446 
RESPIRATION AND EXCRETION 
but by breathing deeply a few times we can remove the larger 
part of the air from the lungs and replace it with fresh air. 
The natural rate of breathing is about eighteen times a 
minute, but the rate is higher in persons with a small lung 
capacity. Exercise increases the rate of breathing. Ex¬ 
plain why exercise out-of-doors is 
better for us than that taken in¬ 
doors. 
All the air passages are lined 
with cells bearing numerous cilia 
(Figure 393), and these cilia are 
constantly in motion. Their work 
is to carry toward the mouth the 
particles of dust and other foreign 
materials brought in by the air. 
This foreign matter is removed 
when we cough or clear our throats. 
Explain why clean air is better for 
us than foul air. 
The purpose of respiration is to 
take the air, rich in oxygen, into 
the lungs, where the oxygen passes 
into the blood as the capillaries 
penetrate the walls of the lungs, 
and to give off the waste carbonic 
acid gas that has come from the 
cells of the body. Thus respiration is a twofold process, the 
supplying of the living cells of the body with oxygen and the 
removal of wastes from the blood. The air that is inhaled 
contains a small amount of carbonic acid gas, but the pro¬ 
portion is much smaller than in the exhaled air. It is well 
to keep in mind that green plants are constantly, during the 
daytime, using carbonic acid gas in manufacturing starchy 
foods and giving off oxygen so that the air is being made 
better for animals and man to breathe, while at the same time 
Figure 395. — Diagram to 
Show the Relation of the 
Diaphragm to the Ribs. 
Between the ribs are shown 
some of the intercostal muscles 
that assist in respiration. 
