External and Internal Features of Heart

Introduction

• The heart is a hollow, conical muscular organ located in the middle mediastinum. It is enclosed within the pericardium.
• The heart is approximately the size of the individual’s clenched fist. It consists of four chambers: the right atrium, right ventricle, left atrium, and left ventricle.
• The right atrium receives deoxygenated blood from the superior and inferior venae cavae and passes it to the right ventricle.
• The right ventricle pumps deoxygenated blood into the pulmonary trunk, which divides into the right and left pulmonary arteries to carry blood to the lungs.
• The left atrium receives oxygenated blood from the lungs through four pulmonary veins and passes it to the left ventricle.
• The left ventricle pumps oxygenated blood into the aorta for distribution throughout the body.

Morphology of Heart

• The heart is conical or pyramidal in shape.
• It measures approximately 12 cm from the base to the apex.
• Its maximum transverse width is about 8–9 cm.
• The heart weighs approximately 300 g in adult males and 250 g in adult females.
• The heart usually attains its adult weight between 17 and 20 years of age.
• The terms right and left used for the cardiac chambers do not correspond exactly to their anatomical positions.
• For example, the left atrium is located mainly posteriorly, whereas the right ventricle lies mainly anteriorly and inferiorly.

External Features

• The heart has an apex, a base, four borders, and four surfaces.
• The four borders are the superior, right, inferior, and left borders.
• The four surfaces are the anterior or sternocostal surface, inferior or diaphragmatic surface, right surface, and left surface.

Apex of the Heart

• The apex is the conical lower end of the heart. It is directed downward, forward, and to the left.
• It is formed by the left ventricle. The apex is overlapped by the left lung and left pleura.
• It is located in the left fifth intercostal space, just medial to the midclavicular line. It lies approximately 9 cm from the midline.

Base (Posterior Surface) of Heart

• The base of the heart is directed posteriorly toward the vertebral column and lies opposite the apex. It is quadrilateral in shape.
• The lateral two-thirds of the base is formed by the posterior surface of the left atrium.
• The medial one-third is formed by the posterior surface of the right atrium.
• In the erect posture, the base lies opposite the sixth to ninth thoracic vertebrae.
• In the supine position, it lies opposite the fifth to eighth thoracic vertebrae.
• A shallow interatrial groove separates the right and left atria.
• The interatrial groove indicates the posterior attachment of the interatrial septum.
• The cardiac crux is the point where the interatrial groove, atrioventricular groove, and inferior interventricular groove meet.

Relations

• The base of the heart is separated from the vertebral column by the oblique pericardial sinus and fibrous pericardium.
• The two right pulmonary veins are also related to the base.
• The principal bronchi and esophagus lie posterior to the base.
• The descending thoracic aorta and other structures of the posterior mediastinum are also related posteriorly.

Surfaces of Heart

• The heart has four surfaces: anterior or sternocostal, inferior or diaphragmatic, left, and right.
• Earlier descriptions commonly recognized three surfaces: anterior, inferior, and left.

Sternocostal (anterior) surface

• The sternocostal surface is formed mainly by the anterior aspects of the right atrium, right auricle, left auricle, right ventricle, and left ventricle. The right ventricle forms the major part of this surface.

Anterior Part of the Atrioventricular Groove

• The anterior part of the atrioventricular groove separates the right atrium from the right ventricle. It runs downward and to the right. It contains the right coronary artery and the anterior cardiac vein.

Anterior Interventricular Groove

• The anterior interventricular groove separates the right ventricle from the left ventricle. It runs downward and to the left. It reaches the inferior border at a notch called the incisura apicis cordis. The groove contains the anterior interventricular branch of the left coronary artery and the great cardiac vein.

Diaphragmatic (inferior) surface

• The diaphragmatic surface of the heart is relatively flat and rests on the central tendon of the diaphragm.
• It is formed mainly by the left ventricle, which contributes about two-thirds of the surface.
• The right ventricle forms approximately one-third of this surface.

Left surface

• The posterior interventricular groove is now termed the inferior interventricular groove.
• It separates the right and left ventricles on the inferior surface of the heart.
• It contains the posterior interventricular branch of a coronary artery.
• It also contains the middle cardiac vein.
• Anastomoses between the interventricular branches of the right and left coronary arteries are present in this groove.

Right surface

• The right surface of the heart is rounded.
• It is formed by the wall of the right atrium.
• The sulcus terminalis separates the right surface from the sternocostal surface.

Borders of Heart

The heart has four borders: right, left, inferior, and superior.

Right border

• The right border is rounded, convex, and almost vertical. It is formed by the right atrium. It extends from the opening of the superior vena cava to the opening of the inferior vena cava. It separates the base of the heart from the sternocostal surface.

