RESPIRATION 167 



corpuscles give up their oxygen — oxyhemoglobin is reduced to 

 hemoglobin — and the blood receives in exchange, as it were, 

 carbon dioxide, and also water and heat resulting from oxidative 

 processes — combustion — in the tissue cells. The carbon dioxide 

 is carried by the red blood corpuscles, and also by the blood plasma 

 in combination with sodium as sodium bicarbonate. (Fig. 126.) 



And now the blood, after its passage — lasting about two sec- 

 onds — through the capillaries, proceeds on its way back to the 

 heart which sends it to the lungs so that it can transfer to the air 

 in the alveoli water, heat, and carbon dioxide. The latter passes 

 to the air because it is under higher tension in the blood than in 

 the air. The respiratory cycle is complete. 



Such, in brief, is the elaborate apparatus present in the higher 

 animals, and Man, to provide for the new conditions arising be- 

 cause of the removal of many of their component cells further and 

 further from the source of oxygen, and the demand for more and 

 more facilities for securing it as their life processes increased in 

 activity. We shall have occasion later to consider the attendant 

 changes in the blood vessels; but now it is only necessary to be 

 sure that the mechanism does not obscure its object — to reiterate 

 that, although one ordinarily thinks of the movements involved 

 in the renewal of the air in lungs as respiration, it is neither in- 

 halation and exhalation, nor the interchange of gases between 

 blood and air or between blood and tissue cells. The essential 

 feature of respiration is the same here as it is in unicellular plants 

 and animals: the protoplasm of each and every cell of the body 

 securing energy from food by combustion, involving the appro- 

 priation of oxygen and the liberation of carbon dioxide. All else 

 is accessory — though necessary. 



