Cardiac Cycle

The Cardiac Cycle and Heart Rate

The cardiac cycle is the term used to describe the relaxation and contraction that occur as a heart works to pump blood through the body. Heart rate is a term used to describe the frequency of the cardiac cycle. It is considered one of the four vital signs. Usually it is calculated as the number of contractions or heartbeats of the heart in one minute and expressed as "beats per minute" (bpm). When restin, the adult human heart beats at about 70 bpm (males) and 75 bpm (females), but this rate varies between individuals. However, the reference range is normally between 60 bpm (if less it is termed bradycardia) to 100 bpm (if greater it is termed tachycardia). Resting heart rates can be significantly lower in athletes, and significantly higher in the obese. The body can increase the heart rate in response to a wide variety of conditions in order to increase the cardiac output (the amoount of blood ejected by the heart per unit in time).

Heartbeat Stages

Every single "beat" of the heart includes three major stages: Atrial systole, ventricular systole and complete cardiac diastole. Atrial systole or atrial contraction leads to ventricular filling, followed by ventricular systole when blood is ejected into both the pulmonary artery and aorta. During cardiac diastole chambers are relaxed and filling passively. Throughout the cardiac cycle, the arterial blood pressure increases during the phases of active ventricular contraction and decreases during ventricular filling and atrial systole. As ventricles contract, pressure rises, causing the AV valves to shut forcefully. This prevents 'backflow' of blood from the ventricles into the atria, ensuring a proper directional flow of blood into the large systemic arteries.

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