How does this delay allow time for atrial systole before the atrioventricular valves close?

Sourav PanNovember 9, 2024

Answered

The delay at the atrioventricular (AV) node is crucial for ensuring that the atria have sufficient time to contract (atrial systole) and effectively fill the ventricles with blood before the ventricles begin their contraction (ventricular systole). Here’s how this delay facilitates proper cardiac function:

Timing of Atrial Systole and Ventricular Filling

1. Sequential Contraction:
   • The heart’s electrical conduction system initiates an action potential at the sinoatrial (SA) node, which spreads through the atria, causing them to contract. This contraction pushes blood into the ventricles. The AV node introduces a delay of approximately 120 milliseconds, allowing the atria to complete their contraction and fully empty their contents into the ventricles before any electrical signal reaches the ventricles to trigger their contraction .
2. Atrial Kick:
   • The contraction of the atria contributes significantly to ventricular filling, often referred to as the “atrial kick.” During this phase, about 20-30% of the total ventricular filling occurs due to atrial contraction. If the AV node did not provide this delay, the ventricles might contract too early, leading to inadequate filling and reduced cardiac output .

Closure of Atrioventricular Valves

1. Pressure Dynamics:
   • As the atria contract, pressure within them increases, which helps push blood through the open atrioventricular (AV) valves (mitral and tricuspid) into the ventricles. Once ventricular contraction begins, pressure in the ventricles rises rapidly.
2. Timing of Valve Closure:
   • The AV valves close when ventricular pressure exceeds atrial pressure, preventing backflow into the atria during ventricular contraction. The AV node’s delay ensures that by the time ventricular systole begins, the atria have finished contracting and are no longer pushing blood into the ventricles. This timing is critical for preventing regurgitation and ensuring effective blood flow from the heart