THE BLOOD 



199 



The following table shows the relationship of these pig- 

 ments to one another : 



RELATIONSHIP OF HB AND ITS DERIVATIVES. 

 Methsemoglobin Hb0 2 HbCO 



Hb 



Haematin 



Globin 



Acid Hsematin 



Alkaline Hsematin 



Oxidised Reduced 



(Hsemochromogen) 



Contain Iron. 



Iron-free Haematin 

 (Hsematoporphyrin) 



Haamatoidin 

 Bilirubin 



j- Iron-free. 



IY. Gases of the Blood. 



The muscles and other active tissues are constantly con- 

 suming oxygen and constantly giving off carbon dioxide. 

 The oxygen must be brought to the tissues by the blood, 

 and the carbon dioxide carried away by the same medium. 



Various methods of carrying out the examination of the 

 gases of the blood have been devised, and many different gas 

 pumps have been invented in which the gases may be col- 

 lected in the Torricellian vacuum over mercury. Haldane and 

 Barcroft have devised a convenient method, which depends 

 upon the fact that the oxygen can be driven off from blood 

 treated with dilute ammonia by the addition of ferricyanide 

 of potassium, and that the carbon dioxide is liberated by 

 adding an acid, and the amount of gas estimated by measur- 

 ing the increased pressure in the tube in which the gas has 

 been given off. 



About 60 c.c. of gas measured at C. and 760 mm. pres- 

 sure can be extracted from 100 c.c. of blood. The proportion 

 of the gases varies in arterial and in venous blood. 



