306 THE KESPIRATION 



thoracic pressure produced by the movements of respiration become 

 more pronounced than under normal conditions. "When the thorax ex- 

 pands with the trachea blocked, the lungs are not able to open up suffi- 

 ciently to fill all the space so that there is excessive dilatation of the 

 veins, auricles and esophagus, as well as drawing in of the intercostal 

 spaces and bulging upwards of the diaphragm. If a manometer is con- 

 nected with the pleural space under these conditions, a very large 

 negative or suction pressure will be observed, amounting often to -70 

 or -80 mm. Hg. It is possible that under such conditions some space 

 might temporarily exist between the parietal and visceral layers of the 

 pleura, but it could not remain long, for it would very soon be filled 

 by fluid exuding from the blood vessels. In the opposite condition, in 

 which the respiratory passages are blocked and a forced expiration is 

 made, as for example in the first stage of coughing or during such acts 

 as defecation and parturition, the thoracic cage is compressed upon the 

 viscera, with the result that the air in the lungs assumes a positive 

 pressure, amounting often to nearly 100 mm. Hg. If a puncture wound 

 is made in the thorax under these conditions, the lungs instead of col- 

 lapsing will bulge out of the wound, for what is really occurring is 

 that the thorax is forcibly contracting on occluded sacs of air. 



It is the alternating changes in intrapleural pressure that are respon- 

 sible for the changes in intrapulmonic pressure and these for the move- 

 ment of air in and out of the lungs with each respiration. In other 

 words, the thorax does not expand on inspiration because air rushes 

 in, as the Uninitiated imagine, but air rushes in because the thorax 

 expands. 



The Influence of Intrapleural Pressure on the Blood Pressure. The 

 movements of respiration produce effects on the vascular system that 

 are of considerable importance in maintaining the circulation of the 

 blood. If an arterial blood-pressure tracing is examined, it will be 

 observed that aside from the cardiac pulsations large waves exist on it that 

 are approximately synchronous with the respiratory movements, the 

 upstroke of each of these waves corresponding in general with inspira- 

 tion, and the downstroke with expiration (Fig. 22). These respiratory 

 variations in blood pressure might be due either to changes in heart 

 rhythm or to a purely mechanical cause. Regarding the first possi- 

 bility, it is indeed the case in most animals that the pulse is quicker on 

 inspiration than on expiration, but that this alone is not an adequate 

 explanation of the rise is shown by the fact that it still persists after 

 the vagus control of the heart has been eliminated, either by cutting 

 the nerve or by the action of atropine. 



The cause must therefore be a mechanical one. Bearing in mind the 



