CHAP, ii.] RESPIRATION. 501 



blood passes through the heart into the aorta, and arterial pres- 

 sure 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 as 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 brains being to in- 

 crease 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 direc- 

 tion on the blood-pressure curve of the pumping action of the 

 chest is unmistakeable. 



In attempting however to estimate the effect of the respira- 

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

 is taking place in the abdomen. In 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 an 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 



