AnatomyThoraxHeartHeart valves

AnatomyThoraxHeartHeart valves

Heart valves

Author: Lorenzo Crumbie, MBBS, BSc • Reviewer: Alexandra Osika
Last reviewed: August 31, 2023
Reading time: 31 minutes

Understanding heart valves anatomy is important in grasping the overall function of the heart. The heart is one of the most important organs in the body. It is responsible for propelling blood to every organ system, including itself. Other articles have discussed at length the gross anatomy of the heart and its four chambers. Special mention has also been made of the fact that the heart has a dual circuit of oxygenated and deoxygenated blood flowing parallel to each other.

The contents of the left and right side of the heart are isolated from each other by the respective interatrial and interventricular septa. Similarly, blood is separated from the atrial and ventricular parts of the heart by the atrioventricular septa. However, it is the heart valves that allow blood to pass from the atria into the ventricles and from the ventricles into systemic or pulmonary circulatory systems.

Key facts about the heart valves
Right atrioventricular valve (tricuspid)	Three cusps - anterior (superior), septal, and posterior (inferior) Associated with four papillary muscles - anterior, septal (medial), inferior (posterior), moderator band (septomarginal trabecula) Prevents blood from flowing from the right ventricle into the right atrium
Left atrioventricular valve (mitral)	Two cusps - anterior (aortic) and posterior (mural) Associated with Inferior (posterior) and superior (anterolateral) papillary muscles Prevents blood from flowing from the left ventricle into the left atrium
Pulmonary valve	Three semilunar cusps - anterior (non-adjacent), right (right adjacent), and left (left adjacent) No associated papillary muscles Prevents backflow of blood from pulmonary circulation into the right ventricle
Aortic valve	Three semilunar cusps - right coronary (right semilunar), left coronary (left semilunar), and a non-coronary cusp (posterior semilunar No associated papillary muscles Prevents backflow of blood from systemic circulation into the left ventricle
Mnemonic	Try Pulling My Aorta (stands for Tricuspid, Pulmonary, Mitral, Aortic valves)
Histology	Semilunar valve layers - ventricularis, spongiosa, and fibrosa Atrioventricular valve layers - atrialis, spongiosa, and fibrosa
Embryology	5th gestational week Fusion of endocardial cushions
Heart sounds	First heart sound (S1) - closure of the atrioventricular valves Second heart sound (S2) - closure of the semilunar valves Third heart sound (S3) - rapid ventricular filling; may be physiological Fourth heart sound (S4) - contraction of the atria against a stiff ventricle; always pathological

The heart valves are uniquely designed gates that promote the unidirectional flow of blood through the heart. They are attached to special muscular appendages that help to keep them stable. This article aims to explore the embryology and gross anatomy of the heart valves. Additional discussion about the heart sounds and their relationship to the status of the valves (open or closed), disorders that affect the valves (valvulopathies or valvular heart disease), and clinical examination of the heart valves will also be included.

Contents

Atrioventricular valves
Semilunar valves
Histology
Embryology
Auscultation of heart sounds
1. Surface markings of the cardiac valves
2. Heart sounds
Sources

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Atrioventricular valves

The mature heart is a muscular tube that is divided into four chambers: two atria (upper chambers) and two ventricles (lower chambers). The flow of blood through the heart is partially regulated by unidirectional valves that exist between the atria and ventricles. The valves between each atrium and ventricle are referred to as atrioventricular valves (right and left). The two valves that regulate the flow of blood from the ventricles to the coronary, pulmonary, and systemic circulation are the semilunar valves (pulmonary and aortic).

For more information about the heart, take a look below:

The atrioventricular valves are regulated pathways that allow blood to flow from the atria to the ventricles. They ensure that blood does not flow back into the atria during ventricular contraction (the systolic phase of the cardiac cycle). There are two such valves, one situated between each atrium and the ipsilateral (on the same side) ventricle. The right atrioventricular valve is also known as the tricuspid valve, and the left atrioventricular valve is also known as the mitral valve. Each valve complex is made up of an orifice that is surrounded by a ring, two or three cusps that extend centrally to close the orifice, and supporting structures known as chordae tendineae and papillary muscles.

