layers of the heart 
Blood Flow Through the Heart
Blood flow follows a typical path: from the right atrium to the right ventricle, then to the lungs for oxygenation, and back to the left atrium before moving into the left ventricle. This is the usual flow of blood through the heart.
The heart valves help ensure the flow remains in the correct direction; valves prevent backflow during contraction.
Atrioventricular Valves (AV Valves): Tricuspid and Mitral
There are two AV valves between the atria and ventricles:
Tricuspid valve (right side)
Mitral valve (left side)
In the diagram, they are labeled as the Tricuspid (T) and Mitral (M).
Both AV valves face downward and are tethered to the walls of the ventricles by chordae tendineae, acting like lifelines.
The tethering is achieved via papillary muscles, which are small muscles in the ventricular walls.
The chordae tendineae and papillary muscles prevent the valve leaflets from flipping backward into the atria during ventricular contraction, maintaining a tight seal.
If a chordae tendineae ruptures (e.g., the cord attached to a papillary muscle snaps), the corresponding valve can’t maintain a proper seal. On the next heartbeat, blood may regurgitate back through the valve, reversing flow and disrupting the normal direction of circulation.
Chordae Tendineae and Papillary Muscles
Chordae tendineae are the tendinous cords that connect valve leaflets to papillary muscles.
Papillary muscles pull on the chordae to keep leaflets taut during ventricular systole, preventing prolapse.
This mechanism is crucial to avoid backflow and to ensure the ventricles push blood forward into the arteries.
Interventricular Septum and Ventricular Septal Defect (VSD)
The interventricular septum (IVS) is the wall between the left and right ventricles.
It has two parts:
Membranous part: thin portion (blue in the illustration).
Muscular part: thick portion (red in the illustration).
The membranous part is particularly notable because many congenital defects occur here.
A common congenital defect is a Ventricular Septal Defect (VSD): an abnormal communication between the left ventricle and the right ventricle, allowing blood to flow from the left ventricle into the right ventricle when it shouldn’t.
The acronym VSD stands for .
VSDs are more common in the membranous part than in the muscular part of the IVS.
The Heart Wall: Endocardium, Myocardium, and Pericardium
The heart wall has three layers, from inside to outside:
Endocardium: the inner lining that lines the chambers and covers the valves; thin and smooth; RBCs rub against it as they move through the chambers. Endocardium is continuous around the valves and also lines the atria and ventricles.
Myocardium: the thick, muscular middle layer; contains the contractile muscle that powers heartbeats. This is where most of the heart’s work and energy use occur; the myocardium is the primary site of contraction.
Pericardium: the outer protective layer surrounding the heart, with two layered components and a potential space in between.
The term endocardium is the inner lining; the myocardium is the muscular layer; the pericardium encases the heart.
The myocardium is the layer where contraction occurs and where the heart consumes the most oxygen.
The Pericardium: Visceral (Epicardium) and Parietal Pericardium
The pericardium is a two-layer sac surrounding the heart:
Visceral pericardium (also called the epicardium): the inner layer that hugs the surface of the heart.
Parietal pericardium: the outer layer that forms the outer boundary of the sac.
Between these two layers is a small gap filled with a small amount of fluid (the pericardial fluid), not cellular.
The entire enclosing structure is called the pericardium.
The visceral pericardium is essentially the same as the epicardium; both terms refer to the layer in direct contact with the heart.
The two layers are continuous around the heart, forming a protective sac that reduces friction during heartbeats.
Developmental Perspective: How the Pericardium Envelops the Heart
In fetal development, the heart grows within a sac that expands around it, resembling a balloon surrounding the heart.
As the heart enlarges, the inner layer of this balloon flattens and folds back on itself, creating a double-layered arrangement (visceral and parietal layers) with a space between them.
This arrangement explains why the pericardium is continuous yet has two layers with a potential space in between.
Quick Reference: Key Terms
Atria: upper chambers
Ventricles: lower chambers
Tricuspid valve (TV): right AV valve
Mitral valve (MV): left AV valve
Chordae tendineae: tendinous cords
Papillary muscles: muscles anchoring the chordae
Interventricular septum (IVS): wall between ventricles
Membranous part of IVS: thin portion
Muscular part of IVS: thick portion
Ventricular Septal Defect (VSD): congenital defect in the IVS
Endocardium: inner heart lining
Myocardium: muscular middle layer
Pericardium: outer sac around the heart
Visceral pericardium: inner layer hugging the heart (epicardium)
Parietal pericardium: outer layer of the sac
Pericardial cavity: fluid-filled space between the visceral and parietal layers for lubrication
Epicardium: synonym for visceral pericardium
Blood Flow Through the Heart
Blood flows from the right atrium to the right ventricle, to the lungs for oxygenation, then to the left atrium, and finally to the left ventricle. Heart valves prevent backflow, ensuring unidirectional circulation.
Atrioventricular Valves (AV Valves): Tricuspid and Mitral
Two AV valves, Tricuspid (right) and Mitral (left), separate atria from ventricles.
They are tethered by chordae tendineae to papillary muscles in the ventricular walls.
This system prevents valve leaflets from prolapsing, maintaining a tight seal and preventing backflow during ventricular contraction. A ruptured chordae tendineae can lead to blood regurgitation.
Interventricular Septum and Ventricular Septal Defect (VSD)
The interventricular septum (IVS) divides the ventricles into a thin membranous and a thick muscular part.
A Ventricular Septal Defect (VSD), an abnormal opening between ventricles, is a common congenital defect, often occurring in the membranous part. The acronym VSD stands for .
The Heart Wall: Endocardium, Myocardium, and Pericardium
The heart wall has three layers:
Endocardium: inner lining of chambers and valves.
Myocardium: thick, muscular middle layer responsible for contraction and most oxygen consumption.
Pericardium: outer protective sac.
The Pericardium: Visceral (Epicardium) and Parietal Pericardium
The pericardium is a two-layered sac:
Visceral pericardium (epicardium): inner layer hugging the heart.
Parietal pericardium: outer layer.
A small fluid-filled pericardial cavity between them reduces friction during heartbeats.
Developmental Perspective: How the Pericardium Envelops the Heart
During fetal development, the heart grows into a sac, causing the pericardium to form continuous double layers (visceral and parietal) with a lubricating space in between.
Quick Reference: Key Terms
Atria: upper chambers
Ventricles: lower chambers
Tricuspid valve (TV): right AV valve
Mitral valve (MV): left AV valve
Chordae tendineae: tendinous cords
Papillary muscles: muscles anchoring the chordae
Interventricular septum (IVS): wall between ventricles
Membranous part of IVS: thin portion
Muscular part of IVS: thick portion
Ventricular Septal Defect (VSD): congenital defect in the IVS
Endocardium: inner heart lining
Myocardium: muscular middle layer
Pericardium: outer sac around the heart
Visceral pericardium: inner layer hugging the heart (epicardium)
Parietal pericardium: outer layer of the sac
Pericardial cavity: fluid-filled space between the visceral and parietal layers for lubrication
Epicardium: synonym for visceral pericardium