394 RESPIRATION. 



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

 and especially during violent and sudden respiratory movements, the influ- 

 ence in this direction on the blood-pressure curve of the pumping action of 

 the chest is unmistakable. 



In attempting, however, to estimate the effect of the respiratory move- 

 ments 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 in 

 inspiration, first tend to drive the blood onward along the vena cava, but sub- 

 sequently 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 dif- 

 ficult to predict what in all cases the effect would be ; and the matter can- 

 not be settled by eliminating the movements of the diaphragm through sec- 

 tion of the phrenic nerves, since in such a case the whole working of the 

 respiratory pump is materially affected. 



326. In addition to the influence thus exerted by the thoracic move- 

 ments on the great veins leading to and the great arteries leading from the 

 heart, we have to consider the behavior of the pulmonary vessels themselves 

 under the varying thoracic pressure. These, like the vense cava3 and aorta, 

 tend to expand under the influence of the inspiratory expansion of the chest, 

 and thus to become fuller of blood, very much as they would if the whole 

 lung were placed under a large cupping-glass. The first effect of this in- 

 creased filling of the pulmonary vessels would be to retain for a while a 

 certain quantity of blood in the lungs and thus to lessen the amount falling 

 into the left auricle. But this would be temporary only, and the widening 

 of the pulmonary vessels would speedily produce an exactly contrary effect, 

 namely, an increased flow through the lungs due to the diminished resistance 

 offered by the widened passages. Conversely, the first effect of expiration 

 would be an increased flow into the left auricle due to the additional quan- 

 tity of blood driven onward by the partial collapse of the pulmonary ves- 

 sels, followed by a more significant diminished flow caused by the greater 

 resistance now offered by the narrower vascular channels. Thus the effect 

 of inspiration in this way would be first to diminish the flow into the left 

 auricle and so into the left ventricle, but afterward, for the rest of the 

 inspiration until the beginning of expiration, to increase the flow into the 

 ventricle ; while conversely the effect of expiration would be first, for a 

 brief period, to increase, and afterward, during the rest of the movement, 

 to diminish the flow of blood into the left ventricle. Further, while this 

 may be considered as the effect on the pulmonary vessels, large and small 

 taken altogether, the influence both of the thoracic negative pressure during 

 inspiration, and the return in a positive direction during expiration, will bear 

 more on the thin-walled pulmonary veins than on the stouter pulmonary 

 artery ; that is to say, as inspiration becomes established, there will be a 

 diminution of pressure'in the pulmonary veins greater than that in the pul- 

 monary artery, and this will be an additional influence favoring the flow into 

 the left ventricle ; during expiration a similar difference of effect will be 

 felt in the contrary direction. During the increase of flow into the ventricle 



