# Chordae Tendineae: Which Heart Valves They Anchor Chordae tendineae are thin, collagen-rich, avascular cords that connect the free edges and ventricular surfaces of the atrioventricular valve leaflets to the papillary muscles of the ventricular wall. They anchor the two atrioventricular valves, the mitral (bicuspid) valve on the left side and the tricuspid valve on the right side, and they never anchor the semilunar valves, which are the aortic and pulmonary valves. That single fact carries a large amount of functional anatomy behind it. The chordae are the reason a valve can slam shut under high pressure without flipping inside out. When a veterinarian hears a systolic murmur, when an echocardiographer watches a leaflet bulge backward, or when a horse develops sudden mitral regurgitation, the chordal apparatus is often the structure in question. Knowing exactly which valves have chordae, and which do not, is the first step in reading any cardiac case correctly. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## The Four Heart Valves and Their Anchoring Systems

The mammalian heart has four valves arranged as two pairs. Each pair solves a different mechanical problem, and each uses a different anchoring strategy. The two atrioventricular (AV) valves sit between the atria and the ventricles. The right AV valve is the tricuspid valve, named for its three leaflets or cusps. The left AV valve is the mitral valve, also called the bicuspid valve, named for its two leaflets. Both are anchored by chordae tendineae. The two semilunar valves sit at the outflow of the ventricles. The pulmonary valve guards the right ventricular outflow into the pulmonary artery. The aortic valve guards the left ventricular outflow into the aorta. Both are built from three pocket-like cusps and a supporting sinus, and neither has chordae tendineae. A useful way to remember the division is by what each valve must resist. AV valves resist the full systolic pressure of the ventricle pushing blood back toward the atrium. Semilunar valves resist the pressure of the great vessels during diastole, and they do it with a cusp-and-sinus design rather than a tether-and-muscle design. The two systems are not interchangeable. ### Why the Distinction Matters Students frequently assume all four valves are built the same way. They are not. The AV valves are anchored structures with a contractile support system. The semilunar valves are passive pocket valves. This difference shows up in disease. Chordae can rupture, elongate, or attach abnormally, and the result is acute AV valve regurgitation. Semilunar valves fail through cusp fibrosis, thickening, or infection, not through chordal rupture, because there are no chordae to rupture. ## The Complete Atrioventricular Valve Apparatus The chordae tendineae are only one component of a functional unit called the valve apparatus, sometimes called the subvalvular apparatus. Four parts work together. ### The Annulus The annulus fibrosus is the fibrous ring at the base of each AV valve. It forms the attachment point for the leaflets and helps maintain the valve orifice shape during the cardiac cycle. Annular dilation stretches the ring and pulls the leaflets apart, which is one mechanism of functional regurgitation [1]. ### The Leaflets The leaflets are thin, flexible membranes that meet in the center of the orifice during closure. The tricuspid valve has three leaflets, usually described as septal, anterior, and posterior. The mitral valve has two, described as septal (or aortic) and parietal (or mural). Leaflet tissue is layered and anisotropic, meaning it stretches differently depending on the direction of the load. Biaxial testing of porcine AV leaflets shows a characteristic nonlinear anisotropic response, with radial stretches averaging about 31 percent higher than circumferential stretches under equal physiological loading [2]. ### The Chordae Tendineae The chordae are the tethers. They arise from the tips and bodies of the papillary muscles and insert onto the ventricular surface, the free edge, or the cleft regions of the leaflets. Chordae are collagen-rich and avascular, which is why they heal poorly after rupture. They come in several functional types. True chordae insert on the leaflets. False chordae, by contrast, are fibrous or fibromuscular bundles that span the ventricular lumen without connecting to a valve leaflet, and they are a separate anatomical entity [3]. Chordal branching is organized rather than random. In the normal mitral valve, a majority of cords divide into three generations, whereas in malformed valves a much higher proportion remain undivided [4]. This branching pattern distributes tension across the leaflet surface instead of concentrating it at a single point. ### The Papillary Muscles The papillary muscles are muscular projections from the ventricular wall that anchor the chordae at their ventricular end. They contract just before the ventricle, which tightens the chordae and pulls the leaflet edges together as pressure rises. Papillary muscle shape varies widely. In the right ventricle, conical, truncated, and flat-topped forms are described. In the left ventricle, conical, truncated, flat-topped, bifurcate, and trifurcate shapes occur [5]. The number of chordae per papillary muscle differs between the two ventricles. In one anatomical series, the anterior papillary muscle of the right ventricle carried about 5.3 chordae, the posterior about 2.7, and the septal about 3.5. In the left ventricle, the anterior and posterior papillary muscles each carried roughly 7.7 chordae [5]. The left-sided apparatus is more heavily tethered because it works against higher pressure. ## How the Anchoring Mechanism Works, Step by Step The valve apparatus functions as a coordinated system across the cardiac cycle. 