CHAPTER XXIV. 



THE EESPIRATORY MECHANISM. 







Definitions. The blood as it flows from the right ven- 

 tricle of the heart, through the lungs, to the left auricle, 

 loses carbon dioxide and gains oxygen. In the systemic 

 circulation exactly the reverse changes take place, oxygen 

 leaving the blood to supply the living tissues; and carbon 

 dioxide, generated in them, passing back into the blood 

 capillaries. The oxygen loss and carbon dioxide gain are 

 associated with a change in the color of the blood from 

 bright scarlet to purple red, or from arterial to venous; and 

 the opposite changes in the lungs restore to the dark blood 

 its bright tint. The whole set of processes through which 

 blood becomes venous in the systemic circulation and 

 arterial in the pulmonary in other words the processes 

 concerned in the gaseous reception, distribution and elimi- 

 nation of the Body constitute the function of respiration; 

 so much of this as is concerned in the interchanges between 

 the blood and air being known as external respiration; 

 while the interchanges occurring in the systemic capillaries, 

 and the processes in general by which oxygen is fixed and 

 carbon dioxide formed by the living tissues, are known as 

 internal respiration. When the term respiration is used 

 alone, without any limiting adjective, the external respira- 

 tion only, is commonly meant. 



Respiratory Organs. The blood being kept poor in 

 oxygen and rich in carbon dioxide by the action of the liv- 

 ing tissues, a certain amount of gaseous interchange will 

 nearly always take place when it comes into close proximity 

 to the surrounding medium; whether this be the atmos- 

 phere itself or water containing air in solution. When an 



