RESPIRATION. 361 



effect of the distension is to increase the flow of blood in the lungs, 

 because the widened arterioles decrease the resistance to the flow. In 

 expiration, the pulmonary capillaries are lessened in diameter, and 

 the narrowing of the capillaries increases the resistance to the flow of 

 blood. Hence it is the quantity of blood in the left ventricle which, 

 during inspiration and expiration, elevates and depresses the blood- 

 pressure. 



Wherefore, on making a tracing of both the respiratory move- 

 ments and the blood-pressure, it is discovered that the blood-pressure 

 falls slightly at the beginning of inspiration, but rises during the 

 rest of the movement. At the beginning of expiration the pressure 

 continues to rise for a short time, and then falls during the rest 

 of the act. 



The arteries and veins are differently affected by the respira- 

 tory movements. According to Foster, the arch of the aorta has an 



Fig. 130. 



inclination to expand, from the decrease of intra-pleural pressure in 

 the thorax during inspiration, which temporarily retards the flow of 

 blood and diminishes aortic pressure. 



The aorta during expiration tends to contract, because expira- 

 tion increases the thoracic pressure outside the aortic arch, which 

 temporarily increases the blood-pressure in the aorta. Hence in in- 

 spiration the arterial pressure temporarily diminishes. During 

 expiration the arterial pressure temporarily increases. 



The blood-vessels of the lungs enlarge during inspiration and 

 thus become more distended with blood, and thus retain for a while 

 a certain quantity of blood in the lungs and thus diminish the 

 amount falling into the left auricle. But this is only temporary, 

 because the widening of the vessels would permit an increased flow 

 of blood in the pulmonary vessels, due to diminished resistance of 

 le dilated passages, and a contrary result would ensue. 



Vice versa, the first effect of expiration would increase the flow 

 the left auricle, due to the additional quantity of blood driven on 

 the partial shrinking of the vessels of the lungs, followed by a 

 >re decided diminished flow caused by great resistance of the con- 



