368 THE HUMAN BODY. 



Influence of the Respiratory Movements upon the 

 Circulation. Suppose the chest in a condition of normal 

 expiration and the external pressure on the blood in the 

 blood-vessels within it and in the heart, to have come, in 

 the manner pointed out in the last paragraph, into equili- 

 brium with the atmospheric pressure exerted on the blood- 

 vessels of the neck and abdomen. If an inspiration now oc- 

 curs, the chest cavity being enlarged the pressure on all of its 

 contents will be diminished. Jn consequence, air enters 

 the lungs from \he windpipe, and blood enters the venae 

 cavae and the right auricle of the heart. Thus not only the 

 lungs, but the right side of the heart, and the intra- thoracic 

 portions of the systemic veins leading to it, are expanded 

 during an inspiration; but the lungs being much the most 

 distensible take far the greatest part in filling up the in- 

 creased space. The left side of the heart is not much in- 

 fluenced as it is filled from the pulmonary veins; and the 

 whole vessels of the lesser circulation lying within the 

 chest, and being all affected in the same way at the same 

 time, the blood-flow in them is not influenced by the aspi- 

 ration of the thorax. Distension of the lungs seems, how- 

 ever, to diminish the capacity of their vessels, and so to a 

 certain extent the flow is influenced; as the lungs expand 

 blood is forced out of their vessels into the left auricle, and 

 when they again contract their vessels fill up from the right 

 ventricle. The pressure on the thoracic aorta being dimin- 

 ished in inspiration, blood tends to flow back into it from 

 the abdominal portion of the vessel, but cannot enter the 

 heart on account of the semilunar valves; and the back-flow 

 does not in any case equal the onflow due to the beat of the 

 heart; so what happens in the aorta is but a slight 

 slowing of the current. The general result of all this is 

 that the circulation is considerably assisted. When the 

 next expiration occurs, and the pressure in the thorax again 

 rises, air and blood both tend to be expelled from the cavity. 

 The aorta thus regains what it lost during inspiration; the 

 pressure on it is increased and it empties itself faster into 

 its abdominal portion. The semilunar valves having pre- 



