392 RESPIRATION. 



the beginning of inspiration (i) the arterial pressure is seen to be falling; it 

 soon, however, begins to rise, but does not reach the maximum until some 

 time after expiration (e) has begun ; the fall continues during the remainder 

 of expiration, and passes on into the succeeding inspiration. This suggests 

 the idea that, while inspiration tends to increase and expiration to diminish 

 the blood-pressure, there are causes at work which in each case delay the 

 effect. 



Extended observations, however, show that such a relation as that shown 

 in the figure, though frequent, is not constant. In fact, the effects of the 

 respiratory movements on blood-pressure are found to vary very widely 

 according as the respiration is quick or slow, easy and shallow, or labored 

 and deep, and especially as the air enters into the chest readily or with diffi- 

 culty. Moreover, respiratory undulations of blood-pressure are seen not 

 only with natural but also with artificial respiration ; in the latter the 

 mechanical conditions are to a large extent the reverse of those of the former, 

 and might fairly be expected to affect the circulation in a different way. 

 The causation of these respiratory undulations is, in fact, complex. The 

 respiratory act affects the vascular system in several different ways, and the 

 general effect varies according as one or other influence is predominant. 

 These several actions are sufficiently interesting and important to deserve 

 discussion. 



325. The heart and great bloodvessels are, like the lungs, placed in the 

 air-tight thoracic cavity, and are subject like the lungs to the pumping action 

 of the respiratory movements. Were there no lungs present in the chest, 

 the whole force of the expansion of the thorax in inspiration would be 

 directed to drawing blood from the extra-thoracic vessels toward the heart, 

 and conversely in expiration the effect of the return of the thorax to its 

 previous dimensions would be to drive the blood thus drawn in back again 

 from the heart toward the extra-thoracic vessels. And, even in the presence 

 of the lungs, some of this effect is still felt. The main purpose and the main 

 result of the expansion of the chest in inspiration is, of course, to draw air 

 into the lungs ; by that expansion the air in the pulmonary alveoli is rare- 

 fied and brought to a lower pressure than that of the atmosphere outside the 

 chest ; and the difference of pressure thus set up leads to an inrush of 

 inspired air until an equilibrium of pressure is established between the air 

 in the lungs and that outside the chest. Before, however, the inspired air 

 can fill a pulmonary alveolus the elastic walls of the alveolus have to be dis- 

 tended, and that distention is effected by means of the pressure which causes 

 the inspired air to enter. Part of the atmospheric pressure, in fact, which 

 causes the entrance of the air into the lung is spent in overcoming the elas- 

 ticity of the pulmonary passages and cells. So that while by the inrush of 

 inspired air the difference of pressure between the air inside the pulmonary 

 alveoli and that outside the chest, brought about by the thoracic expansion, 

 is completely neutralized, the difference between the pressure to which the 

 parts lying within the thorax, but outside the lungs, are exposed and that 

 outside the chest is not so completely neutralized. The pressure on these 

 parts always falls short of the pressure of the atmosphere by the amount of 

 pressure necessary to counterbalance the elasticity of the pulmonary passages 

 and alveoli. Consequently, any structure lying within the thorax, but out- 

 side the lungs, is never, even at the conclusion of an inspiration, when the 

 lungs are filled with air, subject to a pressure as great as that of the atmos- 

 phere. And, since the fraction of the atmospheric pressure which is thus 

 spent in distending the lungs increases as the lungs become more and more 

 stretched, it follows that the fuller the inspiration the greater is the differ- 

 ence between the pressure on structures within the thorax, but outside the 



