56 HUMAN PHYSIOLOGY CHAP. 



Count the number of respirations per minute of different persons 

 without letting them know what you are doing. How does the 

 number vary with age ? Exercise ? Position of body ? 



The average rate for an adult when reclining is about 

 thirteen, sitting, about nineteen, and standing, about 

 twenty-two. 



The breathing is increased very much when we run 

 or do any hard physical work. Why ? We know that 

 when we exercise vigorously the muscle cells work 

 harder. They need more food and oxygen to furnish 

 the energy necessary to keep up the exercise. But 

 you will remember that oxidation manufactures carbon 

 dioxid. When we run, all the large muscles used 

 pour this carbon dioxid and other wastes into the 

 blood very fast. Now, if we did not breathe faster to 

 remove these wastes, they would accumulate in the 

 blood and cause disease and death. 



But how do the muscles of the chest know that they 

 should work harder to remove the animal wastes ? That 

 is a hard question to answer in a simple way. The 

 wastes are, of course, washed along in the blood stream 

 and so reach all parts of the body. When they come 

 in contact with a group of nerve cells at the upper end 

 of the spinal cord they act like a poison to them. The 

 nerve cells send a command to the muscles of the chest 

 by means of several nerves and tell them to contract 

 faster, so that these wastes may not accumulate in the 

 body. Thus we see that the rate of breathing is care- 

 fully regulated and controlled by a special group of 

 nerve cells. 



