Chapter 1: The cardiac cycle, an orchestrated electromechanical system

Introduction

The heart is a muscular pump that drives the human circulatory system. With each heartbeat, the ventricles contract to pass on the energy necessary to circulate the blood by generating an arterial blood pressure wave that drives blood flow through the vascular system. The sequence of mechanical (pumping) and electrical events that are repeated with every heartbeat is called the “cardiac cycle”.

Electrical events linked to the cardiac cycle

The cardiac conduction system is a network of nodes, cells, and signals that controls your heartbeat in an orderly fashion. During an average person’s lifespan, the cardiac system is responsible for approximately 2.5 billion heartbeats. The cardiac action potential originates in a group of cells called the sinoatrial (SA) node, located in the right atrium. Normally, these cells depolarise spontaneously and fire off action potentials at a regular, intrinsic rate that is usually between 60 and 100 beats per minute. The electrical impulses are conducted from cell to cell throughout the atria.

The P-wave on an electrocardiogram (ECG) reflects depolarization of the right and left atrial muscle cells called the cardiac myocytes. About one-tenth of a second later, the electrical impulses arrive at the atrioventricular (AV) node. The impulses will then travel from the AV node to the His-Purkinje fiber system, which is a network of specialized conducting cells that carries the signal to the ventricular myocytes. The QRS complex represents the depolarization of the ventricles. Finally, the cells of the His-Purkinje fiber system will repolarize, which resembles the T-wave on an ECG. The consecutive, orchestrated steps within the electrical conduction system result in one cardiac cycle.

The cardiac conduction system sends thousands of signals each day to keep your heart beating. The following section will cover the mechanical events that result from these electrical impulses.

Mechanical events linked to the cardiac cycle

The heart resembles a pump with a reciprocating piston that alternates between a filling phase (diastole), and an emptying phase (systole). The alterations between both phases occur in an orchestrated manner, caused by the signals from the electrical conduction system. During the P-wave, the atria contract, which enhances ventricular filling. After that, the QRS complex triggers the contraction of the ventricles. The well-organized interaction between the myocytes within the ventricular wall results in the ejection of oxygenated blood from the ventricles into the aorta causing arterial blood pressure waves. These pressure waves cause the pulsatile propagation of blood in the circulatory system. In the peripheral arterial system, the pressure waves result in the expansion and relaxation of blood vessels and the delivery of oxygen and nutrients to the tissues and vital organs. At the end of the cardiac cycle, ventricular contraction is followed by ventricular relaxation.

Alterations in the heart rate and contractility of the heart

Under normal circumstances, the body’s own pacemaker, or SA node, determines the duration of the electrical conduction and the cardiac myocytes determine the relative duration of contraction and relaxation. This pattern remains relatively steady as long as the heart rate remains unchanged. The heart rate and intensity of contraction are modified by the sympathetic and parasympathetic divisions of the autonomic nervous system.

The sympathetic system, a network of nerves that helps your body to activate the “fight-or-flight” response, acts as an accelerator, speeding up the heart rate and increasing the contractility of the heart. Whenever more oxygen is necessary (e.g., during exercise or if blood pressure drops) the sympathetic input increases, causing the heart rate and intensity of the contractions to increase.

The parasympathetic input, a network of nerves that relaxes your body after periods of stress or danger, acts like a brake, slowing down the heart. When you relax, the parasympathetic input becomes dominant and the heart rate slows down.

In some individuals, the electrical conduction system is not working correctly which may cause issues related to the heart rate and/or the heart rhythm. Abnormal signaling results in the fact that the heart beats too fast (tachycardia), too slow (bradycardia), or irregularly. One of the most commonly known cardiac arrhythmias is atrial fibrillation, an irregular rhythm that causes impaired blood flow which results in an increased risk to develop stroke.

Key messages

✔️ The sequence of mechanical and electrical events that is repeated with every heartbeat is called the cardiac cycle.

✔️ The P-wave represents atrial contraction and the start of ventricular filling, while the QRS-complex results in the contraction of the ventricles and ejection of blood into the aorta.

✔️ The pulsatile ejection of blood into the circulatory system causes arterial blood pressure waves, resulting in the expansion and relaxation of blood vessels.

✔️ Under normal circumstances, the sinoatrial node determines the duration of the electrical conduction while cardiac myocytes determine the relative duration of contraction and relaxation.

⏩ Learn more about the ECG signal in the next chapter!