Right atrioventricular (tricuspid) valve

The right atrioventricular valve, or tricuspid valve, is situated between the right atrium and right ventricle. As the name suggests, it has three cusps or leaflets that are supported along the margins of the valve. The leaflets are named for the margin to which they are attached. Hence, there are anterosuperior, septal, and inferior margins as well as similarly named cusps. Note that the septal margin is adjacent to the right fibrous trigone of the cardiac skeleton. For clarity, the right fibrous trigone is continuous with the central fibrous body (part of the cardiac skeleton). Other important facts that should be noted include:

There is a fibrous ring that forms the orifice of the valve; it has a circumference ranging between 10 – 11 cm that varies between males and females. However, the margins of the valve are almost triangular.
In the anatomical position, the valve is positioned roughly at a 45 degrees angle in the sagittal plane.
The ventricular surface of the valve points in the anterolateral direction to the left, toward the cardiac apex.

A leaflet or cusp is a fleshy projection that occludes the valve orifice when apposed with adjacent leaflets, thus preventing retrograde flow of blood. While there is a preference for the term leaflet in newer texts, it is often used interchangeably with the term cusp. There are three cusps of the tricuspid valve that are attached along the margin for which it is named.

The anterior cusp (superior leaflet/cusp) is attached to the anterosuperior margin and is the largest of the three cusps.
The septal cusp (septal leaflet/cusp) is attached to the septal margin and is the smallest of the three cusps. It extends across both the muscular and membranous septa.
The posterior cusp (inferior leaflet/cusp) is attached to the inferior margin and spans the anterosuperior and inferoseptal commissures.

The texture of the cusps is graded from the periphery to the central edge. The basal zone is located at the periphery where the cusps are attached to the margins. This is the thickest part of the cusp, it is heavily vascularized and innervated, and has a larger quantity of connective tissue. The translucent clear zone is much thinner than the basal zone and has little to no chordae tendineae inserted in this part of the valve. It is situated between the basal and rough zones. Finally, the rough zone is opaque, thick, and irregular and is the most central zone of the leaflet. The part of the rough zone of each cusp that is exposed to the atrium comes into contact with each other once the valve is fully closed. The ventricular part of the rough zone of each cusp acts as the point of attachment for the chordae tendineae.

The chordae tendineae, otherwise called the tendinous cords, is a misnomer that refers to support structures that form a bridge between the cusps of the atrioventricular valves and the papillary muscles. Papillary muscles are nipple-like muscular extensions that are intricately involved in the mechanical activity of the valve. Chordae tendineae are collagen-based fibers that can be subdivided into the false and true chords. The true chordae tendineae emerge from the apical third of the papillary muscles or (less frequently) from the base of the papillary muscles. False chordae tendineae are irregular and more commonly are attached either between papillary muscles or from the papillary muscles to the walls of the ventricles. False chords are also seen communicating between the ventricular walls and the septum as well.

The chordae tendineae are also further classified based on their shape and point of attachment to the cusp:

Basal chordae – may either be short and muscular or long and slender. They may also appear as round chords or flattened ribbons. In either case, these chordae arise from either the trabeculated or smooth part of the luminal ventricular wall and will insert along the basal zone of the cusp.
Deep chordae – are long chords that branch to numerous parts of the cups, particularly to the rough zones and occasionally to the clear zones.
Fan-shaped chordae – radiate from short stems towards clear zones of the cusp.
Free-edge chordae – extend from either the base or apex of papillary muscles. They often arise as solitary, slender fibers that are inserted into either the margin or midpoint of the cusp.
Rough zone chordae – start off as single strands that eventually trifurcate. Each branch will insert into either an intermediate region of the cusp, the free margin, or the ventricular surface of the rough zone.

