RESPIRATION AND BLOOD-PRESSURE 165 



also expands them slightly and tends to draw blood from 

 the extrathoracic into the intrathoracic vessels and 

 heart; in fact inspiration is one of the main forces 

 causing a flow of venous blood toward the 

 heart. Now all this, so far as it goes, tends to keep the 

 blood out of the extrathoracic vessels, and so to contradict 

 the statement that inspiration increases arterial tension. 



But, remembering that we are dealing with arterial ten- 

 sion and that our effort is to prove that more blood is sent 

 into the aorta during inspiration than during expiration, it 

 is of value to note that since the walls of the aorta are more 

 resistant than those of the venae cavae there is less expansion 

 of the former than of the latter during inspiration, and con- 

 sequently less tendency for the arterial blood to regurgitate 

 into the thoracic aorta than for the venous blood to enter 

 the thoracic venae cavse. The same expanding force dilates 

 the pulmonary capillaries, pulmonary artery and pulmonary 

 veins the artery least of these. Taking it for granted that 

 more blood is being received by the right side of the heart 

 from the incoming venae cavae, the somewhat dilated pul- 

 monary artery receives more from the right ventricle ; the 

 pulmonary capillaries are more dilated than the artery and 

 this fact greatly encourages (by a suggestive "suction") the 

 increased flow from the pulmonary artery ; they, therefore, 

 receive more blood than usual. The pulmonary veins, being 

 likewise dilated, exert "suction" upon the capillaries, and 

 thus receive and pass on to the heart a larger supply of 

 blood than usual. The heart, receiving more blood, must 

 send more into the aorta, thereby increasing arterial tension 

 in the extrathoracic vessels, unless, by expansion of the 

 chest, the thoracic aorta be so dilated as to accommodate 

 the increased amount which is not true. 



Then, finally, the validity of this argument will hinge on 

 the relative dilatation of the thoracic aorta and of the thor- 

 acic venae cavae. If the veins be less dilated by inspiration 

 than the artery, then they will receive an increase of blood 



