CHAPTEE IX 



RESPIRATION 



THE function of respiration is to provide for an exchange of gases be- 

 tween the tissues and the external air. The blood in its circulation through 

 the lungs takes up oxygen from the alveolar air and gives off to it gaseous 

 products of decomposition,, especially carbon dioxide. In order to renew the 

 supply of oxygen and to free the alveolar air of decomposition products, a 

 constant ventilation of the lungs is kept up by the respiratory movements. 

 We have therefore to study first the movements of respiration and then the 

 exchange of gases in the lungs. 



FIRST SECTION 



MOVEMENTS OF RESPIRATION 



1. ELASTICITY OF THE LUNGS AND INTRATHORACIC 



PRESSURE 



The lungs are inclosed in an air-tight cavity i. e., between them and the 

 thoracic wall or the other organs contained in the thorax there is no air. 

 Since the lungs are hollow sacs with elastic and easily distensible walls, it is 

 obvious that they must dilate every time the thorax is expanded and must 

 become smaller every time it is contracted. Since, further, the lungs are in 

 open communication with the external air by the respiratory passages, it 

 follows that in the former event air must be sucked into the lungs, and in the 

 latter it must be driven out. The former phase of respiration is called 

 inspiration, the latter expiration. 



In the static position of the thorax, the entire atmospheric pressure takes 

 effect through the air passages upon the inner surface of the alveoli. Indi- 

 rectly through the alveoli the air pressure acts upon the inner wall of the 

 thorax and upon the organs heart, oesophagus, etc. lying between it and 

 the lungs. Since now the lungs are elastic, a part of the air pressure is 

 expended in unfolding them, and the pressure taking effect upon the inner 

 wall of the thorax must be less than the atmospheric pressure, by just so 

 much as is necessary to expand the lungs. The intrathoracic pressure is 

 therefore negative. Again, the more the thorax is dilated, the greater is the 

 amount of the air pressure consumed in expanding the lungs, consequently 

 the greater this negative pressure becomes. 

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