The cardiac cycle is a crucial process that involves the rhythmic contraction and relaxation of the heart, specifically the ventricles and atria. It begins with diastole, the phase where the ventricles and atria relax, allowing them to fill with blood. This filling is essential for the subsequent phases of the cycle.

As the cycle progresses, the sinoatrial (SA) node initiates an action potential, leading to atrial systole, where the right and left atria contract to push blood into the ventricles. This contraction is followed by a brief delay caused by the atrioventricular (AV) node, which ensures that the atria have completely emptied their blood into the ventricles before the ventricles contract.

During systole, the action potential travels through the Purkinje fibers, causing the ventricles to contract and eject blood into the pulmonary artery and aorta. This contraction is represented by a significant depolarization on the electrocardiogram (EKG), indicating the powerful electrical signal associated with ventricular contraction. Following systole, the heart returns to diastole, allowing the atria and ventricles to refill with blood, which is also reflected in the EKG.

To quantify the efficiency of the heart's pumping action, we measure cardiac output, defined as the volume of blood pumped by a ventricle per minute. Cardiac output is determined by two key factors: heart rate and stroke volume. Heart rate, measured in beats per minute (BPM), typically averages around 60 BPM at rest. Stroke volume refers to the amount of blood ejected by a single ventricle contraction. By multiplying heart rate by stroke volume, we can calculate cardiac output, providing insight into the heart's performance and overall cardiovascular health.