The Heart – The Engine of Life
The Sinus Node Sets the Heartbeat
Two sounds that can be heard with a stethoscope make up the heartbeat. One sound – the systole phase – is the heart chambers contracting and pumping blood into both circulatory systems. This lasts about one third of a second. The diastole phase – the second sound – is the atria contracting when the lower heart chambers are empty. It lasts approximately two thirds of a second. When the atria are full, the lower chambers are empty, and vice versa.
The sinus node, a plexus in the right atrium, starts the contractions. From there, the impulse travels into the chambers. These electrical currents can be seen with an electrocardiogram (ECG or EKG). The physician uses the ECG to evaluate if the heart is healthy and working well, and see if the patient has previously suffered a heart attack.
The brain and nervous system control the heart. The electrical impulses that create the heartbeat come from the heart itself. The heart will beat as long as it is supplied with oxygen, even if that oxygen is supplied externally, for example during a heart transplant.