400 THE HUMAN BODY 



tory movements influence the circulation of the blood and the 

 flow of the lymph. 



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 equilibrium with the atmospheric pressure exerted 

 on the blood-vessels of the neck and abdomen. If an inspiration 

 now occurs, the chest cavity being enlarged the pressure on all of 

 its contents will be diminished. In consequence, air enters the 

 lungs from the windpipe, and blood enters the venae caya? and the 

 right auricle of the heart from the outlying veins. When the 

 next expiration occurs, and the pressure in the thorax again rises, 

 air and blood both tend to be expelled from the cavity. What- 

 ever extra blood has, to use the common phrase, been " sucked " 

 into the intrathoracic venae cavaB in inspiration and has not been 

 sent already on into the right ventricle before expiration occurs, 

 is, however, on account of the venous valves, prevented from 

 flowing back whence it came, and is imprisoned in the cavse under 

 an increased pressure during expiration; and this tends to make 

 it flow faster into the auricle during the diastole of the latter. How 

 much the alternating respiratory movements assist the venous 

 flow is shown by the dilatation of the veins of the head and neck 

 which occurs when a person is holding his breath; and the black- 

 ness of the face, from distention of the veins and stagnation of 

 the capillary flow, which occurs during a prolonged fit of cough- 

 ing, which is a series of expiratory efforts without any inspira- 

 tions. 



The vencricles and arteries are not directly affected to any 

 appreciable extent by the respiratory movements; their walls 

 are too thick and the arterial pressure too great to respond to 

 these small variations of intrathoracic pressure. The increase 

 in venous flow which occurs during inspiration does, however, 

 by supplying the heart with more blood at that time, bring about 

 a small increase in arterial pressure during each inspiration. The 

 increased blood-supply is handled by the heart through an aug- 

 mentation of its beat. This has been shown to be brought about 

 by an irradiation to the cardiac centers of the influences that 

 govern the act of inspiration. To a marked extent the vigor of 

 breathing and the heart-rate run parallel. 



