CHAPTER XXIII 



RESPIRATION. THE MECHANISM OF BREATHING. THE 

 REGULATION OF BREATHING. 



Definitions. The blood as it flows from the right ventricle of 

 the heart, through the lungs, to the left auricle, loses carbon 

 dioxid and gains oxygen. In the systemic circulation exactly 

 the reverse changes take place, oxygen leaving the blood to supply 

 the living tissues; and carbon dioxid, generated in them, passing 

 back into the blood capillaries. The oxygen loss and carbon 

 dioxid 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 be- 

 comes 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 between the tissues 

 and the systemic capillaries through the lymph, constitute internal 

 respiration, and the processes in general by which oxygen is fixed 

 and carbon dioxid formed by tlie living tissues, are known as 

 tissue 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 oxygen 

 and rich in carbon dioxid 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 contain- 

 ing air in solution. When an animal is small there are often no 

 special organs for its external respiration, its general surface being 

 sufficient (especially in aquatic animals with a moist skin) to 

 permit of all the gaseous exchange that is necessary. In the 



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