CHAPTER XXIII 

 RESPIRATION OF THE TISSUES 



351. Changes in respired air. The air is composed of 

 about 80 per cent of nitrogen, 20 per cent of oxygen, and 

 T$1T P er cent f carbonic acid gas. The nitrogen has no 

 effect upon the body, but acts simply by diluting the oxy- 

 gen. Air which is ordinarily breathed out from the lungs 

 contains 16 per cent of oxygen and 4 per cent of carbonic 

 acid gas, while the amount of nitrogen remains unchanged. 

 Thus, in breathing, the air gains as much carbonic acid 

 gas as it loses oxygen. Expired air is warmer and con- 

 tains more watery vapor than inspired air, and sometimes 

 contains a trace of a very poisonous organic gas. 



352. Blood changes in the lungs. Every 100 cubic 

 inches of venous blood entering the lungs contain 46 cubic 

 inches of carbonic acid gas, and from 8 to 12 cubic inches 

 of oxygen gas. As it leaves the lungs the same amount 

 of blood contains about 40 cubic inches of carbonic acid 

 gas, and 20 cubic inches of oxygen gas, and it has changed 

 its shade from the dark red of venous blood to the bright 

 red tint of arterial blood. It has also lost a small amount 

 of water and some heat. The essential change which 

 occurs in the passage of blood through the lungs is the 

 exchange of carbonic acid for a corresponding amount of 

 oxygen gas. In health, during quiet breathing, the blood 

 becomes completely saturated with oxygen. 



353. Affinity of blood for oxygen. Blood exposed to 



206 