1. **Diastole begins.** The ventricle relaxes, ventricular pressure falls below atrial pressure, and the AV valve leaflets swing open. Blood fills the ventricle. 2. **Papillary muscles contract.** Just before ventricular systole, the papillary muscles shorten. This takes up slack in the chordae. 3. **Ventricular systole begins.** Ventricular pressure rises sharply. Blood pushes the leaflets toward the atrium. 4. **The chordae resist.** The tethers hold the leaflet free edges at the level of the annulus. The leaflets meet and seal. The papillary muscles and chordae prevent the leaflets from prolapsing into the atrium. 5. **Semilunar valves open.** Once ventricular pressure exceeds aortic and pulmonary pressure, the semilunar cusps are pushed open and blood leaves the ventricle. 6. **Semilunar valves close.** When ventricular pressure falls below great vessel pressure, backflow fills the cusps and they seal against each other. No chordae are involved in this step. 7. **Diastole returns.** The AV valves reopen and the cycle repeats. The critical point is step 4. Without chordae, the AV leaflets would be driven into the atrium every systole. The anchoring system converts a pressure-driven flap into a pressure-resistant seal. ### A Concrete Example Consider the left ventricle of a dog during systole. Intraventricular pressure rises well above left atrial pressure. The mitral leaflets are pushed upward. The chordae, already tensioned by papillary muscle contraction, stop the leaflets at the plane of the annulus. The two leaflets meet along their rough zones and seal. If a single major chorda ruptures, that segment of leaflet loses its tether and flips into the atrium, and blood jets backward into the left atrium during systole. This is acute mitral regurgitation, and it can develop suddenly rather than gradually. ## Comparing Atrioventricular and Semilunar Valves The table below summarizes the structural and functional differences between the two valve types, with comparative notes across common domestic species. | Feature | Atrioventricular valves (mitral, tricuspid) | Semilunar valves (aortic, pulmonary) | |--|--|--| | Position | Between atria and ventricles | At ventricular outflow tracts | | Anchoring structure | Chordae tendineae attached to papillary muscles | No chordae. Supported by cusps and the sinus of Valsalva | | Number of leaflets or cusps | Mitral: 2 leaflets. Tricuspid: 3 leaflets | Three cusps each | | Dog | Tricuspid with 3 leaflets, mitral with 2. Chordae present on both [6] | Aortic and pulmonary, 3 cusps each, no chordae | | Cat | Same basic pattern as dog: 3-leaflet tricuspid, 2-leaflet mitral | Aortic and pulmonary, 3 cusps each, no chordae | | Horse | Tricuspid and mitral both anchored by chordae arising from papillary muscles. Papillary muscle number and chordal pattern documented in the equine subvalvular apparatus [7] | Aortic regurgitation from cusp fibrosis is a recognized equine condition. No chordae involved [7] | | Cow | Tricuspid with 3 leaflets and bicuspid with anterior and posterior leaflets. Chordae tendineae present on both AV valves [8] | Aortic and pulmonary semilunar valves with 3 cusps. No chordae [8] | | Collagen content | Chordae are collagen-rich and avascular | Cusps are layered connective tissue without chordal tethers | | Failure mode | Chordal rupture or elongation causes acute regurgitation | Cusp fibrosis, thickening, or infection causes regurgitation or stenosis | The caprine study is worth noting because it confirms the general mammalian pattern in a ruminant: four orifices, AV valves guarded by tricuspid and bicuspid leaflets with chordae, and outflow valves guarded by semilunar cusps without chordae [8]. The pattern holds across the domestic species veterinarians see most often. ## Comparative Notes Across Species ### Dogs The canine right AV valve is anchored to papillary muscles by chordae tendineae, and during ventricular systole these tendineae keep the cusps from being pushed into the atrium [6]. Most chordae follow the typical pattern, but atypical chordae exist. In a study of 39 canine hearts, two hearts had a single-stranded chorda extending from the free edge of the parietal cusp of the right AV valve directly to the ventricular free wall, bypassing the papillary muscles entirely [6]. These atypical tethers are not just anatomical curiosities. Entanglement in normal chordae is a known complication during cardiac catheterization or pacemaker lead placement, and atypical chordae could plausibly cause the same problem [6]. ### Cats The feline heart follows the standard mammalian plan with a three-leaflet tricuspid valve and a two-leaflet mitral valve, both anchored by chordae. Cats are clinically important here because hypertrophic