Atrioventricular (AV) valves are one-way gates within the heart that regulate blood flow. They direct blood through the heart’s chambers, preventing backflow. Their precise closure is essential for efficient heart pumping and proper blood circulation.
Heart Chambers and Valves
The human heart contains four chambers: two upper atria and two lower ventricles. Two primary AV valves control blood flow from the atria into the ventricles.
The tricuspid valve is situated between the right atrium and the right ventricle, composed of three leaflets. On the left side, the mitral valve (bicuspid valve) separates the left atrium from the left ventricle and has two leaflets.
The Cardiac Cycle and Pressure Changes
The closure of atrioventricular valves is directly linked to the heart’s rhythmic cycle of contraction and relaxation, known as the cardiac cycle. During the filling phase (diastole), the atria and ventricles are relaxed. Blood passively flows from the atria into the ventricles through the open tricuspid and mitral valves, driven by a pressure gradient where atrial pressure is higher than ventricular pressure.
As the ventricles fill, they prepare for contraction, marking the beginning of systole. As ventricular contraction begins, pressure inside the ventricles rapidly increases, surpassing atrial pressure. This pressure reversal is the primary mechanical cause forcing the AV valve leaflets shut.
The sudden closure prevents blood from being pushed backward into the atria during ventricular contraction. This phase, known as isovolumetric contraction, involves the ventricles contracting with all valves closed, building pressure without changing blood volume.
Role of Supporting Structures
While pressure changes initiate the closure of the AV valves, specialized supporting structures ensure their effective and complete sealing. These structures include the chordae tendineae, often referred to as “heart strings,” and the papillary muscles. The chordae tendineae are tough, fibrous cords that connect the valve leaflets to the papillary muscles, which are small muscular projections from the ventricular walls.
As the ventricles begin to contract, the papillary muscles also contract. This coordinated contraction generates tension in the chordae tendineae. This tension acts like a restraint, preventing the valve leaflets from prolapsing or turning inside out into the atria under the high pressure generated during ventricular contraction. These structures do not actively close the valves but rather maintain their integrity, ensuring the valves remain securely shut and prevent any backflow of blood.
What You Hear
The sudden closure of the atrioventricular valves produces the first audible heart sound, often described as “lub.” This sound, known as S1, occurs at the very beginning of ventricular contraction. It results from the rapid deceleration of blood and the vibration of the closing tricuspid and mitral valves. The sound indicates the successful prevention of blood backflow into the atria, marking the onset of the heart’s pumping phase.