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THE CIRCULATION OF THE BLOOD AND LYMPH 



Endocardiac Pressure. The function of the heart is to maintain 

 an excess of pressure in the aorta and pulmonary artery sufficient to 

 overcome the friction of the whole vascular channel, and to keep up 

 the flow of blood. So long as the semilunar valves are closed, most 

 of the work of the contracting ventricles is expended in raising the 

 pressure of the blood within them. At the moment when blood 

 begins to pass into the arteries, nearly all the energy of this blood is 

 potential; it is the energy of a liquid under pressure. During a 

 cardiac cycle the pressure in the cavities of the heart, or the endo- 

 cardiac pressure, varies from moment to moment, and its variations 



afford important 

 data for the study 

 of the mechanics of 

 the circulation. 



Manometers. For 



the study of the endo- 

 cardiac pressure, the 

 ordinary mercurial 

 manometer (p. no) 

 is unsuitable, since, 

 owing to the rela- 

 tively great amount 

 of work required to 

 produce a given dis- 

 placement of the mer- 

 cury, it does not 

 readily follow rapid 

 changes of pressure, 

 and the mercurial 

 column, once dis- 

 placed, continues for 

 a time to execute 

 vibrations of its own, 

 which are compoun- 

 ded with the true oscillations of blood-pressure. But by introducing 

 in the connection between the manometer and the heart a valve so 

 arranged as to oppose the passage of blood towards the heart, while it 

 favours its passage towards the manometer, the maximum pressure 

 attained in the cardiac cavities during the cycle may be measured with 

 considerable accuracy. When the valve is reversed the apparatus 

 becomes a minimum manometer. In this way it has been found that 

 in large dogs the pressure in the left ventricle may rise as high as 230 

 to 240 mm. of mercury, and sink as low as - 30 to - 50 mm. ; while in 

 the right ventricle it may be as much as 70 mm., and as little as 

 25 mm. In the right auricle a maximum pressure of 20 mm. of 

 mercury has been recorded, and a minimum pressure of - 10 mm. OF 

 even less. But these results were obtained under somewhat exceptional 

 experimental conditions, and the normal maximum pressures in the 

 heart cavities in man are probably not so high, especially in the right 

 auricle and ventricle. 



Our knowledge of the maximum and minimum pressure attained 

 in the cavities of the heart, even if it were far more precise than it 

 actually is, would only carry us a little way in the study of the endo- 

 cardiac pressure-curve, for it would merely tell us how far above the 



Fig. 29. Curves of Endocardiac Pressure taken with 

 Cardiac Sounds. A ur., auricular curve; Vent., ven- 

 tricular curve; AS, period of auricular systole, in- 

 cluding relaxation ; VS, of ventricular systole, including 

 relaxation; D, pause. 



