128 RESPIRATION 



ratio of the quantity of oxygen absorbed to the carbon dioxide 

 given off is known as the respiratory quotient. While in the 

 lungs the air loses 4.78 volumes of O 2 in 100, and gains 4.34 

 volumes of CO 2 in 100, so that the value of the respiratory 



4.34 

 quotient j^~ is equal to 0.901. This value is subject to 



great variations, because the production of carbon dioxide 

 is to some extent independent of the amount of oxygen absorbed. 

 This is so for several reasons : 



1. CO 2 may result not only from oxidation changes, but 

 from intramolecular splitting, so that the elimination of CO 2 

 in normal quantity may continue when absorption of O 2 has 

 entirely . ceased . 



2. Some foodstuffs require more O 2 for their complete oxida- 

 tion than others. 



The air during its sojourn in the lungs is altered in addition 

 to its O 2 and CO 2 contents by assuming the temperature of 

 the body, regardless of the temperature of the outside atmos- 

 phere; by an increase of its aqueous vapor; and possibly, by 

 the presence of volatile organic bodies. The nitrogen remains 

 unchanged. The quantity of water lost by the lungs varies 

 inversely with the amount in the atmosphere, and directly 

 with the quantity of air inspired. The blood in its passage 

 through the lungs becomes aerated. The O 2 and CO 2 in arterial 

 and venous blood together form about 60 volumes of the blood 

 in 100. The proportions of the gases to each other are constant 

 in the arterial blood, but vary in the venous blood in different 

 localities. 



The oxygen of the air enters the alveoli of the lung, passes 

 into the blood through the delicate epithelial walls, and is 

 carried to the tissues, where it is taken up by the cells. Very 

 little is used up in the blood. The CO 2 given off by the cells 

 is carried to the alveoli of the lungs by the blood, and is there 

 given off. It becomes necessary, therefore, to consider the 

 methods by means of which the interchange of gases is brought 

 about. The air currents that are set up mechanically probably 

 help to equalize the composition of the air in the lungs. Besides, 

 the heart with each contraction as it shrinks in size expands 

 the lungs slightly and causes a movement of air into the dies! 

 synchronous with its beat. These are known as cardiopneurnatic 