cardiomyopathy can alter left ventricular geometry and papillary muscle position, which changes the tension on the mitral apparatus and can produce systolic anterior motion of the mitral leaflet. The chordae are part of that mechanical story. ### Horses Equine cardiology has been slower to develop than small animal cardiology, but the anatomy is well described. In a study of 20 domestic horse hearts, researchers examined the normal and comparative anatomy of the left and right subvalvular apparatus, including the number of muscular bellies of the papillary muscles, the type of muscle connection, the height of muscle originating from the ventricle wall, and the chordae tendineae arising from the papillary muscles [7]. Mitral regurgitation in horses is particularly dangerous because of the possibility of sudden death, and it has a better prognosis when the horse has valve prolapse rather than chordal rupture [7]. Tricuspid regurgitation usually does not pose a clinical problem in horses, though severe cases may lead to heart failure [7]. ### Cattle In cattle, gross morphological examination confirms four primary orifices. The atrioventricular orifices feature tricuspid and bicuspid valves, while the aorta and pulmonary arteries are guarded by semilunar valves [8]. The tricuspid valve has three leaflets and the bicuspid valve has anterior and posterior leaflets [8]. Chordae tendineae are present on both AV valves. The semilunar cusps respond differently to decellularization than AV leaflets, which reflects their different structural composition [8]. ### A Note on Chordal Numbers Chordal counts differ between the two AV valves and between species. In sheep, pig, and bovine hearts, the number of chordae was up to twice as many for the tricuspid valve compared with the mitral valve [9]. Counts for the three tricuspid leaflets and the two mitral leaflets were almost identical between species, and chordae attaching to the posterior papillary muscle were almost double those on the septal and anterior papillary muscles [9]. In porcine hearts, the mitral valve averaged about 30.5 chordae and the tricuspid valve about 35.3 [2]. These numbers are useful for understanding why the right-sided apparatus handles a larger leaflet area with a more distributed tether system. ## How the Anchoring System Is Observed in Practice Veterinarians assess the chordae and the valve apparatus mainly through echocardiography, supplemented by gross anatomy during necropsy. ### Echocardiography Two-dimensional echocardiography shows the leaflets, the papillary muscles, and the chordae as thin echogenic lines. The echocardiographer looks for leaflet prolapse, meaning a leaflet that bows past the plane of the annulus during systole, and for flail segments, where a portion of leaflet has lost its chordal support entirely. Color Doppler shows the regurgitant jet that results. The distinction between prolapse and flail matters because flail usually indicates chordal rupture, which carries a different prognosis than simple prolapse. ### Gross Anatomy At necropsy, the ventricles are opened and the subvalvular apparatus is examined directly. The number, shape, and attachment pattern of the papillary muscles are recorded, and the chordae are traced from their papillary origin to their leaflet insertion. This is how the comparative data in the literature were generated. In horses, for example, the number of muscular bellies and the type of muscle connection are specifically analyzed because they affect valve function [7]. ### What the Chordae Look Like Chordae are pale, glistening, and avascular. They have no blood supply of their own, which is why they depend on diffusion from surrounding fluid for nutrition and why they heal poorly after injury. Their collagen composition gives them high tensile strength with limited elasticity, which is exactly what a tether needs. ## Clinical Relevance, Limitations and Common Mistakes Chordal rupture is the most direct clinical consequence of chordal anatomy. When a chorda ruptures, the leaflet segment it supported loses its anchor and flips into the atrium during systole. The result is acute regurgitation, which can be severe and can develop rapidly. In horses, mitral regurgitation from chordal rupture carries a worse prognosis than mitral regurgitation from valve prolapse [7]. Chordal elongation without rupture produces a similar but more gradual picture. The leaflet prolapses progressively, and regurgitation worsens over time. Functional regurgitation, in which the valve apparatus is structurally normal but the ventricle or atrium has dilated, is a different mechanism. In functional regurgitation, annular dilation and leaflet tethering combine to prevent proper coaptation [1]. The chordae are not diseased, but they are pulled out of position by a changing ventricular geometry. Dysplasia of the AV valve apparatus is another category. In a meerkat with right AV valve dysplasia, the valve had shortened or absent chordae tendineae and direct attachment of the valve to the papillary muscles, which were fused and abnormally positioned [10]. The result was right atrial dilation, right ventricular hypertrophy, and right-sided heart failure. This case illustrates that the chordae are not optional accessories. When they are absent or malformed, the valve cannot function. The most common student mistakes are worth naming directly. **Mistake 1: Thinking all four valves have chordae.