The valves of the heart are structures which ensure blood flows in only one direction. They are composed of connective tissue and endocardium (the inner layer of the heart).
There are four valves of the heart, which are divided into two categories:
- Atrioventricular valves: The tricuspid valve and mitral (bicuspid) valve. They are located between the atria and corresponding ventricle.
- Semilunar valves: The pulmonary valve and aortic valve. They are located between the ventricles and their corresponding artery, and regulate the flow of blood leaving the heart.
In this article, we will look at the anatomy of these valves – their structure, function, and their clinical correlations
Fig 1 – The four valves of the heart, visible with the atria and great vessels removed.
Atrioventricular Valves
The atrioventricular valves are located between the atria and the ventricles. They close during the start of ventricular contraction (systole), producing the first heart sound. There are two AV valves:
- Tricuspid valve – located between the right atrium and the right ventricle (right atrioventricular orifice). It consists of three cusps (anterior, septal and posterior), with the base of each cusp anchored to a fibrous ring that surrounds the orifice.
- Mitral valve – located between the left atrium and the left ventricle (left atrioventricular orifice). It is also known as the bicuspid valve because it has two cusps (anterior and posterior). Like the tricuspid valve, the base of each cusp is secured to fibrous ring that surrounds the orifice.
The mitral and tricuspid valves are supported by the attachment of fibrous cords (chordae tendineae) to the free edges of the valve cusps. The chordae tendineae are, in turn, attached to papillary muscles, located on the interior surface of the ventricles – these muscles contract during ventricular systole to prevent prolapse of the valve leaflets into the atria.
There are five papillary muscles in total. Three are located in the right ventricle, and support the tricuspid valve. The remaining two are located within the left ventricle, and act on the mitral valve.
Semilunar Valves
The semilunar valves are located between the ventricles and outflow vessels. They close at the beginning of ventricular relaxation (diastole), producing the second heart sounds. There are two semilunar valves:
- Pulmonary valve – located between the right ventricle and the pulmonary trunk (pulmonary orifice). The valve consists of three cusps – left, right and anterior (named by their position in the foetus before the heart undergoes rotation).
- Aortic valve – located between the left ventricle and the ascending aorta (aortic orifice). The aortic valve consists of three cusps – right, left and posterior.
- The left and right aortic sinuses mark the origin of the left and right coronary arteries. As blood recoils during ventricular diastole, it fills the aortic sinuses and enters the coronary arteries to supply the myocardium.
The pulmonary and aortic valves have a similar structure. The sides of each valve leaflet are attached to the walls of the outflow vessel, which is slightly dilated to form a sinus. The free superior edge of each leaflet is thickened (the lunule), and is widest in the midline (the nodule).
At the beginning of ventricular diastole, blood flows back towards the heart, filling the sinuses and pushing the valve cusps together. This closes the valve.
Fig 3 – The aortic valve cusps, aortic sinuses, and the origin of the coronary arteries.
Clinical Relevance: Aortic Stenosis
Aortic stenosis refers to narrowing of the aortic valve, restricting the flow of blood leaving the heart. The main three causes are:
- Age-related calcification
- Congenital defects
- Most commonly a bicuspid aortic valve, which predisposes the valve to calcification later in life.
- Rheumatic fever
The classical triad seen in severe aortic stenosis is shortness of breath, syncope and angina. The increasing workload for the left ventricle can also result in left ventricular hypertrophy.
Definitive treatment is surgical, and can be achieved via valve replacement or balloon valvuloplasty.
Fig 4 – Aortic stenosis, secondary to rheumatic heart disease. The aorta has been removed to show thickened, fused aortic valve leaflets and opened coronary arteries from above.
