RESPIRATION AND EXCRETION 347 



The air loses 4.94 parts of oxygen in the lungs, and* 

 gains 4.34 parts carbon dioxide and .6 parts of water. 

 Notice that the amount of gases inspired equals the whole 

 amount of gases and water expired. This indicates the 

 character and amount of exchange that has taken place in 

 the blood during its pulmonary circulation. The blood is 

 thus aerated in the lungs. The presence of carbon dioxide 

 in expired air may be demonstrated by breathing through 

 a straw into lime water. Carbonate of lime will be 

 formed, giving the liquid a milky appearance. The pres- 

 ence of water, as vapor, is easily demonstrated by breathing 

 upon glass. 



Quantity of air breathed. — The lung capacity after the 

 deepest inspiration is, in a grown person, about 330 cubic 

 inches of air. About 30 cubic inches are inspired and ex- 

 pired during each ordinary act of breathing. This is known 

 as tidal air. During the deepest possible inspiration about 

 230 cubic inches of air enter the lungs. This represents the 

 vital capacity of the lungs. There is therefore at all times at 

 least 100 cubic inches of air in the lungs. This is the residual 

 air which is kept aerated by diffusion from the tidal air. 

 Thus there is a constant supply of air in the lungs. This 

 allows continuous exchange between the blood and lungs, 

 while the exchange between the outer air and the lungs is 

 intermittent. 



Hygienic habits of breathing. — Breathing should be 

 habitually deep in order to use all the parts of the lungs and 

 to have as large an amount of tidal air as possible passing 

 in and out of the lungs. 



Occasionally the lungs should be tested to their fullest 

 capacity by the deepest possible inspirations. This not 

 only effects as perfect ventilation as possible, but exercises 

 the respiratory muscles to their utmost extent, and exercise 

 of these develops the capacity- of the lungs. 



Effects of exercise upon breathing. — During exercise 