There are two major and one minor papillary muscle associated with the tricuspid valve. Of note, there may be several additional (albeit smaller and less consistent) muscles associated with the valve as well:

The anterior papillary muscle is the largest of the three papillary muscles. It emerges from the right anterolateral ventricular wall (below the anteroinferior commissure) and merges with the right part of the moderator band (also called the septomarginal trabecula).
The moderator band acts as a conduit for part of the atrioventricular bundle which conducts electrical impulses from the sinuatrial node (cardiac pacemaker) to the rest of the ventricle.
The smallest of the three muscles is the medial or septal (or medial) papillary muscle. It is medially located and has its attachment (along with the other minor papillary muscles) to the ventricular septum.
The inferior (or posterior) papillary muscle may arise as two or three bands from the inferoseptal commissure.

Right atrioventricular (tricuspid) valve in a cadaver: Dissecting the atrium of the heart clearly revealvs the heart valves.

Left atrioventricular (mitral) valve

The left atrioventricular valve, or mitral valve, is very similar to the tricuspid valve in terms of its structure and composition. The key difference lies in the fact that the mitral valve is made up of only two cusps instead of three as seen in the tricuspid valve. Another important difference between the valves is that the mitral valve is smaller than the tricuspid valve; it ranges from 7 cm in females to 9 cm in males.

It also has a circular orifice that is supported by anterior and posterior fila coronaria arising from the corresponding left and right fibrous trigone. The supporting annulus (ring) of cartilage varies in consistency so that it may alter its shape during the different phases of the cardiac cycle. The valve is oriented at approximately 45o in the sagittal plane with the ventricular surface facing anterolaterally toward the ventricular apex. Anatomically, the mitral valve is posterosuperior to the tricuspid valve and posteroinferior to the aortic opening.

Historically, the valve was described as being bicuspid. This term has since been abandoned based on the fact that there may be small accessory cusps between the two major cusps; therefore it’s not truly “bicuspid”. Like the cusps of the tricuspid valve, those that form the mitral valve are also made of a fibrous core covered by endocardium. Additionally, the leaflets are not truly cusped or peaked, as the suffix suggests. The leaflets, therefore, have the appearance of an uninterrupted sheath that attaches circumferentially to the valve orifice. The free ends of the cusps are defined by a posterolateral and anteromedial commissure, as well as additional inconsistent indentations. The simple labels of anterior and posterior cusps may be misleading based on the orientation of the valve. Nevertheless, the mitral valve has two cusps:

The anterior cusp is also referred to as the aortic leaflet/cusp. The core of the anterior cusp is continuous with both fibrous trigones and the associated fibrous coronaria. The rough zone of the anterior cusp is shaped like a half-moon, while the clear zone extends from the inner limit of the rough zone to the annulus. There is no basal zone on the anterior cusp.
The posterior cusp, otherwise called the mural leaflet/cusp, has a wider attachment base than its anterior counterpart. The indentations on the posterior cusp give it an appearance of a scallop; there is an anterolateral and posteromedial commissural scallop and a large middle scallop. Unlike the anterior cusp, the posterior cusp has all three zones. The rough zone receives chordae tendineae on the ventral surface of the valve and the basal zone receives basal chordae tendineae. The clear zone has no chordae tendineae inserted on it.

The mitral valve also has a similar composition of chordae tendineae throughout its surface. The true chordae tendineae arise from one of two papillary muscles in the left ventricle. There is a inferior papillary muscle (also known as the posterior papillary muscle) that arises from the diaphragmatic part of the ventricular wall and an superior papillary muscle (otherwise called the anterolateral papillary muscle) that arises from the sternocostal surface of the cardiac wall. There are also false chordae that are haphazardly distributed throughout the left ventricle. However, the degree of variation with the chordae tendineae among normal hearts makes it difficult to assign nomenclature or give an accurate anatomical description of these structures.

Atrioventricular valves mnemonic

If you keep mixing your atrioventricular valves we can help! A simple way to remember which valve is which is to use a mnemonic LAB RAT.

Left Atrium: Bicuspid

Right Atrium: Tricuspid

Semilunar valves

The semilunar valves are the “doorways” leading out from the ventricles to the major vessels; they carry blood away from the heart to pulmonary and systemic circulations. They remain closed at the end of the diastolic phase of the cardiac cycle (relaxation) to prevent blood from flowing back into the ventricles. There are two semilunar valves:

on the right, there is the pulmonary valve at the root of the pulmonary artery.
on the left side, there is the aortic valve found at the root of the ascending aorta.

