268 A MANUAL OF PHYSIOLOGY. 



thickness of its walls, to a smaller extent than on the 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 may not be great, they will, of course, be better 

 marked in small animals with comparatively yielding arteries 

 than in large animals. Yet, whether great or small, 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 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 is found in the changes of vascular 

 capacity which take place in the lungs when they pass from 

 the expanded to the collapsed condition. The expansion of 

 the lungs in natural respiration causes a widening of the pul- 

 monary capillaries, with a consequent increase of their capacity 

 and diminution of their resistance. When the vessels at the 

 base of the heart are 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 is found that they invariably contain more blood 

 in inspiration than in expiration. 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 fall of arterial pressure. As soon as a 

 more copious stream begins to flow through the lungs, this is 

 succeeded by a rise. In like manner the first effect of expiration, 

 which increases the resistance and diminishes the capacity of 

 the pulmonary vessels, is to force out of the lungs into the left 

 auricle the blood for which there is no room. This causes a 

 rise of arterial blood-pressure, succeeded by a fall as soon as 

 the lessened blood-flow through the lungs is established. 



The changes in the diastolic capacity of the chambers of the 

 heart itself, with the changes of pericardial pressure, must also 

 act in the same direction. It is obvious, then, how greatly the 

 rate and depth of respiration in relation to the size of the animal 

 and the other circumstances already mentioned may influence 

 the time relations of the respiratory oscillations in the arterial 



