THE MECHANISM OF THE HEART PUMP 1009 



the restraining influence of the pericardium can be effective. The con- 

 traction of the auricles drives their contents into the ventricles and so 

 still further increases their distension, no resistance being offered by the 

 widely dilated auriculo-ventricular orifices or by the flaccid wall of the 

 ventricles. As the blood rushes from auricle into ventricle through the 

 funnel-shaped opening of the membranous tube formed by the valves, 

 eddies are set up in the ventricle tending to close the valves, so that they 

 are held, as the resultant of the two opposing currents, in a condition 

 midway between closure and opening. The onset of the ventricular 

 contraction is extremely rapid. There is a quick rise of pressure in 

 the ventricle, which presses together the flaps of the mitral or tricuspid 

 valves, while the bases of these valves are approximated by the con- 

 traction of the circular fibres at the base of the ventricles. As the 

 heart shortens in systole the papillary muscles also shorten, so that 

 the valves are drawn further down into the ventricles and prevented 

 from eversion into the auricles, while the blood is pressed, so to speak, 

 between the cone of the ventricular wall and the cone formed by the 

 tubular valves. 



The outflow of blood from the ventricles does not, however, com- 

 mence immediately. Whereas at the beginning of systole the pressure 

 in the ventricle cavity is quite small (only 2 or 3 mm. Hg), there is a 

 pressure in the aorta of 50 to 80 mm. Hg. Before the semilunar 

 valves separating the lumen of the aorta from the ventricular cavity 

 can be opened, the pressure in the left ventricle must rise to a point 

 which is greater than that in the aorta, and similarly on the right side 

 of the heart. As soon as this happens the valves open and the out- 

 flow of blood commences, and continues so long as the pressure in the 

 ventricles is higher than that in the great arteries. Directly, however, 

 the ventricular pressure falls below the arterial pressure the valves must 

 close and the output of blood come to an end. 



In order to obtain an accurate idea of the exact duration of each 

 of these events in the cardiac cycle it is necessary to study the changes 

 occurring in the pressure within the auricles and ventricles during the 

 various phases of the heart-beat. 



THE ENDOCARDIAC PRESSURE 



A manometer which shall register accurately the changes in the 

 pressure within the heart must be capable of responding to very 

 rapid changes. Thus in the left ventricle at the beginning of 

 the systole there may be a rise of 130 mm. Hg in -06 sec., i.e. 

 2170 mm. Hg per second. A mercurial manometer with its great 

 inertia would be quite unequal to registering such rapid changes of 

 pressure and would moreover tend to enter into oscillations which would 

 quite deform the curve. We require an instrument with very small 



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