CIRCULATORY SYSTEM 337 



out into the tissues. They are always found in large 

 numbers in the region of wounds. Here they may aid 

 in repairing the injured part, but they are most useful 

 in destroying the many bacteria which collect about 

 wounds. This they do by flowing around the bacteria 

 and then by digesting them. 



Coagulation. When blood escapes from the blood 

 vessels into the tissues, or when a vessel is ruptured 

 and it escapes from the body, it soon coagulates or clots. 

 This process is the result of the action of an enzyme. 

 The function of clotting is to plug up the wound and 

 so prevent excessive bleeding. 



Amount of blood. The blood constitutes about one- 

 thirteenth of the weight of the "body. The distribution 

 of this to the parts of the body varies at different times. 

 Just after a meal more blood is needed in the digestive 

 organs. During exercise more blood is sent to the 

 working muscles. 



The amount of blood going to all parts of the body 

 depends upon the rate of the heart beat. The amount 

 sent to each organ, however, can not be regulated by 

 the heart. This regulation is brought about by a 

 change in the diameter of the blood vessels leading 

 to or from the organ in question. If more blood is 

 needed in the stoma.ch, for example, the diameter of 

 the blood vessel going to the stomach is enlarged. 

 This is the result of the action of the muscles in its 

 wall, which is controlled in turn by the nervous 

 system. There is less resistance to the flow of blood 



