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A MANUAL OF PHYSIOLOGY 



The next table illustrates the difference in the intensity oi 

 metabolism in different kinds of animals, a difference, however, 

 largely dependent upon relative size : 



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 respira- 

 tion. Within wide limits the oxygen consumption of the 

 organism is independent 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 under- 

 stand 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 

 gas is in a closed vessel, the molecules will be constantly striking its 



