RESPIRATION AND BLOOD-PRESSURE 165 



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 consequently less tendency for 

 the arterial blood to regurgitate into the thoracic aorta than for 

 the venous blood to enter the thoracic venae cavae. 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 

 pulmonary 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 thoracic venae 

 cavae. If the veins be less dilated by inspiration than the artery, 

 then they will receive an increase of blood which will not com- 

 pletely occupy the increase of space in the dilated thoracic aorta, 

 and there will be a backward " suction" made upon the contents 

 of the arterial tree with a consequent decrease in pressure; but 

 a condition just opposite to this seems to obtain. 



During expiration contrary conditions in general are opera- 

 tive with contrary results. The intrapulmonary vessels and 

 heart are compressed, but the veins and capillaries more than 

 the aorta, with the result that less blood reaches the heart than 



