290 HANDBOOK OF PHYSIOLOGY. 



the veins is to increase their extra- vascular and so their intra-vascular 

 pressure, and to diminish the flow of blood into the left side of the 

 heart, and with it the general blood-pressure,, but this is almost exactly 

 balanced by the necessary increase of arterial tension caused by the 

 increase of the extra- vascular pressure of the aorta and large arteries, so 

 that the arterial tension is not much affected during expiration either 

 way. Thus, ordinary expiration does not produce a distinct obstruction 

 to the circulation, as even when the expiration is at an end the intra- 

 thoracic pressure is less than the extra-thoracic. 



The effect of violent expiratory efforts, however, has a distinct action 

 in obstructing the current of blood through the lungs, as seen in the 

 blueness of the face from congestion in straining, this condition being 

 produced by pressure on the small pulmonary vessels. 



We may summarize this mechanical effect of respiration on the blood- 

 pressure therefore, and say that inspiration aids the circulation and so 

 increases the arterial tension, and that although expiration does not 

 materially aid the circulation, yet under ordinary conditions neither does 

 it obstruct it. Under extraordinary conditions, however, as in violent 

 expiration, the circulation is decidedly obstructed. 



We have seen, however, that there is no exact correspondence between 

 the point of highest blood -pressure and the end of inspiration, and we 

 must suppose that there are other mechanical factors, such, for example, 

 as the effect of the abdominal movements, both in inspiration and in 

 expiration, upon the arteries and veins within the abdomen and of the 

 lower extremities, and the influence of the varying intrathoracic pres- 

 sure upon the pulmonary vessels, both of which ought to be taken into 

 consideration. As regards the first of these, the effect during inspira- 

 tion as the cavity of the abdomen is diminished by the descent of the 

 diaphragm should be two-fold: on the one hand, blood would be sent 

 upward into the chest by compression of the vena cava inferior; on the 

 other hand, the passage of blood downward from the chest in the 

 abdominal aorta, and upward in the veins of the lower extremity, would 

 be to a certain extent obstructed. In ordinary expiration all this would 

 be reversed, but if the abdominal muscles are violently contracted, as in 

 extraordinary expiration, the same effect would be produced as by in- 

 spiration. The effect of the varying intrathoracic pressure, which occurs 

 during inspiration upon the pulmonary vessels is to produce an initial 

 dilatation of both artery and veins, and this delays for a short time the 

 passage of blood toward the left side of the heart, and the arterial 

 pressure falls, but the fall of blood-pressure is soon followed by a steady 

 rise, since the flow is increased by the initial dilatation of the vessels : 

 the converse is the case with expiration. As, however, the pulmonary 

 veins are more easily dilatable than the pulmonary artery, their greater 

 distensibility increases the flow of blood as inspiration proceeds, while 



