360 



PHYSIOLOGY. 



outside the chest exceeds the air-pressure within the chest. Hence a 

 larger amount of blood enters the right auricle during inspiration. 

 This being ejected by the right ventricle, the pulmonary capillaries, 

 having less pressure externally, let the blood pass in larger quan- 



Fig. 128. 



tity and the left ventricle forces out more blood into the aorta and the 

 arterial blood-pressure rises. During expiration, the pressure on the 

 heart and blood-vessels inside the chest returns to normal ; hence the 

 atmospheric pressure outside the chest does not drive the blood from 

 the veins into the chest, as in inspiration, hence less blood goes into 

 the right side of the heart, and, as the pulmonary vessels are also 



Fig. 129. Comparison of Blood-Pressure Curve with Curve of Intra- 

 thoracic Pressure. (M. FOSTER.) 



(To be read from left to right.) a is the curve of blood-pressure with its respira- 

 tory undulations, the slower beats on the descent being very marked ; 6 is the curve of 

 the intra-thoracic pressure obtained by connecting one limb of a manometer with the 

 pleural cavity. Inspiration begins at ?- and expiration at e. The intra-thoracic pressure 

 rises very rapidly after the cessation of the inspiratory effort, and then slowly falls as 

 the air issues from the chest ; at the beginning of the inspiratory effort the fall becomes 

 more rapid. 



pressed upon, a less quantity of blood goes to the left ventricle and 

 out into the aorta ; hence a fall of blood-pressure. 



The pulmonary capillaries in the lungs contain more blood in 

 inspiration because the inspiratory act tends to dilate them. The 



