172 Life and Health 



260. Effects of Respiration upon the Air in the Lungs. 

 It is well known that if two different liquids be placed in 

 a vessel in contact with each other and left undisturbed, 

 they do not remain separate, but gradually mix, and in 

 time will be perfectly combined. This is called diffusion 

 of liquids. 



Diffusion occurs with gases also, though the process 

 is not usually visible. Two liquids will mingle even when 

 separated from each other by a membrane. In a simi- 

 lar manner two gases, especially if of different densities, 

 may mingle even when separated from each other by a 

 membrane. 



In a general way this explains the respiratory changes 

 that occur in the blood in the lungs. Blood containing 

 oxygen and carbon dioxide is flowing in countless tiny 

 capillaries through the walls of the air sacs of the lungs. 

 The air sacs themselves contain a mixture of the same two 

 gases. A thin, moist membrane, well adapted to allow 

 gaseous diffusion, separates the blood from the air. This 

 membrane is the delicate wall of the capillaries and the 

 epithejhmi of the air sacs. 



By experiment it has been found that the pressure of 

 oxygen in the blood is less than that in the air sacs, and 

 that the pressure of carbon dioxide gas in the blood is 

 greater than that in the air sacs. A diffusion of gases ensues. 



The results of respiration upon the air in the lungs are 

 thus twofold : 



The blood gains oxygen and loses carbon dioxide. 



The air sacs lose oxygen and gain carbon dioxide. 



blood, in passing through the tissues, loses only a part of its oxygen. In 

 blood, however venous, there is in health always some oxygen ; and in 

 even the brightest arterial blood there is actually more carbonic acid than 

 oxygen. T. H. HUXLEY. 