Each valve has associated crescentic cusps and a supporting fibrous skeleton. However, they do not have the typical chordae tendineae or papillary muscle attachments as the atrioventricular valves do.

Pulmonary valve

The pulmonary valve is located at the base of the pulmonary artery as it leaves the right ventricle. The valve is oriented in an oblique plane, directed posterosuperiorly toward the left-hand side. The cusps of the pulmonary valve are attached to the crescent-shaped arches of the cardiac skeleton at the root of the pulmonary artery. This attachment is situated near the commissure of the cusps and extends to the sinutubular junction or ridge of the pulmonary trunk–between the sinuses of the pulmonary trunk.

In the fetal heart, the pulmonary cusps are officially referred to as the anterior, posterior, and septal cusps. However, following further rotation and folding during intrauterine life, the names of the cusps change and are referred to as anterior (non-adjacent), right (right adjacent), and left (left adjacent) semilunar cusps. The cusp itself is made up of a thick fibrous core covered by endocardial folding. The extensive folding of the overlying endocardium also contributes to the hill-and-valley appearance of the cusps, also known as nodules and lunules. This feature facilitates a snug apposition of cusps while the valve is closed.

It should be noted that the fibrous skeleton of the mitral, tricuspid, and aortic valves are intimately related to each other as they arise from the central fibrous body. On the other hand, the fibrous skeleton of the pulmonary valve is separated from the supporting valve structures.

Aortic valve

The aortic valve is the larger of the two semilunar valves. It has well-defined sinuses that each associated cusp is named after. Its three semilunar cusps mark the termination of the left ventricular outflow tract. The aortic valve does not have a continuous collagenous ring that serves as points of attachment for the cusps. Therefore, the constant reference to the valve having an annulus is misleading. Instead, there are three fibrous, triangular arches that function as points of attachment for the cusps of the aortic valve. The left posterior loop is in communication with the left fibrous trigone, and the right posterior loop is in continuity with the right fibrous trigone. Therefore, the aortic and mitral valves are in continuity with each other. The left and right posterior arches also function as points of origin for the subaortic curtain. This is a fibrous sheath that provides a point of origin for the anterior cusp of the mitral valve as well as parts of the cusps of the aortic valve.

Heart valves in a cadaver: Notice three of the four heart valves and how the cardiac vessels are related to them. (LMCA: Left main coronary artery, LCx: Left circumflex artery, RCA: Right coronary artery)

The aortic valve cusps are named according to the origin of the coronary vessels emerging from the root of the aorta. There is a right coronary (right semilunar), left coronary (left semilunar), and a non-coronary cusp (posterior semilunar). This nomenclature is also shared with the sinuses associated with each cusp. However, during intrauterine life, the cusps are named posterior, left, and right prior to rotational changes of the heart during development. As is the case with other valves, the aortic cusps are made of a fibrous core with surrounding folds of endocardium. The base of each cusp is thickened and collagenous at the ventricular origin, and the central aspect is defined by a deep concavity toward the aortic surface. The free margin of the valve (the part associated with the left ventricular free wall) is horizontal. Recall that the left ventricular free wall is the part of the left ventricle not associated with the apex or interventricular septum. There are areas of fibrous build-up along the free border that form valvular nodules. The nodules are sandwiched between slender fibrous cores that form translucent and sometimes fenestrated lunules.

Ready to test your knowledge about the heart valves? Take this quiz!

Heart valves mnemonic

Need an easy way to remember the four heart valves? Memorise the phrase 'Try Pulling My Aorta', which stands for:

Tricuspid
Pulmonary
Mitral
Aortic

Also make sure to take a look at the following study unit can help you master the anatomy of heart valves!

Histology

The leaflets and cusps that make up the heart valves are made up of layers of connective tissue that are histologically divided into three layers. The semilunar valves are divided into:

Ventricularis - which is in direct contact with pulsatile blood and directed toward the ventricles. It is the most caudal layer.
Spongiosa - which is between the ventricularis and fibrosa layers.
Fibrosa - which is directed toward the outflow vessel lumen. This is the most cranial layer.

