Valves—Bi and Tricuspid. During auricular contraction the force of the blood propelled into the ventricle is transmitted in all directions, but being insufficient to raise the semilunar valves, it is expended in distending the ventricle and in raising and gradually closing the auriculo-ventricular valves (tricuspid and bicuspid valves). These when the ventricle is full form a complete septum (partition) between it and the auricle.
The arterial or semilunar valves are brought into action by the pressure of the arterial blood forced back towards the ventricles, when the elastic walls of the arteries recoil after being dilated by the blood propelled into them in the previous contraction of the ventricle.
The sounds. When the ear is placed over the region of the heart two sounds may be heard at every beat of the heart, which follow in quick succession, and are succeeded by a pause or a period of silence. The first sound is dull and prolonged; its commencement coincides with the impulse of the heart and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a somewhat flapping character, and follows close after the arterial pulse.
First sound. The chief cause of the first sound of the heart appears to be the vibration of the auriculo-ventricular valve, and also, but to a less extent, of the ventricular walls, and the coats of the aorta and pulmonary artery, all of which parts are suddenly put into a state of tension at the moment of ventricular contraction.
The second sound is more complete than that of the first. It is probably due entirely to the sudden closure and consequent vibration of the semilunar valves when they are pressed down across the orifice of the aorta and pulmonary artery.
Pulse. The heart of a healthy adult man in the middle period of life acts from seventy to seventy-five times per minute. The frequency of the heart’s action gradually diminishes from the commencement to near the end of life.
In persons of sanguine temperament, the heart acts somewhat more frequently than in those of the phlegmatic; and in the female sex more frequently than in the male; in children, more frequently still.
Capacity. The capacity of the two ventricles is probably exactly the same. From the mean of various estimates taken, it may be inferred that each ventricle is able to contain on an average about three ounces of blood, the whole of which is impelled into the respective arteries at each contraction.
Every time the ventricles contract three ounces of blood are pumped out of the heart into the lungs and heart respectively.
Calculating seventy pulses per minute, the quantity of blood passing through the heart would be about 211 ounces, or 14¼ pints per minute; or 895 pints per hour, or 21,480 pints in 24 hours.