Mechanism of Heart Action

Mechanism of Action

The mechanism of heart action involves a rhythmic, electrical impulse driven cycle of muscular contraction (systole) and relaxation (diastole) that pumps blood through the body. The SA node (natural pacemaker) triggers electrical signals, causing atria to contract, followed by ventricular contraction.

Background

Anatomy

The human heart is a four chambered organ that pumps blood throughout the body. It consists of two upper chambers (atria) and two lower chambers (ventricles), separated by a muscular wall called the septum. Four valves ensure one-way blood flow, while an electrical system controls the heart rate.

The right atrium receives deoxygenated blood from the systemic circulation via the superior and inferior vena cava and passes it to the right ventricle, which pumps it to the lungs through the pulmonary artery for oxygenation.

The left atrium receives oxygenated blood from the lungs via the pulmonary veins and transfers it to the left ventricle, the strongest chamber, which pumps blood into the systemic circulation through the aorta.

Electrical Conduction

The heart’s pacemaking system consists of specialized cardiomyocytes that generate and coordinate electrical impulses to maintain rhythmic contraction.

The primary pacemaker, the sinoatrial (SA) node, located in the right atrium near the superior vena cava, initiates impulses that spread across the atria.

These signals reach the atrioventricular (AV) node in the lower right atrium, the secondary pacemaker, where a brief delay allows ventricular filling.

The impulse then travels through the bundle of His, down the right and left bundle branches,Tertiary pacemaker of the heart

Finally, into the Purkinje fibers, ensuring rapid and synchronized ventricular contraction and the final pacemaker of the heart.

This organized conduction system enables efficient cardiac function and maintains normal heart rhythm. The SA node overrides the other pacemakers of the heart, meaning he sets the tempo. If the SA node malfunctions, the rhythm is controlled by the secondary pacemaker and so on.

Diastole: Ventricles relax and fill with blood returning from the body and lungs.

Systole: Ventricles contract, closing the mitral and tricuspid valves to prevent backflow and opening the aortic/pulmonary valves to eject blood.

Propagation

Propagation of the action potential between cardiomyocytes occurs through electrical coupling via gap junctions.

These junctions, located in the intercalated discs, form low-resistance channels that allow ions (mainly Na⁺, K⁺, and Ca²⁺) to flow directly from one cell to the next.

When an action potential is generated in one cardiomyocyte, local depolarizing currents spread through these gap junctions to adjacent cells, bringing them to threshold and triggering a new action potential.

This cell-to-cell conduction enables the myocardium to function as a functional syncytium, ensuring coordinated contraction of the atria and ventricles.

Diagnostic

Electrocardiogram

History

In 1887, Augustus D. Waller recorded the first human electrocardiogram using a capillary electrometer and famously demonstrated cardiac electrical activity in public using his dog “Jimmy,” placing its paws in saline-filled jars, although the recordings lacked precision. Building on this work, Willem Einthoven advanced the field by introducing the term electrocardiogram and inventing the string galvanometer in 1901, enabling accurate recordings and the first telecardiogram in 1903, thereby establishing modern electrocardiography.

Physics

The concept of vectors in ECG is fundamental to understanding how the heart’s electrical activity is represented on the surface of the body.

A vector in this context refers to the direction and magnitude of electrical depolarization as it spreads through the myocardium. During each cardiac cycle, electrical impulses originate in the SA node and propagate through the atria, atrioventricular node, and ventricles, generating a series of instantaneous vectors.

These vectors combine to form a mean electrical vector, which reflects the overall direction of depolarization at a given moment. ECG leads act as viewpoints that record the projection of these vectors onto specific axes. when the electrical wave moves toward a lead, a positive deflection is produced, and when it moves away, a negative deflection appears.

Thus, by analyzing the orientation and magnitude of these deflections across different leads, clinicians can determine the heart’s electrical axis and identify abnormalities such as hypertrophy, conduction blocks, or ischemia.

Echocardiography

An echocardiogram uses sound waves propagation create pictures heart structure, blood flow and valve estimation. It is a non-invasive, safe diagnostic test used in daily healthcare clinics. Healthcare professional can use the pictures from the test to find heart disease and other heart conditions.

• Two-dimensional (2D) echocardiography is the most widely used modality, offering real-time cross-sectional images of cardiac structures. It allows assessment of cardiac anatomy, chamber size, valve morphology, and global and regional wall motion.

• M-mode (motion mode) echocardiography records the motion of cardiac structures along a single ultrasound line over time. Owing to its high temporal resolution, it is particularly useful for precise measurements of chamber dimensions and wall thickness.

• Doppler echocardiography is used to evaluate blood flow within the heart and great vessels. it provides a visual representation of blood flow overlaying on the 2D image, indicating direction and velocity.

Imaging

X-ray imaging of the heart is most commonly performed using chest radiography (chest X-ray), which provides a rapid and non-invasive overview of cardiac size, shape, and position within the thorax.

On a standard chest X-ray, the cardiac image represents the combined outlines of the cardiac chambers and great vessels. Although individual chambers are not directly visualized in detail, characteristic contours allow indirect assessment.

For example, the right heart border is primarily formed by the right atrium, while the left heart border is mainly formed by the left ventricle.

Chest X-ray can also provide important information about associated findings, such as: Enlarged Heart , Fluid Accumulation , Fluid abnormality, Calcifications (Detects calcium buildup in valves, coronary arteries, or the pericardium)

Source

Guyton and Hall- The heart

Costanzo- cardiovascular system