252 A MANUAL OF PHYSIOLOGY 



aorta, although, on account of the greater thickness of its 

 walls, to a much smaller extent than on the thoracic 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, cannot be great, 

 the tendency will be to diminish the pressure in the one 

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

 of pressure produced by alterations in the flow of venous 

 blood into the chest and through the lungs are thoroughly 

 established, the slight 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 may be found in the changes 

 of vascular resistance and capacity which take place in the 

 lungs when they pass from the expanded to the collapsed 

 condition. 



According to the most careful of recent observations, 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 

 (De Jager). This is supported by experiments on the 

 rabbit, in which the vessels at the base of the heart were 

 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 was found that the lungs invariably contained 

 more blood in inspiration than in expiration (Heger and 

 Spehl). 



During inspiration, as we have seen, the right ventricle 

 is sending an increased supply of blood into the pulmonary 

 artery ; but before any increase in the outflow through the 

 pulmonary veins can take place, the vessels of the lung must 

 be filled to their new capacity. The first effect, then, of the 

 lessened vascular resistance of the lungs in inspiration is a 

 temporary falling off in the outflow through the aorta, and 

 therefore a temporary fall of arterial pressure. As soon as 



