PRESSURE IN THE AIR-PASSAGES DURING RESPIRATION. 247 



118. Pressure in the Air-Passages During 



Respiration. 



Normal Respiration. If a manometer be tied into the trachea of an 

 animal, so that the respiration goes on completely undisturbed, during 

 every inspiration there is a negative pressure ( 3 mm. Hg.) and dur- 

 ing expiration a positive pressure (Bonders). Bonders placed the 

 U-shaped manometer tube in one nostril, closed his mouth, leaving 

 the other nostril open, and respired quietly. Buring every quiet 

 inspiration, the mercury showed a negative pressure of 1 mm., and 

 during expiration a positive pressure of 2-3 mm. (Hg.) 



Forced Eespiration. As soon as the air was inspired or expired 

 with greater force, the variations in pressure became very much greater, 

 e.g., during speaking, singing, and coughing. The inspiratory pressure 

 was= 57 mm. (36-74). the greatest expiratory pressure-}- 87 (82-100) 

 mm. Hg. (Bonders). The pressure of forced expiration therefore, is 30 

 mm. greater than the inspiratory pressure. 



Resistance to Inspiration. Notwithstanding this, we must not con- 

 clude that the expiratory muscles act more powerfully than the inspira- 

 tory; for during inspiration, a variety of resistances has to be overcome, 

 so that after these have been met, there is only a residue of the 

 force for the aspiration of the mercury. The resistances to be overcome 

 by the inspiratory muscles are: (1.) The elastic tension of the lungs, 

 which during the deepest expirations =r 6 mm. ; during the deepest in- 

 spirations =30 mm. Hg. ( 107). (2.) The raising of the weight of the 

 chest. (3.) The elastic torsion of the costal cartilages. (4.) The depression 

 of the abdominal contents, and the elastic distension of the abdominal 

 walls. All these not inconsiderable resistances, which the inspiratory 

 muscles have to overcome, act during expiration, and aid the expiratory 

 muscles. The forces concerned in inspiration are decidedly much greater 

 than those of expiration. 



As the lungs within the chest, in virtue of their elasticity, con- 

 tinually strive to collapse, necessarily they must cause a negative 

 pressure within the chest. This amounts in dogs during inspiration, 

 to 7'1 to 7*5 mm. Hg., and during expiration to 4 mm. Hg. (Heynsius). 

 The analogous values for man have been estimated at 4*5 mm. Hg. and 

 3 mm. Hg., by Hutchinson. 



Even the greatest inspiratory or expiratory pressure is always much less than the 

 blood-pressure in the large arteries; but if the pressure be calculated upon the 

 entire respiratory surface of the thorax, very considerable results are obtained. 



Effects of the first Respiration on the Thorax. Until birth, the airless 

 lungs are completely collapsed (atelectic) within the chest, and fill it, so that on 

 opening the chest in a dead fcetus, pneumo-thorax does not occur (Bernstein). 



