EFFECT OF RESPIRATION ON THE CIRCULATION 



281 



its minimum is less by from 7 to 30 mm. than the normal atmospheric pres- 

 sure, 760 mm. of mercury. It will be understood from the accompanying 

 diagram how an increase in the volume of the thorax will have the effect of 

 pumping blood into the heart from the veins. During inspiration the pressure 

 outside the heart and great vessels is diminished, and they, by virtue of their 

 elasticity, have therefore a tendency to expand and to diminish the intra- 

 vascular pressure. The diminution of pressure within the veins passing 

 to the right auricle and within the right auricle itself, will draw the blood 

 into the thorax, and so assist the circulation. This suction action of the thorax 

 is the cause of the slight negative pressure of the ventricle previously de- 

 scribed. The effect of more blood in the right auricle will, cczteris paribus, 

 increase the amount passing through the right ventricle, and through 

 the lungs into the left auricle and ventricle, and thus into the aorta. 

 This all tends to increase the blood pressure. The effect of the 



FIG. 240. Diagram of an Apparatus Illustrating the Effect of Inspiration upon the M >art and 

 Great Vessels within the Thorax. I, The thorax at rest; II, during inspiration; D represents the 

 diaphragm when relaxed; D', when contracted (it must be remembered that this position is a mere 

 diagram), i.e., when the capacity of the thorax is enlarged; H, the heart; V, the veins entering 

 it, and A, the aorta; Rl, LI, the right and left lung; T, the trachea; M, mercurial manometer in 

 connection with pleura. The increase in the capacity 9f the box representing the thorax is seen to 

 dilate the heart as well as the lungs, and so to pump in blood through V, whereas the valve pre- 

 vents reflux through A . The position of the mercury in M shows also the suction which is taking 

 place. (Landois.) 



diminished pressure upon the pulmonary vessels will also help toward the 

 same end, an increased flow through the lungs, so that, as far as the mechani- 

 cal effects on the heart and its veins are concerned, inspiration increases 

 the blood pressure in the arteries. The effect of inspiration upon the aorta 

 and its branches within the thorax would be, however, contrary; for as the 

 external pressure is diminished, the vessels would tend to expand, and thus 

 to diminish the tension of the blood within them, but, inasmuch as the rela- 

 tive variation in pressure on the large arteries is slight, the diminution of 

 arterial tension caused bv this means will be insufficient to counteract the 



