oxygen pressure, ~J-- or 150 mm. the as- 

 sociation would be complete. The partial pressure of the oxygen in 

 the lung is about 100 mm., or about 13-2 per cent, of the atmospheric 

 pressure, so that in the lungs the haemoglobin becomes nearly satu- 

 rated with oxygen; at 10 mm. partial pressure about equal parts 

 of haemoglobin and oxyhaemoglobin are present. Over 30 per cent 

 of oxyhaemoglobin is still present at 5 mm. partial pressure. 



The haemoglobin in the body is never completely saturated with 

 oxygen, except when it is in contact with the air of the lungs. A sample 

 of blood whether taken from a vein or artery always contains a mix- 

 ture of oxyhaemoglobin and haemoglobin. 



The Estimation of Oxygen in Blood and the Oxygen Capa- 

 city of Blood. 



Haldane has shown that when oxyhaemoglobin is treated with 

 potassium ferricyanide the whole of the oxygen of the oxyhaemo- 

 globin is evolved and methaemoglobin is formed (p. 482). The amount 

 of oxygen in blood can thus be readily determined. The circulating 

 blood contains a mixture of oxyhaemoglobin and haemoglobin. The 

 total amount of oxygen which is obtainable from the blood when it 

 has been exposed to the air so that it contains only oxyhaemoglobin 

 is the oxygen capacity of the blood. The oxygen capacity depends 

 on the total amount of haemoglobin in the blood and thus indirectly 

 it will give the haemoglobin content (p. 489). 



