The heart functions as a synchronized, four-chambered pump, ensuring blood travels in a single direction. The flow between the upper chambers (atria) and the lower chambers (ventricles) is regulated by the atrioventricular (AV) valves. These valves act as one-way gates, allowing forward movement while preventing backward flow. The tricuspid valve separates the right atrium and right ventricle, and the mitral valve separates the left atrium and left ventricle.
The Mechanics of Atrioventricular Valves
The movement of the AV valves is a passive process governed entirely by differences in blood pressure across them, not muscle contraction. Blood naturally flows from higher pressure to lower pressure. The valve flaps open or close in response to this dynamic: higher pressure behind the valve pushes it open, while higher pressure in front pushes it shut.
Attached to the ventricular side of the valve flaps are tough, fibrous cords called chordae tendineae. These cords connect the valve leaflets to muscular projections on the ventricular wall known as papillary muscles. The papillary muscles contract simultaneously with the ventricles, maintaining tension on the chordae tendineae. This system prevents the valve leaflets from prolapsing back into the atria when ventricular pressure rises significantly.
Opening the AV Valves: The Diastolic Phase
The AV valves open during diastole, the heart’s relaxation phase when the ventricles are filling. Diastole begins after ventricular contraction is complete and the pressure within the ventricles drops. This decrease in ventricular pressure eventually falls below the pressure of the blood accumulated in the atria.
This pressure reversal pushes the AV valves open, allowing blood to pour from the atria into the relaxed ventricles. The initial stage is a period of rapid ventricular filling, where the majority of blood flows quickly through the open valves into the expanding chambers. Slower filling follows as the pressure gradient lessens. Filling continues until the atria contract toward the end of diastole, pushing a final volume of blood across the open AV valves.
Closing the AV Valves: Preventing Backflow
The AV valves close when the ventricles begin to contract, initiating systole. As the ventricular walls shorten, the blood inside is compressed, causing ventricular pressure to increase rapidly. This pressure quickly exceeds the pressure remaining in the atria.
The sudden pressure reversal pushes the valve leaflets closed, preventing blood from being ejected backward into the atria. The nearly simultaneous closure of the mitral and tricuspid valves creates the first sound of the heartbeat, the “lub” sound. This closure forces the contracting ventricles to direct blood flow forward, out of the heart and into the major arteries.