Heart rate

An electrocardiogram (ECG) cycle consists of 5 waves: PQRST.

The P wave corresponds to the atrial depolarization and the consequent pumping of blood from the atrium to the ventricle. The QRS complex corresponds to the depolarization of the ventricle and the consequent pumping of blood to the body and lungs. The T wave corresponds to the repolarization of the ventricle and its recovery for the next cycle.

Figure 1 - PQRST wave

Heart frequency or heart rate is based on the frequency of ventricular contraction. One special adaptation that seals have to dive for long periods of time is their ability to lower the heart frequency, thus reducing their oxygen consumption, metabolism and saving energy.