THE CHEMISTRY OF EXTERNAL RESPIRATION 



245 



Forced respiration, although it will temporarily increase the 

 quantity of carbon dioxide given off by the lungs, and thus raise 

 for a short time the respiratory quotient, 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. 

 Within wide limits the oxygen consumption of the organism is in- 

 dependent of the supply of oxygen offered to it. 



How it is that the depth of 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. 



SECTION IV. 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 essen- 

 tial 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 direc- 

 tion 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 mole- 

 cules 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 gas is in a closed 

 vessel, the molecules will be constantly striking its sides and rebounding 

 from them. If a very small opening is made in the vessel, some mole- 

 cules will occasionally hit on the opening and escape altogether. If the 

 opening is made larger, or the experiment continued for a longer time 



