130 THE BLOOD 



changed in character. For example, the bright red 

 blood which comes from the lungs is rich in oxygen 

 and fairly poor in carbon dioxide. After its contact with 

 the tissues, the amount of oxygen is diminished, whereas 

 the carbon dioxide has increased. As a result, the blood 

 becomes a dark bluish-red. This blood again passes 

 through the lungs, where it replenishes its supply of 

 oxygen and loses much of its carbon dioxide, thereby 

 regaining its brilliancy. 



Food elements. The character of the blood is also 

 changed by the rapid absorption of food materials when 

 we eat and drink. Some of these it stores up and later 

 doles them out to the tissues as they need them for their 

 activity. 



Carbohydrates. In the storage of the sugar absorbed, 

 the blood is assisted by the liver. The liver takes from 

 the blood the excess sugar which the blood absorbed 

 in the villi during digestion and stores it in its cells in 

 the form of a starch, called glycogen. As the blood be- 

 comes poor in sugar through loss to the tissues, the liver 

 changes the glycogen once again into sugar and restores 

 it to the blood. In this way, the supply of sugar in the 

 blood is kept fairly uniform, provided that meals are 

 taken regularly. 



Hydrocarbons. The fat absorbed into the blood 

 after each meal remains there until removed by the 

 tissues and oxidized. If, however, an excessive, quantity 

 of fat is eaten, the blood is assisted in taking care of 

 it by the various cells of the body, especially those of 

 the connective tissue, which store it within themselves 

 in the form of fat globules. 



Proteids. The proteid material which is absorbed 

 during digestion is retained in the blood until given 

 up to the tissues for repair, growth, or oxidation. Dur- 



