166 BLOOD AND LYMPH 



globin gives them an important function as carriers of 

 oxygen from the lungs to the tissues, and their physiology 

 is largely concerned with the action of this pigment. 



In structure they are found to consist of a colorless 

 substance called the stroma, which' gives shape to the cor- 

 puscles, and in this stroma is deposited the htemoglobin 

 together with some water and salts. They contain no 

 nucleus, and are to be considered as cells which have been 

 modified to meet the needs of the blood. 



Hwmoglobin. This pigment consists of two chemical 

 compounds, hsematin and a proteid body. The ' hsematin 

 has the power when in contact with oxygen to combine 

 with it and to form an unstable compound which breaks 

 down when it reaches the tissues, and gives up its oxygen 

 to the cells. This power of hsematin to unite with oxygen 

 is due to the iron which forms a large part of its chemical 

 composition. An interesting demonstration of the power 

 of the red corpuscles to take up oxygen is furnished when- 

 ever we mix blood with air. For example, shake violently 

 a tube half filled with blood. The increased redness of the 

 shaken mixture is due to the taking up of oxygen. In 

 the body the red corpuscles are brought in contact with 

 the air in the lungs, and the oxygen which they pick up is 

 carried by them to the tissues and there given up to the 

 cells as needed. It is evident, then, that one function of 

 the red corpuscles is to carry oxygen to the tissues. This, 

 however, is only one of their functions. 



Haemoglobin forms compounds not only with oxygen 

 but also with carbon dioxide. This compound is called car- 

 bohoemoglobin to distinguish it from oxyhcemoglobin. This 

 compound is formed when the carbon dioxide is absorbed 

 from the tissue cells. In this way the red corpuscles are 



