342 ELEMENTS OF GENEKAL SCIENCE 



The blood is able to carry oxygen because the red blood 

 corpuscles, which are very numerous, contain a chemical 

 substance haemoglobin which combines with oxygen and 

 later gives it up to any cell where it is required. The blood 

 not only brings to the cell food and oxygen which are neces- 

 sary for its life processes but also carries away the waste 

 products from the cell. Carbon dioxide is carried away from 

 the cells to the lungs ; urea and other substances are carried 

 to the kidneys and excreted. The water which is lost to the 

 body in the breath, in perspiration, and in the excretion of 

 the kidneys is also carried by the blood. 



349. Digestion and exercise. The blood vessels under pres- 

 sure from the arteries expand or contract and thus regulate 

 the volume of blood flowing through an organ. During 

 the height of digestion the capillaries in the intestinal wall 

 are very much dilated and the proportion of blood flowing 

 through them is therefore large. This explains why it is 

 so hard to study after a full meal. There is not enough 

 blood flowing through the capillaries of the brain to make 

 it alert and active. During digestion excessive exercise re- 

 tards the digestive processes. When the muscles are being 

 used, the blood flow through them is large and there is not 

 enough blood in the intestinal capillaries. 



Wild animals which feed at irregular intervals and eat to 

 distention when they do feed usually become quiet and often 

 sleep for some time after feeding. In such cases the chief 

 activities of the body are for a time centered about digestion 

 and the circulatory distribution of food. Animals which feed 

 oftener and eat smaller quantities man, for example may 

 safely proceed with some sort of mild exercise after eating. 



350. Food and work. A large part of the food we eat 

 corresponds to the gasoline in the automobile. It combines 

 with oxygen in the cells of the body and releases energy 

 by which we do our work. This is the process of respiration, 

 the same process that was described as taking place in plant 