Left border (obtuse margin or margo obtutus)

• The left border is also called the obtuse margin or margo obtusus.
• It is curved, oblique, and less sharply defined. It is formed mainly by the left ventricle and partly by the left auricle. It extends from the left auricle to the apex of the heart. It separates the sternocostal surface from the left surface.

Inferior border (acute margin or margo acutus)

• The inferior border is also called the acute margin or margo acutus.
• It is sharp, thin, and nearly horizontal. It is formed mainly by the right ventricle.
• A small part near the apex is formed by the left ventricle. It extends from the opening of the inferior vena cava to the apex of the heart. It separates the sternocostal surface from the diaphragmatic surface.
• The incisura apicis cordis is a small notch on the inferior border near the apex.
• The anterior interventricular branch of the left coronary artery passes through this notch.

Upper (superior) border

• The superior border is overlapped by the pulmonary trunk and ascending aorta. It is formed by the right and left atria. It extends from the upper end of the right border to the upper end of the left border.

Grooves or Sulci

• The atrioventricular groove, also called the coronary sulcus, separates the atria from the ventricles.
• Anteriorly, part of the atrioventricular groove is obscured by the ascending aorta and pulmonary trunk.
• The interatrial groove is a faint groove on the base, or posterior surface, of the heart. It separates the right atrium from the left atrium.
• The anterior interventricular groove runs nearly parallel to the left border of the heart. It separates the right ventricle from the left ventricle on the anterior surface.
• The inferior interventricular groove, previously called the posterior interventricular groove, lies on the diaphragmatic surface. It separates the right ventricle from the left ventricle inferiorly.
• The interatrial groove corresponds to the attachment of the interatrial septum. The interventricular grooves correspond to the attachment of the interventricular septum.

Right Atrium

• The right atrium is a quadrilateral chamber of the heart.
• It receives deoxygenated blood from the systemic circulation and passes it to the right ventricle through the right atrioventricular opening.
• The right atrioventricular opening is guarded by the tricuspid valve.

Tributaries

1. The right atrium receives blood through the superior vena cava.
2. It receives blood from the inferior vena cava.
3. The coronary sinus also opens into the right atrium.
4. The anterior cardiac veins drain directly into the right atrium.
5. The venae cordis minimae open into the right atrium through numerous small openings.
6. The right marginal vein may occasionally drain directly into the right atrium.

External Features

• The right atrium is vertically elongated and extends between the openings of the superior and inferior venae cavae.

Right Auricle

• The right auricle is a small, ear-shaped projection of the right atrium.
• It extends to the left from the right border of the heart onto the sternocostal surface.
• Its margins are irregular and notched.
• The right auricle overlaps the root of the ascending aorta and partly covers the infundibulum of the right ventricle.

Sulcus Terminalis

• The sulcus terminalis is a shallow external groove extending between the superior and inferior venae cavae along the right side of the atrium.
• Internally, it corresponds to the muscular ridge called the crista terminalis.
• The sinoatrial node lies near the upper end of the sulcus terminalis.
• The sinoatrial node normally acts as the pacemaker of the heart.

Atrioventricular Groove

• The atrioventricular groove separates the right atrium from the right ventricle.
• It runs downward and to the right.
• It contains the right coronary artery and the small cardiac vein.

Internal Features

The interior of the right atrium can be divided into three main regions: the sinus venarum, the pectinate part with the right auricle, and the interatrial septum.

Sinus venarum

• The sinus venarum forms the smooth posterior part of the right atrium.  It develops from the right horn of the sinus venosus.
• The opening of the superior vena cava lies in the upper posterior part of the right atrium. The opening of the superior vena cava has no valve.
• The opening of the inferior vena cava lies in the lower posterior part of the right atrium. The opening of the inferior vena cava is partly bordered by the Eustachian valve, or valve of the inferior vena cava. In adults, this valve is usually rudimentary and functionally insignificant.
• The coronary sinus opens into the posteroinferior part of the right atrium between the opening of the inferior vena cava and the right atrioventricular opening. Its opening is partly guarded by the Thebesian valve, which is usually rudimentary.
• The foramina venarum minimarum are small openings through which the venae cordis minimae drain directly into the right atrium.

Developmental Features

• During fetal life, the valve of the inferior vena cava helps direct oxygenated blood from the inferior vena cava toward the foramen ovale and into the left atrium.
• The intervenous tubercle of Lower is a small elevation on the posterior wall of the right atrium between the openings of the superior and inferior venae cavae.
• It is usually poorly developed or indistinct in adults.
• During fetal life, it helps separate the streams of blood entering through the superior and inferior venae cavae.

Pectinate part and right auricle

• The anterior rough part of the right atrium is called the pectinate part. It develops from the primitive atrium.