The atrioventricular valves have fibrosa and spongiosa layers as well, but there is an atrialis layer instead of a ventricularis layer. The layers of the atrioventricular valves are arranged such that:

The atrialis layer is the most cranial and is directed toward the atrium.
The spongiosa is in the middle.
The fibrosa layer is the most caudal and directed toward the ventricles.

The valves are filled with an extracellular matrix that contains a mixture of proteoglycans in the spongiosa layer and collagen fibers in the fibrosa layer. These layers are encased in a sheath of endocardial endothelial cells interlaced with valve interstitial cells. Together, these cells have a homeostatic activity that aids in the daily function of the valve.

The histology of heart valves is explained in detail in the following resources:

Embryology

Valvulogenesis is the process by which the heart valves develop. It involves the formation and maturation of the endocardial cushions within the atrioventricular canal and the outflow tract. Endocardial cushions are masses composed specialized extracellular matrix called cardiac jelly. They grow towards each other and fuse, forming a physical barrier within the heart lumen during the 5th gestational week. As a result of this fusion, the heart tube is divided into left and right atrioventricular canals.

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The primitive atria and ventricles are only partially isolated from each other as the endocardial cushions continue to function as valves. Exposure of part of the endocardium to inductive agents (compounds that promote morphological changes) promotes epithelial to mesenchymal transition. The cells also produce an extracellular matrix that contributes to the growth and development of the valves as well.

Recall that the valves are made up of leaflets. Each valve is made up of septal and mural leaflets. Septal leaflets are attached to the midline septum and originate from the fusion of the inferior and superior endocardial cushions. Mural leaflets are attached to the walls of the heart and are derived from mesenchyme.

Auscultation of heart sounds

Auscultation is a clinical technique that describes listening to sounds generated within certain areas of the body. For example, the effect of air passing through the airway during breathing can be appreciated using a stethoscope. The heart sounds that are appreciated on auscultation are the audible effects of the closure of the heart valves. They are classically described as “lub-dub” sounds that are best heard while auscultating the precordium (area of the anterior chest wall around the heart). The “lub” part of the heart sound arises from the atrioventricular valves, while the “dub” part of the sound is generated by the semilunar valves.

The advent of the stethoscope has vastly improved the process of identifying normal and abnormal heart sounds which may be linked to valvular heart disease. Abnormal sounds generated by diseased heart valves are referred to as cardiac murmurs. Murmurs can be high or low pitched; therefore it is important to know which side of the stethoscope (the bell or the diaphragm) to use to isolate the sounds. The bell of the stethoscope is the smaller of the two circular areas on the end farthest away from the earbuds, while the diaphragm is the larger circular area opposite the bell.

Surface markings of the cardiac valves

Like all other clinical examinations, evaluation of the cardiovascular system involves inspection, palpation, and auscultation. Percussion is not regularly practiced in this instance. Knowing where to place the stethoscope during auscultation is important so that the clinician can adequately relate any abnormality to specific anatomical structures. The precordium is the region of the anterior chest wall immediately anterior to the heart. There are specific landmarks, also known as the surface projections of the heart, that are ideal for listening to heart sounds.

Under normal conditions, the apex of the heart is located in the left, fifth intercostal space, along the midclavicular line, a vertical, imaginary line extending craniocaudally from the middle of the clavicle. In individuals with non-pendulous breasts, this line usually passes through the nipple. This is the most ideal point to listen to the mitral valve. The sound generated by the tricuspid valve is best heard at the lower left sternal edge – this happens to coincide with the most medial part of the fifth intercostal space. Both the pulmonary and aortic valves are audible in the second intercostal space. The only difference is that the aortic valve is heard in the right second intercostal space at the upper right sternal edge, while the pulmonary valve is auscultated in the left second intercostal space at the upper left sternal edge.

