96 THE CIRCULATION OF THE BLOOD 



the blood-pressure is independent of the vagus and is 

 purely mechanical in origin. There is a rise in blood- 

 pressure during inspiration, and a fall during expiration. 

 The blood-pressure and respiratory changes are, however, 

 not synchronous^ — the blood-pressure is at its highest just 

 after the end of inspiration, and at its lowest just after the 

 end of expiration. With the distension of the pulmonary 

 circulation more blood is presented to the left side of the 

 heart, the output of the left ventricle is increased, and the 

 blood-pressure in this way raised. The delay in the rise 

 of pressure is due to the fact that at the beginning of 

 inspiration blood first has to occupy the increased capacity 

 of the pulmonary circulation before it affects the left side 

 of the heart. 



With the diminution in the capacity of the lung capil- 

 laries which occurs in expiration, there is first a further 

 increase of the blood reaching the left auricle. Later, as 

 the piilmonary vessels have constricted, the amount of 

 blood fed to the left side of the heart' is diminished and 

 the blood-pressure falls. 



These effects are reinforced by the movement of the 

 diaphragm. As this muscle contracts it forces blood from 

 the abdomen into the thorax, as already described. 



In man the effects of the respiratory movement upon 

 blood-pressure are exceedingly complex, varying with the 

 form and depth of respiration. 



Whether or no the pulmonary arterioles are subject to 

 nervous control was for long a matter of controversy. 

 By direct stimulation of nerves no positive evidence can 

 be procured. Since, however, the vessels constrict to 

 adrenalin, it is inferred that they receive constrictor fibres 

 from the sympathetic. 



