RESPIRATION. 383 



THE EFFECT OF THE RESPIRATORY MOVEMENTS ON THE FLOW OF 



BLOOD THROUGH THE INTRA-THORACIC VESSELS AND 



ON THE ARTERIAL PRESSURE. 



1 . On the Intra-thoracic Vessels. The forces which cause the 

 air to flow into and out of the lungs will at the same time and in the 

 same way cause the blood of the systemic vessels to flow into, through, 

 and out of the intra- thoracic vessels. From the tendency of the pul- 

 monary elastic tissue to recoil, the blood-vessels in the thorax at the 

 end of an expiration sustain a positive pressure, 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 standing 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. Since during inspiration, with the increasing tendency 

 to pulmonary recoil, the positive pressure on the veins and heart may 

 diminish by thirty millimeters of mercury, the blood will flow in 

 increased volume from the systemic to the intra-thoracic vessels. 

 The right heart, being more generally filled with blood, will discharge 

 a larger volume with each contraction 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 that of 

 the intra-pulmonary air; and at the end of an inspiration it will be 

 considerably less. With the inspiration there will occur a dilatation 

 of the vessels, a larger flow of blood through them and into the pul- 

 monary 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 un- 

 dulations 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 



