

CHAPTER XXV. 



THE RESPIRATORY MECHANISM. 



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 3J 



Definitions. Tttie blood as it flows from the right ventri- 

 cle of the heart, through the lungs, to the left auricle, loses 

 carbon dioxide and gains oxygen. In the systemic-- circula- 

 tion exactly the reverse changes take place, oxygen leaving 

 the blood to supply the living tissues; and carbon dioxide, gen- 

 erated 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 elimination 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 liv- 

 ing tissues, are known as internal respiration. When the 

 term respiration is used alone, without any limiting adjective, 

 the external respiration only, is commonly meant. 



Respiratory Organs. The blood being kept poor in oxy- 

 gen and rich in carbon dioxide by the action of the living 

 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 atmosphere itself 

 .or water containing air in solution. When an animal is 

 .small there are often no special organs for its external res- 

 piration, its general surface being sufficient (especially in 

 aquatic animals with a moist skin) to permit of all the gas- 

 eous exchange that is necessary. In the simplest creatures, 

 .indeed, there is even no blood, the cell or cells composing 



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