Heart auscultation points
Mitral valve	Left fifth intercostal space
Tricuspid valve	Lower left sternal edge
Pulmonary valve	Upper left sternal edge (second intercostal space)
Aortic valve	Upper right sternal edge (second intercostal space)

Heart sounds

It is important to keep a checklist of what needs to be identified while auscultating the heart:

First, identify the first (S1) and second (S2) heart sounds.
Then listen for additional heart sounds, the third (S3) and fourth (S4) heart sounds.
There are additional sounds that can be generated by diseased valves such as snaps and clicks that should also be identified.
Finally, identify the cardiac murmurs. Ensure that the murmurs are timed. State whether or not they occur in systole (phase of cardiac contraction) or diastole (phase of cardiac relaxation). Other features of the murmur (intensity, location, and changes with respiration) should also be noted. These details will be explained in a subsequent article on valvular heart disease.

First and second heart sounds

Keep this checklist in mind while conducting the auscultating part of the examination to avoid missing any of the steps. Start by listening at the cardiac apex with the diaphragm for the first heart sound ‘lub’ followed by the second heart sound ‘dub’. Continue with the diaphragm at the lower left, as well as the upper left and right, sternal borders. That way, all low frequency sounds can be appreciated. It is important to have two fingers of the left hand palpating the carotid artery for the duration of the auscultation as well. This will help the clinician in timing the heart sounds and murmurs (when present). Return to the cardiac apex, this time using the bell of the stethoscope to detect any high-frequency sounds that would have been missed by the diaphragm. For example, the murmur of mitral stenosis is a high pitched sound best heard with the bell, not the diaphragm.

Prior to this, it is important to ask the patient to roll over on their left side. This brings the apex closer to the chest wall and enhances the volume of the heart sound. Listen at the lower left sternal border with the bell as well. The next important step involves the patient sitting up and leaning forward. While in this position, auscultate at the second right intercostal space and over the pulmonary area (the upper left sternal edge) as the patient fully expires (breathes all the way out) and holds their breath. This accentuates regurgitant murmurs of the aortic valve.

Third and fourth heart sounds

There are additional heart sounds that may either be physiological or pathological in nature. The more commonly encountered physiological heart sound is called the third heart sound (S3). This heart sound is the result of rapid ventricular filling heard in patients with hyperdynamic circulation (increased circulatory volume). Examples of non-pathological hyperdynamic circulation include pregnancy, exercise, and anxiety. This heart sound is best heard at the cardiac apex using the bell of the stethoscope. While it may not be associated with pathological processes in younger patients, it is almost always pathological in origin if heard in patients older than 40 years.

There is a fourth heart sound (S4) which, when heard, is always pathological. It arises as a result of forceful contraction of the atria against a non-compliant ventricle. Disorders such as left ventricular hypertrophy, hypertrophic cardiomyopathy, or aortic stenosis may also result in a fourth heart sound.

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

References:

Combs, M., & Yutzey, K. (2009). Heart Valve Development. Circulation Research, 105(5), 408-421. doi: 10.1161/circresaha.109.201566
Douglas, G., Macleod, J., Nicol, E., & Robertson, C. (2014). Macleod's Clinical examination(13th ed.). Churchill Livingstone.
Loukas, M., Wartmann, C., Tubbs, R., Apaydin, N., Louis, R., & Easter, L. et al. (2009). The clinical anatomy of the sinutubular junction. Anatomical Science International, 84(1-2), 27-33. doi: 10.1007/s12565-008-0011-0
MITRAL VALVE ANATOMY. Retrieved from https://www.cthsurgery.com/mitral-valve-anatomy.html
Moore, K., Persaud, T., & Torchia, M. (2013). The developing human (9th ed.). Philadelphia, PA: Elsevier-Saunders.
Netter, F. (2014). Atlas of Human Anatomy (6th ed.). Philadelphia, PA: Saunders.
Standring, S. (2009). Gray's Anatomy (40th ed.). Edinburgh: Elsevier Churchill Livingstone.

Illustrators:

Anatomy of heart valves (superior view) - Yousun Koh
Right atrioventricular valve (cadaveric dissection) - Prof. Carlos Suárez-Quian
Heart valves (cadaveric dissection) - Prof. Carlos Suárez-Quian

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