RESPIRATION AND RELEASE OF ENERGY 119 



Internal Respiration. — When the oxygen is finally presented to the 

 tissues or cells in which it is to be consumed, its introduction to those cells 

 is again dependent on relative pressures. Oxygen is at higher pressure 

 in the plasma of the blood than in the adjoining tissue cells, which have 

 used their oxygen. As the plasma gives up its oxygen to the cells its 

 oxygen pressure is lowered; and in response to this reduction, oxygen is 

 released from chemical combination in the red cells, and is dissolved in the 

 plasma. The plasma thus maintains a higher oxygen pressure as long 

 as there is oxygen in loose combination in the red cells; and before the red 

 cells have lost all their loosely combined oxygen, the blood has passed 

 on and been replaced by fresh blood which has not yet been called upon 

 to give up its oxygen. So there is a continual diffusion of oxygen from 

 the blood to the tissue cells. The transfer is very rapid, for the oxygen 

 pressure in the blood is reduced by half in one second of time. The cells 

 nearest the capillaries pass some of their oxygen on to cells farther away, 

 again in response to differences in pressure but aided by a fluid (see next 

 chapter) bathing the cells, and no cell is very far from the nearest blood 

 vessel. 



Respiration Also an Excretory Process. — While we are not yet ready 

 to discuss the general phenomenon of removal of Avastes, it should be 

 pointed out in passing that certain wastes are removed in respiration. 

 These wastes are carbon dioxide and a small amount of water. Carbon 

 dioxide results from the very abundant oxidation going on everywhere 

 in living things. It leaves the tissues where it is produced because its 

 pressure is higher than in the near-by blood plasma. The resulting 

 increase of pressure in the plasma causes the chief protein of the red cells 

 to combine with carbon dioxide. Delivered by the blood to the lungs, 

 the carbon dioxide is at greater pressure in the blood than in the air of the 

 lungs; hence the plasma gives up carbon dioxide to the air on the other 

 side of the two thin walls w^hich separate blood and air, and red cells 

 yield more carbon dioxide to the plasma. Since the blood moves on, no 

 equilibrium can be reached; always carbon dioxide passes from blood to 

 air in the lungs. This elimination of carbon dioxide is regarded as part 

 of respiration, even though it is also excretion. Excretion in general is 

 treated in another chapter. 



Release of Energy. — Energy for all sorts of work in living things is 

 obtained, as stated earlier, by combustion of foods. These substances 

 are literally burned, just as coal is burned in a boiler, with the difference 

 that combustion in living things is carried on at relatively low tempera- 

 tures. The reason for the ability of animals to burn their fuel without 

 great heat lies in their possession of enzymes. The burning is simple 

 oxidation, and the enzymes serve to bring oxygen and the foods together 

 in chemical reaction. One of the chief functions of respiration is to 



