THE MECHANISM OF THE HEART PUMP 895 



Each beat of the heart begins by a simultaneous contraction of both 

 auricles associated with a retraction of the auricular appendages, which 

 become pale and bloodless. After a pause of not more than a tenth of a 

 second the contraction of the auricles is followed by that of the ventricles, 

 and blood is thrown out into the large arteries. The contraction of the auricles 

 lasts about a tenth of a second, that of the ventricles about three-tenths of a 

 second. The period of relaxation or diastole lasts about four-tenths of a 

 second. During this cycle of changes the following events are taking place 

 within the heart : 



In the diastolic period the aortic valves are closed and the arterial 

 system is open only towards the capillaries. In consequence of the high 

 pressure established within the arteries by the previous heart-beats the 

 blood flows steadily through the arterioles, capillaries, and veins into the 

 right heart, and similarly the pressure in the pulmonary artery causes a 

 partial emptying of this vessel with its branches through the pulmonary 

 capillaries into the left heart. The flow into the heart is assisted by the 

 elastic retraction of the lungs, which causes a negative pressure in the 

 structures between them and the chest wall, so that the blood is sucked from 

 the other parts of the body towards the thorax. During diastole there is a 

 continuous flow of blood from veins into auricles and from auricles into 

 ventricles, and as the walls of both these cavities are relaxed there is no 

 impediment to the inflow of the blood until the dilating heart begins to 

 stretch the pericardium. 



Under normal circumstances the diastole conies to an end before the 

 restraining influence of the pericardium can be effective. The contraction 

 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 contraction 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 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, commence 

 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 



