RESPIRATION. 427 



tion sustain a positive pressure, the intra-thoracic pressure (see page 386), 

 about six millimeters of mercury less than that in the lungs, or, in other words, 

 a pressure negative to that of the atmosphere by six millimeters. As a result 

 the blood in the systemic vessels under atmospheric pressure will flow 

 steadily toward the intra-thoracic veins, the venae cavae, and the right 

 side of the heart, i.e., from a point of high to a point of low pressure. During 

 inspiration there is a decrease in the intra-thoracic pressure, the decrease 

 being proportional to the extent of the inspiration. With this decrease 

 of pressure, the intra-thoracic veins expand and their internal pressure falls. 

 As the systemic or extra- thoracic veins are subjected to atmospheric pressure, 

 the blood in these vessels is forced, by reason of the difference of pressure 

 between these two regions, to flow more rapidly and freely into the intra- 

 thoracic veins and right side of the heart. The right heart being more 

 generously filled with blood will discharge a larger volume with each con- 

 traction into the pulmonary artery. 



Coincident with these effects a similar effect is produced in the arterioles 

 and capillaries of the pulmonary alveoli. Situated between the two elastic 

 layers of the alveolar wall, embedded in a meshwork of connective tissue, 

 the pressure to which they are subjected at the end of an expiration will 

 also be a few millimeters less than the intra-pulmonic pressure; and at the 

 end of an inspiration it will be considerably less. With the inspiration there- 

 fore there will occur a dilatation of these vessels, and hence a larger flow of 

 blood through them and into the pulmonary veins. The left heart, being 

 in consequence more generously filled with blood, will discharge a larger 

 volume into the aorta at each contraction. During expiration the flow of 

 blood through the intra-thoracic vessels will be diminished for the reverse 

 reasons. 



2. On the Arterial Pressure. An examination of a tracing of the arterial 

 pressure will show that it is characterized by small undulations due to the 

 cardiac beat and large undulations due to the respiratory movements, 

 inspiration being accompanied by a rise, expiration by a fall of pressure. 

 These results are readily accounted for by the difference in the volume of 

 blood discharged by the left heart into the aorta during the time of the two 

 movements. If a tracing of the respiratory movements and of the blood- 

 pressure be taken simultaneously, it will be found that the rise of pressure 

 does not exactly correspond with inspiration, nor the fall of pressure with 

 expiration; for a certain period after the beginning of an inspiration the 

 pressure continues to fall, and for a certain period after the beginning of an 

 expiration the pressure continues to rise. During the remainder of the 

 period, however, inspiration is attended by a rise, expiration by a fall of 

 pressure. The explanation of these results lies in the fact that at the begin- 

 ning of the inspiration, when the vessels dilate, the blood-flow momentarily 

 slows; the left heart continuing to discharge small volumes into the aorta, 

 the pressure continues to fall. So soon as the left heart begins to be better 

 filled, the pressure at once begins to rise. At the end of an expiration the 

 flow of blood into the left heart continues and the pressure rises, but with the 

 return of the intra-thoracic pressure the vessels diminish in caliber, the 

 volume of blood transmitted by them becomes less, the output of the left heart 

 declines, and the pressure falls. 



