584 PHYSIOLOGY OF RESPIRATION. 



nasal cavities. If the air passages are abnormally constricted at 

 any point the fall of pressure during inspiration will be correspond- 

 ingly magnified in the parts below the constriction, as happens, for 

 instance, in bronchial asthma, edema of the glottis, cold in the 

 head, etc. Under normal conditions the fall of pressure during a 

 quiet inspiration is not large. Donders determined it in man by 

 connecting a water manometer with one nostril and found that it 

 was 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 collapse 

 of the chest wall takes place with sufficient rapidity to compress the 

 air somewhat during its escape and cause a temporary rise of pres- 

 sure. In normal expiration Donders estimated this rise as equal to 

 7 or 8 mms. water. The intrapulmonic pressure may vary greatly 

 from these figures in the positive or negative direction according to 

 the factors mentioned above, especially the intensity of the respira- 

 tory movement and the size of the opening to the exterior. The 

 extreme variations are obtained when the opening to the outside is 

 entirely shut off. When an inspiration or an expiration is made 

 with the glottis firmly closed the pressure in the lungs, of course, 

 rises and falls with the rarefaction or compression of the contained 

 air. A strong inspiration under such conditions may lower the 

 pressure by 30 to 80 mms. of mercury, while a strong expiration 

 raises the pressure by an amount equal to 60 to 100 mms. Hg. In 

 the act of coughing we get a similar result: the strong spasmodic 

 expirations are made with a closed glottis and consequently cause a 

 marked rise in the intrapulmonic pressure. Such great variations 

 in pressure have a marked influence on the heart and the circula- 

 tion, as is explained below. 



Intrathoracic Pressure. When a reference is made to the 

 pressure within the thorax, it is the intrathoracic pressure that is 

 meant, that is, the pressure in the pleural cavity and mediastinal 

 spaces. This pressure, under normal conditions, is always negative, 

 that is, is always less than one atmosphere. The reason for this 

 is simply that the lungs are distended to fill the thoracic cavity, and 

 consequently the organs, like the heart, which lie in this cavity 

 outside the lungs, are exposed to a pressure of one atmosphere, 

 minus the force of elastic recoil of the lungs (see Fig. 241) . The heart 

 and other intrathoracic organs are protected from the direct pres- 

 sure of the air by the thoracic walls; they are pressed upon, how- 

 ever, through the lungs, but naturally the atmospheric pressure is 

 reduced by an amount equal to the elastic force of the distended 

 lungs. Intrathoracic pressure, in fact, may be defined as intra- 

 pulmonic pressure minus the elastic pull of the lungs, and since 

 under usual conditions the intrapulmonic pressure is equal to that 



