RESPIRATION 251 



end of the inspiration, since the pulmonary circulation-time 

 (four to five seconds in a small dog, two to three seconds 

 in a rabbit) is longer than the time of a complete inspira- 

 tion at any ordinary rate. The increase in the quantity of 

 blood pumped into the pulmonary artery will, if not counter- 

 acted by other circumstances, tend to raise the blood- 

 pressure in the artery and its branches, and therefore at 

 once to accelerate the outflow through the pulmonary vein. 

 This will be greatly aided if at the same time the vascular 

 resistance in the lungs is reduced, as there seems good reason 

 for believing is the case. 



The increased blood-flow into the left ventricle will of 

 course correspond to better filling of the systemic arteries ; 

 that is, to a rise of arterial blood-pressure. 



In expiration the contrary will happen. The return of 

 blood to the thorax will be checked. This is well shown by 

 the swelling of the veins at the root of the neck in expiration, 

 their shrinking in inspiration, the so-called pulsus venosus. 

 Less blood being drawn into the right heart, less will be 

 pumped into the pulmonary artery, in which the pressure 

 will, of course, fall. The outflow into the left auricle will 

 thus be diminished all the more as in the expiratory phase 

 the vascular resistance in the lungs is increased and the 

 systemic arterial pressure will be lowered. Now, this is 

 just what is seen on the blood-pressure curve, except that 

 in both cases the change is somewhat belated, and does not 

 coincide exactly with the commencement of the inspiration 

 or the expiration. But this delay may be explained on 

 several grounds. First, we cannot expect the curve of 

 pressure to alter its course quite suddenly, at the very 

 moment when the respiration changes its phase ; for the 

 change in the blood-flow through the lungs must require 

 time to establish itself, in the face of the opposite tendency 

 to which it succeeds. The same is true of the systemic 

 arteries, in which at the end of expiration the movements of 

 the blood associated with the falling pressure are going on. 

 It is impossible that these movements can be checked at 

 once ; inertia must carry them on into inspiration. 



The negative pressure of the thorax acts also on the 



