CHAP. IL] RESPIRATION. 649 



consequence less blood passes through the heart into the aorta, 

 and arterial pressure falls again. During forced expiration, the 

 intra-thoracic pressure may be so great as to afford a distinct 

 obstacle to the flow from the veins into the heart. 



The effect of the respiratory movements on the arteries is 

 naturally different from that on the veins. During inspiration 

 the diminution of pressure in the thorax around the aortic arch 

 tends to expand the aortic arch, and thus to check the onward 

 flow of blood, and to diminish the pressure of blood within the 

 aorta. During expiration, the increase of pressure outside the 

 aortic arch of course tends to increase also the blood-pressure 

 within the aorta, acting in fact just in the same way as if the 

 coats of the aorta themselves contracted. Thus so far as arterial 

 blood-pressure is concerned the effects of the respiratory move- 

 ments on the great veins and great arteries respectively are 

 antagonistic to each other ; the effect on the veins being to increase 

 arterial pressure during inspiration and to diminish it during 

 expiration, while the effect on the arteries is to diminish arterial 

 pressure during inspiration and to increase it during expiration. 

 But we should naturally expect the effect on the thin-walled veins 

 to be greater than that on the stout thick-walled arteries, so 

 much so that the direct effect on the arteries may be neglected. 

 That is to say, we should expect the blood-pressure to rise during 

 inspiration and to fall during expiration. This as we have seen 

 is frequently the case, and indeed when the breathing is deep and 

 laboured, and especially during violent and sudden respiratory 

 movements, the influence in this direction on the blood-pressure 

 curve of the pumping action of the chest is unmistakeable. 



In attempting however to estimate the effect of the respiratory 

 movements on blood-pressure we must bear in mind what is 

 taking place in the abdomen. ID inspiration the descent of the 

 diaphragm compresses the abdominal viscera, and so, while at the 

 very first it drives a quantity of blood onward along the inferior 

 vena cava, subsequently hinders the upward flow from the 

 abdomen and lower limbs ; at the same time by compressing the 

 abdominal aorta, it tends to raise the pressure in the thoracic 

 aorta and its branches, while lowering that of the abdominal 

 aorta and its branches. The effect of easy expiration would be 

 the converse of this; but in forced expiration the pressure of 

 the contracting abdominal muscles would, as in inspiration, first 

 tend to drive the blood onward along the vena cava but subse- 

 quently to hinder the flow both along the vena cava and the 

 aorta. The effect of the abdominal movements therefore is 

 mixed and variable, and their influence on the blood-pressure in 

 the femoral artery must be different from that on the radial artery 

 or other branch of the thoracic aorta. It is difficult to predict 

 what in all cases the effect would be ; and the matter cannot be 

 settled by eliminating the movements of the diaphragm through 



F. n. 42 



