A text-book of physiology for medical students and physicians . designated asintrapulmonic pressure. Therelations of the two pressureswith reference to the outsideatmosphere is indicated sche-matically in Fig. 270. The Intrapulmonic Pres-sure and its Variations.—Theair passages and the alveoli ofthe lungs are in free communi-cation with the external air;consequently in every positionof rest, whether at the end ofinspiration or expiration, thepressure in these cavities isequal to that of the atmos-phere outside. During the actof inspiration, however, the in-trapulmonic pressure falls tem-porari
A text-book of physiology for medical students and physicians . designated asintrapulmonic pressure. Therelations of the two pressureswith reference to the outsideatmosphere is indicated sche-matically in Fig. 270. The Intrapulmonic Pres-sure and its Variations.—Theair passages and the alveoli ofthe lungs are in free communi-cation with the external air;consequently in every positionof rest, whether at the end ofinspiration or expiration, thepressure in these cavities isequal to that of the atmos-phere outside. During the actof inspiration, however, the in-trapulmonic pressure falls tem-porarily below that of the atmosphere,—that is, during the inflow ofair. The extent to which the pressure falls depends naturally uponthe rapidity and amplitude of the inspiratory movement and uponthe size of the opening to the exterior. The narrowest portion ofthe air passages is the glottis; consequently the variations in pres-sure below this point are probably greater than in the pharynx ornasal cavities. If the air passages are abnormally constricted at 649. Fig. 270.—Diagram to illustrate howthe pressure of the air is exerted throughthe lung walls upon the heart (H) and otherorgans in the mediastinal space. The pres-sure on these organs (intrathoracic pressure)is equal to one atmosphere minus the amountof the opposing pressure exerted by the ex-panded lungs. 650 PHYSIOLOGY OF RESPIRATION. any point the fall of pressure during inspiration will be correspond-ingly magnified in the parts below the constriction, as happens, forinstance, in bronchial asthma, edema of the glottis, cold in thehead, etc. Under normal conditions the fall of pressure during aquiet inspiration is not large. Donders determined it in man byconnecting a water manometer with one nostril and found that itwas equal to —9 or —10 mms. water. At the end of an inspiration,if there is a pause, the pressure within the lungs again rises, of course,to atmospheric. During expiration, on the other hand, the coll
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