The heart’s four chambers manage the flow of blood using heart valves. These valves operate like one-way gates, ensuring blood travels in the correct direction and preventing backward movement. The Atrioventricular (AV) valves are positioned between the upper chambers (atria) and the lower chambers (ventricles). They regulate the flow of blood moving into the main pumping chambers. Their opening and closing are passive events governed entirely by fluid pressure.
What the AV Valves Do
The heart contains two AV valves, one serving each side. The Mitral valve, also called the bicuspid valve because it has two cusps, is located between the left atrium and the left ventricle, managing oxygenated blood flow. On the right side, the Tricuspid valve, which has three cusps, controls the passage of deoxygenated blood from the right atrium into the right ventricle.
These valves are anchored to the ventricular walls by chordae tendineae, which attach to papillary muscles. This structure ensures blood moves one-way from the atria into the ventricles. When the ventricles contract to push blood out to the body or lungs, the AV valves must close tightly to prevent backflow into the atria.
The Pressure Difference That Opens the Valves
The opening and closing of heart valves, including the AV valves, are passive processes driven by pressure differences on either side of the valve flaps. A valve opens only when the pressure behind it is greater than the pressure in the chamber in front of it. For the AV valves, this means they open when the pressure within the atrium exceeds the pressure inside the ventricle on the same side.
This pressure gradient is the sole trigger for the valve to swing open. As blood fills the atria from the returning veins, the atrial pressure begins to rise. Simultaneously, the ventricles must be in a relaxed state for their internal pressure to be at its lowest point, creating the necessary pressure differential across the valve. The moment the ventricular pressure drops below the atrial pressure, the flow of blood pushes the valve cusps apart, allowing the blood to rush in.
If a valve opening were to narrow, a condition known as stenosis, the pressure required in the atrium to push blood into the ventricle would have to be much higher than normal. In a healthy heart, the pressure gradient needed to open the AV valves is typically small, allowing for a smooth and effortless flow of blood into the relaxed ventricles.
When the Ventricles Fill With Blood
The period when the AV valves are open corresponds directly to the heart’s relaxation phase, known as ventricular diastole. This is the time when the ventricles are filling with blood. Diastole begins after the ventricles have finished contracting and their internal pressure has fallen below the pressure in the atria. This drop in ventricular pressure is what initiates the opening of the AV valves.
The filling process is divided into several sub-phases, and the AV valves remain open throughout all of them. Initially, there is a phase of rapid filling, where approximately 70–80% of the blood quickly flows from the atria into the ventricles simply due to the pressure difference. This passive movement occurs because the ventricles are relaxed and expanding, pulling blood in like a slight suction.
Following the rapid phase, the rate of blood flow slows down during a period of reduced filling as the pressures begin to equalize. The final portion of ventricular filling occurs with the contraction of the atria, known as atrial systole or the atrial kick, which pushes the remaining 20–30% of blood into the ventricles just before the valves close. The AV valves are forced shut at the very beginning of the next cycle, the instant the ventricles start to contract and their pressure sharply reverses the gradient by exceeding the atrial pressure.