** Only the two AV valves do. The aortic and pulmonary valves are semilunar valves with cusps and sinuses, and they have no chordae tendineae. **Mistake 2: Confusing the mitral and tricuspid names.** The mitral valve is on the left and has two leaflets. The tricuspid valve is on the right and has three. The bicuspid valve is another name for the mitral valve, not a separate structure. **Mistake 3: Assuming chordae attach only to papillary muscles.** Most do, but atypical chordae can attach directly to the ventricular free wall [6]. False chordae do not attach to valve leaflets at all [3]. **Mistake 4: Believing chordae are passive strings.** They are tensioned by papillary muscle contraction before systole, and the AV valves have been shown to contract independently during different moments of the cardiac cycle, suggesting active adaptation mechanisms [11]. The apparatus is dynamic, not static. **Mistake 5: Thinking semilunar valve disease involves chordae.** Aortic and pulmonary valve disease involves the cusps and sinuses. Chordal rupture is an AV valve event. Individual cases require a veterinarian for diagnosis and treatment. The anatomy described here provides the framework, but clinical decisions depend on the specific patient. ## Quick Review - Chordae tendineae anchor the mitral (bicuspid) and tricuspid valves, the two AV valves. - Chordae tendineae never anchor the aortic or pulmonary valves, which are semilunar valves built from cusps and sinuses. - The AV valve apparatus has four parts: annulus, leaflets, chordae tendineae, and papillary muscles. - Chordae are collagen-rich and avascular, which is why they heal poorly after rupture. - Chordal rupture causes acute AV valve regurgitation because the leaflet loses its tether. - The tricuspid valve has three leaflets and the mitral valve has two, and the tricuspid typically has up to twice as many chordae as the mitral [9]. - Chordal counts and papillary muscle patterns are similar across sheep, pig, and cattle, and the same basic plan applies to dogs, cats, and horses [9][7][8]. ## Frequently Asked Questions ### Which valves are anchored by chordae tendineae? The mitral (bicuspid) valve and the tricuspid valve, the two atrioventricular valves, are anchored by chordae tendineae. The aortic and pulmonary semilunar valves are not. ### Do semilunar valves have chordae tendineae? No. The semilunar valves in the heart, the aortic and pulmonary valves, rely on three cusps and a supporting sinus rather than chordae. They close when backflow fills the cusps, not when a tether pulls them shut. ### What happens if a chorda tendinea ruptures? The leaflet segment it supported loses its anchor and flips into the atrium during systole, causing acute regurgitation. In horses, mitral regurgitation from chordal rupture has a worse prognosis than regurgitation from valve prolapse [7]. ### How many leaflets does each heart valve have? The tricuspid valve has three leaflets. The mitral or bicuspid valve has two. The aortic and pulmonary semilunar valves each have three cusps. ### Are chordae tendineae the same in dogs, cats, horses, and cows? The basic plan is the same across these species. All have a three-leaflet tricuspid valve and a two-leaflet mitral valve, both anchored by chordae, plus two semilunar valves without chordae [9][7][8]. Chordal counts and papillary muscle shapes vary somewhat between species and between the left and right sides of the heart. ### What is the difference between true and false chordae tendineae? True chordae insert on valve leaflets and anchor them to papillary muscles. False chordae are fibrous or fibromuscular bundles that span the ventricular lumen without connecting to a valve leaflet [3]. They are a separate anatomical structure with their own clinical implications. ## Related Articles - [Canine Heart Anatomy: Chambers, Valves, and Conduction System](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-heart-anatomy-chambers-valves-conduction-system) - [Golden Retriever Health Problems: Cancer Risk Ichthyosis and Heart Valve](/knowledge/veterinary-medicine/breed-specific-health/golden-retriever-health-problems-cancer-risk-ichthyosis-and-heart-valve) - [Dog Heart Disease: What Are the Stages and What Do They Mean?](/knowledge/veterinary-medicine/senior-and-chronic-care/dog-heart-disease-stages) - [Congestive Heart Failure CHF in Dogs: Mitral Valve Disease Treatment Progression](/knowledge/veterinary-medicine/internal-medicine/congestive-heart-failure-chf-in-dogs-mitral-valve-disease-treatment-progression) - [Lernaea (Anchor Worm)](/knowledge/parasites/fish-parasites/lernaea) - [Aquarium Fish Anchor Worms: Identification and Removal](/knowledge/veterinary-medicine/aquarium-fish-care/aquarium-fish-anchor-worms-identification-removal) - [Normal Pulse in Dogs: Heart Rate Ranges by Size](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/normal-pulse-in-dogs-heart-rate-ranges-by-size) - [Anatomical Heart: Structure and Chambers](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/anatomical-heart-structure-and-chambers) ## Sources 1. 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