RESPIRA TION 267 



the atmospheric pressure, which is readily transmitted through 

 their thin walls, while the heart and thoracic veins are under 

 a smaller pressure. The venous blood both in inspiration 

 and expiration will, accordingly, tend to be drawn into the right 

 auricle. In inspiration the venous flow will be increased, since 

 the pressure in the thorax, and therefore in the pericardial 

 cavity, is diminished ; and upon the whole more venous blood 

 will pass into the right heart during inspiration than during 

 expiration. Now, the right ventricle is not in general working 

 as hard as it can work. Hence, the excess of blood which 

 reaches it during an inspiration is at once sent into the lungs, 

 although not even the first of it can have passed through to the 

 left side of the heart at the end of the inspiration, since the 

 pulmonary circulation-time (four to five seconds in a small dog, 

 two to three seconds in a rabbit) is longer than the time of a 

 complete inspiration at any ordinary rate. The increase in the 

 quantity of blood pumped into the pulmonary artery will, if not 

 counteracted by other circumstances, tend to raise the blood- 

 pressure in the artery and its branches, and therefore at once to 

 accelerate the outflow through the pulmonary veins. This will 

 be aided if at the same time the vascular resistance in the lungs 

 is reduced, as is known to be the case. The left ventricle, like 

 the right, is capable of discharging more blood than it ordinarily 

 receives. The excess of blood coming to it is easily and promptly 

 ejected. The systemic arteries are better filled and the arterial 

 pressure rises. 



In expiration the contrary will happen. The return of blood 

 to the thorax will be checked. This is well shown by the swelling 

 of the veins at the root of the neck in expiration, their shrinking 

 in- inspiration, the so-called respiratory venous pulse. Less 

 blood being drawn into the right heart, less will be pumped into 

 the pulmonary artery, in which the pressure will, of course, fall. 

 The outflow into the left auricle will thus be diminished all 

 the more as in the expiratory phase the vascular resistance in 

 the lungs is increased and the systemic arterial pressure will 

 be lowered. In both cases, however, the change seen in the 

 blood-pressure curve will be belated, and will not coincide 

 exactly with the commencement of the inspiration or the expira- 

 tion. If it is delayed for a period about equal to the length of 

 an inspiration or an expiration, the blood-pressure will be seen 

 to sink in inspiration and to rise in expiration. If the period of 

 delay is less than this, the pressure will be mounting during a 

 part of each respiratory phase and falling during the rest. As 

 to the explanation of the delay, several factors may be concerned. 



The negative pressure of the thorax acts on the aorta, as well 

 as on the thoracic veins, although, on account of the greater 



