INFLUENCE OF RESPIRATION ON THE BLOOD-PRESSURE 285 



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 pul- 

 monary artery, in which the pressure will, of course, fall. The out- 

 flow into the left auricle will thus be diminished all the more if 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 co.nmencement of the inspira- 

 tion or the expiration. 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 thickness 

 of its walls, to a smaller extent than on the veins. The diminution 

 of pressure in inspiration tends to expand the thoracic aorta, and to 

 draw blood back out of the systemic arteries, while expiration has 

 the opposite effect. And although the hindrance caused in this way 

 to the flow of blood into the arteries during inspiration, and the 

 acceleration of the flow during expiration may not be great, they 

 will, of course, be better marked in small animals with compara- 

 tively yielding arteries than in large animals. Yet, whether great 

 or small, the tendency will be to diminish the pressure in the one 

 phase and increase it in the other. As soon as the changes of pres- 

 sure produced by alterations in the flow of venous blood into the 

 chest and through the lungs are thoroughly established, the arterial 

 effect will be overborne; but before this happens that is, at the 

 beginning of inspiration and expiration it will be in evidence, and 

 will help to delay the main change. 



Another factor in this delay is found in the changes of vascular 

 capacity which take place in the lungs when they pass from the 

 expanded to the collapsed condition. The expansion of the lungs 

 in natural respiration causes a widening of the pulmonary capillaries, 

 with a consequent increase of their capacity and diminution of their 

 resistance. When the vessels at the base of the heart are ligatured 

 either at the height of inspiration or the end of expiration, so as 

 to obtain the whole of the blood in the lungs, it is found that they 

 invariably contain more blood in inspiration than in expiration. 

 During inspiration, as we have seen, the right ventricle is sending 

 an increased supply of blood into the pulmonary artery ; but before 



