CHAPTER XXXV. 



THE PRESSURE CONDITIONS IN THE LUNGS AND 



THORAX AND THEIR INFLUENCE UPON THE 



CIRCULATION. 



In considering the pressure changes in respiration the distinction 

 between the pressure in the thorax outside the lungs and the pres- 

 sure within the lungs and air passages must be kept clearly in mind. 

 The pressure in the thoracic cavity outside the lungs may be 

 designated as the intrathoracic pressure; it is the pressure exerted 

 upon the heart, great blood- 

 vessels, thoracic duct, esopha- 

 gus, etc. The pressure in the 

 interior of the lungs and air 

 passages may be designated as 

 intrapulmonic pressure. The 

 relations of the two pressures 

 with reference to the outside 

 atmosphere is indicated sche- 

 matically in Fig. 249. 



The Intrapulmonic Pres- 

 sure and its Variations. The 

 air passages and the alveoli of 

 the lungs are in free communi- 

 cation with the external air; 

 consequently in every position 

 of rest, whether at the end of 

 inspiration or expiration, the 

 pressure in these cavities is 

 equal to that of the atmos- 

 phere outside.* During the act 

 of inspiration, however, the in- 

 trapulmonic pressure falls tem- 

 porarily below that of the atmosphere, that is, during the inflow of 

 air. The extent to which the pressure falls depends naturally upon 

 the rapidity and amplitude of the inspiratory movement and upon 

 the size of the opening to the exterior. The narrowest portion of 

 the air passages is the glottis; consequently the variations in pres- 

 sure below this point are probably greater than in the pharynx or 



* Strictly speaking, a correction must be made for the aqueous tension 

 in the lungs (see p. 628). 



604 



Fig. 249. Diagram to illustrate how 

 the pressure of the air is exerted through 

 the lung walls upon the heart (H) and other 

 organs in the mediastinal space. The pres- 

 sure on these organs (intrathoracic pressure) 

 is equal to one atmosphere minus the amount 

 of the opposing pressure exerted by the ex- 

 panded lungs. 



