CHAPTER VIII. 

 RESPIRATION. 



Object. The object of respiration is to furnish oxygen to 

 the tissues and remove carbon dioxide from them. The inter- 

 vention of the lungs and blood is necessary to accomplish this 

 end. At each inspiration a certain volume of air is taken into 

 the lungs, and from it, while in these organs, is removed a cer- 

 tain amount of oxygen which enters the blood of the pulmonary 

 capillaries. At each expiration there is removed from the lungs 

 a certain volume of air, and it contains a proportion of carbon 

 dioxide over and above that contained in the ordinary atmosphere, 

 i. e., in the inspired air; this carbon dioxide is removed from the 

 blood of the pulmonary capillaries and enters the air in the lungs. 

 The entrance and exit of air to and from the lungs, in obedience 

 to movements to be noticed later, constitutes what is commonly 

 called respiration ; but the mere tide of the air inward and out- 

 ward is of no significance unless the interchange of oxygen and 

 carbon dioxide takes place. 



Internal Respiration. Nor is this interchange of value unless 

 another occurs in the tissues. The oxygen which has entered the 

 pulmonary blood is conveyed by the circulation to a point where 

 the fluid is brought into very close relationship with the tissues 

 (namely, in the capillaries), and is here given up to the cells; 

 furthermore, at the same place the cells give up carbon dioxide to 

 the capillary blood. It is only for the purpose of effecting this 

 last interchange that there is any respiration, or any respiratory 

 apparatus. Inspiration and expiration, the pulmonary inter- 

 change of gases, the transportation of oxygen and carbon dioxide 

 to and away from the cells, are all equally immaterial except as 

 being means to the accomplishment of this end. It would make 



