RESPIRATION 



229 



Forced respiration, although it will temporarily increase 

 the quantity of carbon dioxide given off by the lungs, does 

 not sensibly affect the production ; it is only the store of 

 already formed carbon dioxide in the body which is drawn 

 upon. The amount of oxygen taken up is little altered by 

 changes in the movements of respiration except for a very 

 short time. 



How it is that the depth ot the respiration may affect the 

 rate at which carbon dioxide is eliminated, we can only 

 understand when we have examined the process by which 

 the gaseous interchange between the blood and the air of 

 the alveoli is accomplished; and before doing this it is 

 necessary to consider the condition of the oxygen and carbon 

 dioxide in the blood. 



The Gases of the Blood. 



Physical Introduction. Matter may be assumed to be made up 

 of molecules beyond which it cannot be divided without altering its 

 essential character. A molecule may consist of two or more particles 

 of matter (atoms) bound to each other by chemical links. The kinetic 

 theory of matter supposes the molecules of a substance to be in 

 constant motion, frequently colliding with each other, and thus having 

 the direction of their motion changed. 



In a gas the mean free path, that is, the average distance which a 

 molecule travels without striking another, is comparatively long, and 

 far more time is passed by any molecule without an encounter than 

 is taken up with collisions. Although the average velocity of the 

 molecules is very great, these collisions will produce all sorts of 

 differences in the actual velocity of different molecules at any given 

 time. Some will be moving at a greater, some at a slower rate, 

 than the average ; while some may be for a moment at rest. If the 



