PRESSURE CONDITIONS IN LUNGS AND THORAX. 



611 



volume of blood flowing through the lungs during inspiration. The 

 increased capacity of the lung capillaries during the expansion of 

 the lungs was shown experimentally by Heger-and Spehl. They 

 opened the anterior mediastinum without wounding the pleura and 

 proved that if the lungs are tied off at the end of inspiration they con- 

 tain more blood than when tied off at the end of expiration. The in- 

 creased velocity of the blood-flow through the lungs during inspira- 

 tion is explained by the fact that the greater negative pressure affects 

 the thin- walled pulmonary veins more than the pulmonary artery; 

 consequently the head of pressure driving the blood through the 

 lungs, that is, the difference in pressure between the blood in the 

 pulmonary artery and veins is increased. These data explain 

 satisfactorily the general fact regarding the respiratory waves, 

 namely, that during inspiration there is a rise of aortic pressure due 

 to a greater output of blood from the heart, and during expiration 



Fig. 251. Diagram to represent the time relation between the respiratory waves of 

 blood-pressure and the respiratory movements (dog): A represents the blood-pressure 

 record, showing the heart-beats and the larger respiratory waves. B represents a simul- 

 taneous record of the respiratory movements. At the beginning of inspiration there is a 

 fall of blood-pressure, but the final and main effect is a nse. At the beginning of expi- 

 ration there is a rise of pressure, but the final and main effect is a fall. 



the reverse. To account for the subsidiary fact that at the begin- 

 ning of inspiration the pressure falls and at the beginning of expira- 

 tion it rises for a time two explanations are offered. De Jager 

 refers these temporary effects to the changes in capacity of the blood- 

 bed in the lungs. At the end of inspiration there is a certain ca- 

 pacity of the bed; when expiration comes on, the lungs shrink, the 

 capacity of the blood-vessels is thereby diminished, and consequently 

 some blood is squeezed out of the lungs in the direction of least 

 resistance, that is, toward the left auricle. This accounts for the 

 initial rise of pressure during expiration. At the beginning of inspi- 

 ration, on the other hand, the sudden increase in capillary capacity 

 in the lungs retards for a moment the flow of blood to the left auricle, 

 and thus accounts for the temporary fall of pressure. Tigerstedt,* 

 on the other hand, finds that shutting off the entire circulation of 

 one lung may have little or no influence upon the pressure in the 



* See Tigerstedt, " Ergebnisse der Physiologic, " Vol. ii, part u, 560, 1903. 



