The continuous function of your heart is probably easiest to understand if you break it down into a single unit of pumping action, the heartbeat. A healthy heart beats between 50 and 75 times per minute, so a single heartbeat may occur in less than a second. It involves two distinct phases, the systole and the diastole. The systole is the pumping phase and the diastole is the resting phase.
The systole actually occurs in two sequential pumping actions: the atrial systole and the ventricular systole. The lub-dub that is heard through a stethoscope is the sound of the heart valves closing during the heartbeat’s pumping cycle. The ?rst heart sound, the “lub,” coincides with the closing of the mitral and tricuspid valves. The second heart sound, the “dub,” occurs with the closing of the aortic and pulmonary valves.
A single heartbeat moves a quantity of blood through the heart in two phases: a resting, or dilating (diastolic), phase and a pumping, or squeezing (systolic), phase. During the diastolic phase, the heart relaxes and fills, as oxygen-depleted blood flows into the right atrium from the body, and oxygen-rich blood flows from the lungs into the left atrium. The ventricles fill partially. Then, during the systolic phase (right), an electrical impulse causes the heart to contract. First, the atria contract and completely fill the ventricles with blood. Then the ven- tricles contract, pumping blood out of the heart.
The diastole, the ?rst and longer resting phase, occurs as blood col- lects in the two (right and left) atria. In the right atrium, depleted blood enters from the body, and in the left atrium, oxygen-rich blood ?ows in from the lungs.
Systole begins when an electrical signal from the heart’s pacemaker cells stimulates the atria to contract and empty. The tricuspid and mitral valves open and blood ?ows into the two ventricles.
When the ventricles are full, the electrical impulse passes into an area just above the ventricles and triggers the ventricular systole, the third and ?nal step. All four valves are in action: the tricuspid and mitral valves close to prevent back?ow from the ventricles to the atria, and the pulmonary and aortic valves are pushed open as blood surges out. On the right side of the heart, oxygen-poor blood travels from the right ventricle into the pulmonary artery on its way to the lungs to acquire oxygen. On the left side of the heart, oxygen-enriched blood ?ows from the left ventricle through the aorta and into the general and coronary circulation.
After the blood has left the ventricles, they relax, and the pulmonary and aortic valves close. As the ventricles relax, the pressure in the ven- tricles lowers, allowing the tricuspid and mitral valves to open, and the cycle begins again.
Throughout this cycle, the two adjacent pumps move exactly the same amounts of blood; the volume of blood that enters and leaves the right chambers is the same as the volume that passes through the left chambers. Any change in the amount of blood entering the right side of your heart—in response to exertion, stress, or temperature changes, for example—causes a corresponding change in the amount of blood passing through the left side. Your brain is constantly monitoring the conditions that might require a change in blood supply and adjusting your heart’s function accordingly.
