362 PHYSIOLOGY. 



tracted pulmonary vessels. Hence inspiration first diminishes the 

 flow of blood into the left auricle and necessarily in the left ven- 

 tricle, but afterwards for the rest of inspiration, until the beginning 

 of expiration, it increases the flow into the ventricle. 



Vice versa, expiration temporarily, at first, increases and after- 

 wards diminishes the flow of blood into the left ventricle. 



The influence of thoracic negative pressure during inspiration 

 and the return in- a positive direction during expiration will have 

 more effect on the pulmonary veins with their thin walls than on the 

 thicker-walled pulmonary artery that is, during inspiration there 

 will be a diminution of pressure in the pulmonary veins greater than 

 that in the pulmonary artery, and this will be an added influence in 

 favoring the flow into the left ventricle. In expiration, a similar dif- 

 ference will be observed in the contrary direction. The left ventricle, 



Expiration. 

 Inspiration. 



Fig. 131. 



from the increased flow of blood, will throw a larger amount of 

 blood and the arterial pressure will rise. 



The respiratory movement on the vessels of the lungs at the 

 beginning of inspiration will continue the lowering of the blood- 

 pressure which was taking place during expiration, but afterwards 

 will raise it. Vice versa, at the beginning of expiration it continues 

 the rise of arterial pressure which was going on during inspiration, 

 but afterwards lowers the tension in the arteries. Tigerstedt does not 

 believe with De Jager, that changes in the capacity of the pulmonary 

 vessels are the cause of the temporary fall of pressure at the beginning 

 of inspiration and the temporary rise at the beginning of expiration. 

 He found that cutting off half of the pulmonary circulation in an 

 animal breathing naturally caused no fall in the systemic circulation. 

 He believes that the variations in filling of the right heart with blood 

 are the most powerful agents in producing temporary fall of pressure 

 at the beginning of inspiration and a temporary rise at the beginning 

 of expiration. The variations of blood-supply to the right auricle are 

 due to the effect of respiratory movements on the veins outside the 

 chest. 



