THE CIRCULATION 173 



be suddenly set up in the erect position the blood may 

 settle down into the most dependent parts by hypostasis, 

 and the heart stop owing to its being no longer kept 

 filled with blood. This has already been referred to 

 when the mechanism of shock was described (p. 168). 



The less the hsemodynamic pressure, the more pro- 

 nounced is the effect of the haemostatic pressure. Hence 

 sudden changes of position are most dangerous in those 

 whose arterial blood-pressure is low. 



Influence of Respiration upon the Circulation. 



Owing to the negative pressure within the thorax 

 (p. 190), the heart and part of the large veins are really 

 placed in a partial vacuum, which has the effect of 

 helping to fill the heart during diastole. By the act 

 of inspiration this suction effect is greatly increased, 

 whilst at the same time blood is squeezed out of the 

 large abdominal veins into the inferior vena cava. As 

 a result the current of blood in the large veins is 

 accelerated, and the right heart more rapidly filled. 

 At the same time the pathway for the blood through 

 the lungs is opened up, and the escape of blood from 

 the right side round to the left is greatly facilitated. It 

 can readily be understood from this what a great aid the 

 action of the respiratory pump is to the proper carrying 

 on of the circulation. If, however, the action of the 

 respiratory pump be interfered with, as it is, for example, 

 in emphysema, the circulation is apt to become laboured, 

 and the large veins engorged with blood. One conse- 

 quence of the orthopnoea of cardiac disease is to facilitate 



