THE HUMAN MACHINE 



What Causes the Blood to Circulate ? But the blood 

 does not just flow around in the vessels. It is under pres- 

 sure from a double force pump we call the heart. This 



to head, anct 



to righb arm and: band 



pulmonary 

 circulation 



to right lung 



systemic 

 Circulation. 



to left arm andhanct 



pulmonary 

 circulation 



renal Circulation 

 to right 



systemic circulation 



to lejb 



right auri 

 right ventricle 



left auricle 

 left ventricle 



Tenal Circulation 



to left "kidney 



systemic circulation 



The blood circulates throughout the body by means of a continuous closed 

 system of tubes, called the circulatory system. 



organ, though not bigger than the fist of a good-sized man, 

 pumps about a gallon of blood a minute, day in and day 

 out, during our lives. When we exercise, the heart pumps 

 faster, and during a game of handball or tennis it may 

 pump five or six gallons a minute to the working cells. 

 Blood vessels which leave the heart are elastic with 

 rather thick walls in order to withstand the pressure. 

 These are called arteries. They are the vessels which 

 pulsate as the heart beats and from them we get our pulse. 

 Arteries branch out, getting smaller and smaller until 



