Heart

Overview

The heart is a fist-sized retrosternal muscular organ found in the middle mediastinum and is composed of four chambers which provide blood flow to the systemic and pulmonary circulations. The heart is enclosed in a protective sac called the pericardium. The heart wall is made up of three layers: epicardium, myocardium and endocardium.

Chambers

Right atrium

A reservoir for the collection of deoxygenated blood from systemic venous circulation via the superior and inferior vena cavae, as well as the heart muscle via the coronary sinus. It forms the right cardiac border.

• Right auricle (atrial appendage) – muscular outpouching that increases the capacity of the atrium
• Atrioventricular orifice
• Coronary sinus – opens into the atrium between the inferior vena cava orifice and right atrioventriventricular orifice

The internal surface of the right atrium is divided by the crista terminalis into two parts which differ in embryological origin:

• Sinus venerarum – located posterior to the crista terminalis. Smooth walled. Receives superior and inferior vena cavae. Derived from the embryonic sinus venosus.
• Atrium proper – located anterior to the crista terminalis. Rough muscular walls formed by pectinate muscles. Formed by the primitive atrium.

Right ventricle

Triangular-shaped chamber which receives deoxygenated blood from the right atrium via the atrioventricular orifice. It then pumps blood through the right ventricular outflow tract, across the pulmonary valve, into the pulmonary trunk

• Papillary muscles – muscles which anchor to the ventricular wall and attached at their apices to the chordae tendineae, which are in turn attached to the three tricuspid valve cusps. Contraction of the chordae tendinae prevents prolapses of the valve during ventricular systole.

The right ventricle is divided by a muscular ridge supraventricular crest, into an inflow and outflow portion.

• Inflow part – The internal surface is covered by irregular muscular elevations called trabeculae carnae
• Outflow part (conus arteriosus) – located in the superior aspect of the ventricle and derived from the embryonic bulbus cordis. It is smooth walled and does not have trabeculae carneae.

Interatrial septum

• Interatrial septum – solid muscular wall that separates the right and left atria
• Fossa ovalis – oval-shaped depression on the septal wall in the right atrium, a remnant of the foramen ovale in the foetal heart. The foramen ovale allows right to left shunting of blood to bypass the lungs, closing once the newborn takes its first breath.

Left Atrium

The left atrium receives oxygenated blood from the four pulmonary veins, and pumps it through the left atrioventricular orifice into the left ventricle.

• Forms the posterior border (base) of the heart
• Left auricle – extends from the superior aspect of the chamber, overlapping the root of the pulmonary trunk.

The interior surface of the left atrium can be divided into two parts, each with a distinct embryological origin:

• Inflow part – receives blood from the pulmonary veins. Its internal surface is smooth and it is derived from the pulmonary veins
• Outflow part – located anteriorly, includes the left auricle. It is lined by pectinate muscles, and is derived from the embryonic atrium.

Interventricular Septum

Separates the right and left ventricles and is composed of a superior membranous part and an inferior muscular part.

• Membranous part – thinner, part of the fibrous skeleton
• Muscular part – same thickness as left ventricular wall

Left Ventricle

Receives oxygenated blood from the left atrium, and pumps it through the aortic orifice into the aorta.

• Forms the left and diaphragmatic borders aswell as the apex of the heart

It can be divided into an inflow portion and an outflow portion:

• Inflow part – lined by trabeculae carneae. There are two papillary muscles which attach to the cusps of the mitral valve
• Outflow part (aortic vestibule) – smooth-walled with no trabeculae carneae. Derived from the embryonic bulbus cordis

Valves

There are four valves of the hearts, divided into two categories:

• Atrioventricular valves: Includes the tricuspid valve and mitral (bicuspid) valve, located between the atria and corresponding ventricle. 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 attached to papillary muscles, located on the interior surface of the ventricles, which function to prevent prolapse of the valve leaflets into the atria during ventricular systole.
• Semilunar valves: Includes the pulmonary valve and aortic valve, located between the ventricles and their corresponding artery. 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).

Tricuspid Valve

• Located between the right atrium and right ventricle (right atrioventricular orifice).
• 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 (bicuspid valve)

• Located between the left atrium and left ventricle (left atrioventricular orifice)
• Consists of two cusps (anterior and posterior), with the base of each cusp anchored to a fibrous ring that surrounds the orifice

Pulmonary Valve

• Located between the right ventricle and pulmonary trunk (pulmonary orifice).
• Consists of three cusps (left, right and anterior)

Aortic Valve

• Located between the left ventricle and the ascending aorta (aorta orifice)
• Consists of three cusps (left, right and posterior)

Surfaces

Sternocostal (right ventricle), diaphragmatic (mostly right ventricle, portion of left ventricle), pulmonary (cardiac impression) surfaces

Arterial Supply

Right main coronary artery

• sinoatrial nodal branch
• right marginal branch
• Atrioventricular nodal branch
• posterior interventricular branch

Left main coronary artery (80% of flow)

• Circumflex branch – supplies lateral and posterior left ventricle
• Left anterior descending (anterior interventricular) – supplies anterior two-thirds of interventricular septum and adjoining part of the left ventricular anterior wall

Venous Drainage

Coronary sinus

• Great, middle and small cardiac veins
• Left marginal vein
• Left posterior ventricular veins

Lymphatic Drainage

Small lymphatic vessels form a dense network beneath the epicardium and endocardium of the ventricles and open into a lymphatic duct in the atrioventricular groove. However, the detailed lymphatic anatomy of the human heart has not been worked out.

Innervation

• Sinus node
• Atrioventricular node
• The sinus node and the AV node are both supplied by sympathetic nerve fibres from the sympathetic ganglia and parasympathetic fibers through the vagus nerve and parasympathetic ganglia behind the heart.

Development

• Develops from two endocardial tubes that merge, loop, and septate to form the heart.
• During the intrauterine stage, the septum between the two atrial is open, and a ductus connects the pulmonary artery to the aorta, effectively bypassing the pulmonary circulation because the lungs are not functional. Rapidly after birth, these two connections close, establishing separate pulmonary and system circulations.

Variants

Left coronary artery

• Left dominant variant, large left circumflex supplies posterior interventricular artery
• Left coronary artery supplying SA nodal branch (40%)
• LCx supplying AV node
• Coronary artery arising from non-coronary sinus
• Co-dominance of the arteries
• Ramus intermedius – an additional artery arising at the bifurcation of the left main artery

Right coronary artery

• Right dominance: RCA gives of posterior descending artery (80%)
  • Left dominance/Co-dominance: LCA gives PDA or both contribute (20%)
• RCA supplies SA nodal artery (60%)
  • LCx supplies SA nodal (40%)
• Anomalous courses of the RCA e.g. origin from the left aortic sinus (rare)