Cardiac Conduction System
The cardiac conduction system is a highly specialized and intricate network of nodes, fibers, and pathways embedded within the heart’s muscular tissue. Its primary role is to generate and propagate electrical signals that precisely coordinate the rhythmic contraction and relaxation of the heart chambers. This system ensures that the heart functions as an efficient pump, delivering oxygen-rich blood to the body and returning oxygen-poor blood to the lungs for reoxygenation. The five main components responsible for this electrical activity are the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, right and left bundle branches, and Purkinje fibers.
See also: Anatomy of the heart
1. Sinoatrial (SA) Node: The SA node is often referred to as the heart’s natural pacemaker because it initiates the electrical impulses that set the rhythm for the entire cardiac cycle. It is located in the upper wall of the right atrium, close to the entrance of the superior vena cava. The SA node consists of specialized autorhythmic cells that generate electrical signals spontaneously, typically firing 60 to 100 times per minute in a healthy adult at rest.
Once generated, these impulses rapidly spread through the walls of both atria, causing the right and left atria to contract simultaneously. This atrial contraction (known as atrial systole) helps push blood into the relaxed ventricles, completing the atrial phase of the heartbeat. The impulse then travels toward the atrioventricular node for further conduction.
3. Atrioventricular (AV) Node: Located in the interatrial septum, near the lower part of the right atrium and just above the tricuspid valve, the AV node acts as a gatekeeper of the electrical conduction system. It receives the electrical impulse from the SA node after the atria contract and delays it briefly—typically by about 0.1 seconds.
This brief pause is essential because it allows the ventricles sufficient time to completely fill with blood from the atria before they contract. Without this delay, the ventricles might contract prematurely, leading to inefficient blood flow. Once the delay ends, the AV node sends the impulse to the bundle of His, initiating ventricular contraction.
4. Bundle of His (AV Bundle): The bundle of His, also known as the atrioventricular bundle, is a slender collection of specialized muscle fibers that originates from the AV node and passes into the interventricular septum—the wall separating the left and right ventricles. It serves as the only electrical connection between the atria and the ventricles.
The bundle of His conducts the electrical impulse rapidly from the AV node down into the ventricular septum, acting as a conduit for transmitting signals from the upper chambers to the lower chambers of the heart. This ensures that the ventricles contract immediately after the atria, but only after they’ve had time to fill with blood.
5. Right and Left Bundle Branches: As the bundle of His travels through the interventricular septum, it divides into two main branches—the right bundle branch and the left bundle branch. These branches extend downward along the respective sides of the septum and carry the electrical signal toward the apex (bottom tip) of the heart.
The right bundle branch conducts the signal to the right ventricle, ensuring it contracts properly to pump deoxygenated blood into the pulmonary artery.
The left bundle branch carries the signal to the left ventricle, which contracts forcefully to send oxygenated blood into the aorta and throughout the body.
This coordinated impulse delivery ensures that both ventricles contract simultaneously, maximizing the efficiency of the heartbeat.
6. Purkinje fibers: The Purkinje fibers are a network of fast-conducting fibers that originate from the bundle branches and spread throughout the inner walls of the ventricles, particularly in the subendocardial region. These fibers allow the impulse to be distributed rapidly and evenly across the ventricular myocardium.
When the electrical signal reaches the Purkinje fibers, it triggers the rapid and powerful contraction of both ventricles (called ventricular systole). This contraction forces blood out of the heart:
The right ventricle pumps deoxygenated blood into the pulmonary artery, leading to the lungs.
The left ventricle pumps oxygenated blood into the aorta, which delivers it to the rest of the body.
This final step completes one full cycle of the heartbeat and enables continuous blood circulation throughout the body.
The Heartbeat (Cardiac Cycle)
The heartbeat, also known as the cardiac cycle, is the repeating sequence of electrical and mechanical events that result in the pumping of blood by the heart. It is composed of three main phases: atrial systole, ventricular systole, and diastole.
Atrial Systole: Triggered by the SA node, the atria contract to push blood into the relaxed ventricles. This is the first phase of the cardiac cycle and ensures that the ventricles are filled with blood before they contract.
Ventricular Systole: Shortly after the atria contract, the electrical impulse reaches the AV node, bundle of His, bundle branches, and Purkinje fibers, prompting the ventricles to contract. This powerful contraction pumps blood from the right ventricle to the lungs and from the left ventricle to the entire body.
Diastole: After contraction, both the atria and ventricles relax in this resting phase. During diastole, the heart chambers refill with blood from the veins (the superior and inferior vena cava, and the pulmonary veins), preparing for the next heartbeat.
This entire cycle repeats itself 60–100 times per minute in a normal resting adult, ensuring a continuous and efficient supply of blood throughout the body.