The Atrio-Ventricular valves (or A-V valves) are exactly what their name implies. They are valves located between the atria and ventricles.
Since there are two atria and two ventricles, it follows naturally that there must be two A-V valves. The one on the right is called the Tricuspid Valve and the one on the left is called the Mitral Valve.
Why should we have A-V valves ?
In earlier articles, we have seen how the heart contracts forcefully and ejects blood from its lower chambers called the ventricles. This blood leaves the heart and flows to the other organs of the body.
We have also seen that the blood enters the ventricles from the upper chambers of the heart called the atria. Now, what then keeps the blood flowing in a forward direction into the arteries to the other organs, rather than backwards into the atria?
You’ve got it right – the A-V Valves!
The A-V valves perform a very important function in the heart by maintaining the blood flow in one direction.
They are subjected to tremendous strain during every heart beat, and are uniquely designed to withstand this strain through a person’s lifetime. Their importance becomes very obvious when they are affected by disease and become narrow or leaky. When this happens, there is a rapid failure of the heart’s function, and the patient develops symptoms of heart failure.
The Structure of A-V Valves
The atrioventricular valves have a special organization of structure which helps them fulfill their functions. Each valve has leaflets, chordae tendineae, and papillary muscles. I will describe each of these components now.
The leaflets are thin fibrous membranes that cover the area of the opening between the atria and ventricles. You can think of them as something like a “door” in the gateway between the upper and lower heart chambers.
These leaflets in normal life are white, thin and translucent, and yet are extremely strong and tough tissues. When affected by disease, these leaflets become thick, opaque and sometimes calcium gets deposited in them.
The papillary muscles are stalks of heart muscle that arise from the wall of the ventricle and attach to the valve leaflets through the chordae tendineae. These muscles contract as a part of the ventricle, and are responsible for keeping the valve leaflets pulled shut during the period of ventricle contraction.
This keeps the valve closed during the time when pressure inside the ventricles is high, and this prevents blood from leaking backwards into the atria. The left AV valve has two papillary muscles and the right has three.
The chordae tendineae are thin cords that connect the papillary muscles to the free margin of the valve leaflets. They are non-elastic and act as struts to transmit the force of the papillary muscle to the valve leaflets.
Each leaflet has a variable number (about 5 to 60) chordae attached to it. These chordae fan out from the papillary muscle to insert into the leaflet, with the interstices between them allowing space for blood to flow from the atria to ventricles.
The Mitral Valve
The mitral valve is the left sided A-V valve that guards the opening between the left atrium and ventricle. Due to the high pressures generated on the left side of the heart, this is the single most-stressed heart valve of all.
It was named the mitral valve because it’s two leaflets, when viewed in position inside the heart, suggested to early anatomists a “Bishop’s miter”. Those olden anatomists sure had some imagination!
The mitral valve has two leaflets, one posterior leaflet that is longer and narrower than the anterior leaflet. Due to its high stress exposure, disease of the mitral valve is very common, and has serious consequences.
The Tricuspid Valve
The tricuspid valve is the right sided A-V valve, located between the right atrium and ventricle. It is called tricuspid, because it has three leaflets – one anterior, one inferior and one septal (named so because it is attached to the inter-ventricular septum).
Being on the right side, which has lower pressures than the left, the valve is not exposed to great stresses, and so has a very low incidence of disease. In fact, in some diseases like endocarditis, one operative measure is to completely excise the tricuspid valve, a procedure that is surprisingly well tolerated.