RESPIRATION OF THE TISSUES 211 



through the capillaries too rapidly, there is no time for 

 either giving or receiving oxygen, and the body may be 

 actually starved of oxygen. So the average amount of 

 oxygen which the blood can carry is found to be about 

 25 ounces daily. 



It is possible to educate the respiratory muscles so that 

 during physical exertion they act more regularly and 

 strongly. As a result, the lungs are expanded more, and 

 a greater area of capillaries is exposed to the air. The 

 heart also may be trained to restrain its violent action, so 

 that the blood is not shot through the capillaries of the 

 lungs too rapidly to take up oxygen. An athlete trains 

 his body so that it can absorb more than 25 ounces of 

 oxygen daily, and thus he can put forth a greater 

 amount of exertion. Such a person is said to be long 

 winded. 



361. Causes of shortness of breath. The sensation of short- 

 ness of breath is usually due to a deficiency of oxygen in the blood 

 which circulates through the respiratory center. The blood contains 

 too. little oxygen when an extra amount of oxygen is used during great 

 physical exertion. At first, the heart pumps the blood faster so that it 

 carries more oxygen in a given time, but when the blood is pumped 

 very rapidly, the red blood cells are shot through the lungs so quickly 

 that they cannot obtain the necessary oxygen. When, as in heart dis- 

 ease, the blood is pumped too slowly, only a small amount of oxygen will 

 be carried through the respiratory center, and there will be continuous 

 difficulty in breathing. Shortness of breath is often the first sign of 

 heart failure. After severe hemorrhage there are too few red blood 

 cells to carry the full amount of oxygen, and so shortness of breath will 

 be felt. Death by bleeding is due to suffocation and lack of oxygen. 

 In the disease called anemia there are too few red blood cells to carry 

 oxygen, and so there is shortness of breath on exertion. When the 

 larynx or the trachea is compressed or obstructed, as in choking, or 

 when the smaller bronchi are filled with mucus, as in bronchitis, oxygen 

 j is prevented from entering the blood, and the respiratory center feels a 



iat shortness of breath. 