Crista Terminalis

• The crista terminalis is a smooth, C-shaped muscular ridge. It separates the smooth sinus venarum from the rough pectinate part of the right atrium.
• It extends from the region anterior to the opening of the superior vena cava toward the opening of the inferior vena cava. It develops from the right venous valve of the sinus venosus.

Musculi Pectinati

• The musculi pectinati are parallel muscular ridges arising from the crista terminalis. They extend toward the atrioventricular region. Their arrangement resembles the teeth of a comb. Within the right auricle, the pectinate muscles form a prominent muscular network.

Interatrial septum

• The interatrial septum develops mainly from the septum primum and septum secundum.

Fossa Ovalis

• The fossa ovalis is an oval depression in the lower part of the interatrial septum. Its floor is formed mainly by the septum primum.

Limbus of the Fossa Ovalis

• The limbus of the fossa ovalis, also called the annulus ovalis, forms the prominent margin around the fossa ovalis. It is derived mainly from the septum secundum.
• Inferiorly, its margin is continuous with the region of the valve of the inferior vena cava.

Triangle of Koch

• The triangle of Koch is a triangular area on the septal wall of the right atrium.
• Anteriorly, it is bounded by the base of the septal leaflet of the tricuspid valve.
• Posteriorly, it is bounded by the opening of the coronary sinus.
• Superiorly, it is bounded by the tendon of Todaro.
• The atrioventricular node lies within the triangle of Koch.

Tendon of Todaro

• The tendon of Todaro is a subendocardial fibrous ridge. It forms an important boundary of the triangle of Koch. It extends from the central fibrous body toward the region of the valve of the inferior vena cava.

Torus Aorticus

• The torus aorticus is an elevation in the anterosuperior part of the interatrial septal region. It is produced by the bulging of the adjacent aortic root.

Right Ventricle

• The right ventricle is a triangular, thick-walled chamber of the heart.
• It receives deoxygenated blood from the right atrium through the right atrioventricular orifice, which is guarded by the tricuspid valve.
• It pumps blood into the pulmonary trunk through the pulmonary orifice, which is guarded by the pulmonary valve.
• The wall of the right ventricle is thinner than that of the left ventricle, with an approximate thickness ratio of 1:3.

External Features

• The right ventricle forms most of the sternocostal surface of the heart.
• It also forms most of the inferior border and part of the diaphragmatic surface.
• The atrioventricular groove separates the right ventricle from the right atrium externally.
• The anterior and inferior interventricular grooves separate the right ventricle from the left ventricle externally.

Internal Features

• The right ventricle has two main parts: a rough inflow part and a smooth outflow part.  The smooth outflow part is called the infundibulum or conus arteriosus. The inflow and outflow parts are separated by a muscular ridge called the supraventricular crest or infundibuloventricular crest.
• The supraventricular crest extends from the interventricular septum toward the right atrioventricular orifice.

Inflowing rough part

• The inflow part forms the lower, rough portion of the right ventricle. It receives blood from the right atrium. Its internal surface is rough because of the presence of trabeculae carneae. It develops from the primitive ventricle.

Trabeculae Carneae

• Trabeculae carneae are irregular muscular projections from the ventricular wall. They give the inner surface of the right ventricle a sponge-like appearance.
• They are classified into three types.
• Ridges are muscular elevations attached to the ventricular wall throughout their length.
• Bridges are attached to the ventricular wall only at their ends.
• Pillars are attached at their bases, while their apices remain free.
• Papillary muscles are examples of the pillar type of trabeculae carneae.

Papillary Muscles

• Papillary muscles are conical muscular projections arising from the ventricular wall. The base of each papillary muscle is attached to the ventricular wall.
• Its apex projects into the ventricular cavity. The apex is connected to the cusps of the tricuspid valve by thin fibrous cords called chordae tendineae.
• The right ventricle contains three groups of papillary muscles.
• The anterior papillary muscle is the largest and arises from the anterior wall of the right ventricle.
• The posterior or inferior papillary muscle arises from the inferior wall of the right ventricle.
• The septal papillary muscle arises from the interventricular septum.

• During ventricular systole, the papillary muscles contract and tighten the chordae tendineae.
• This action prevents prolapse or inversion of the tricuspid valve cusps into the right atrium.
• It therefore helps prevent regurgitation of blood from the right ventricle into the right atrium.

Moderator Band

• The moderator band, or septomarginal trabecula, is a thick muscular band in the right ventricle. It extends from the lower part of the interventricular septum to the base of the anterior papillary muscle. It carries part of the right bundle branch of the atrioventricular bundle.

Outflow part (infundibulum)

• The outflow part of the right ventricle is called the infundibulum or conus arteriosus. It is a smooth, conical region that leads upward toward the pulmonary orifice. The pulmonary orifice is guarded by the pulmonary valve. The infundibulum is separated from the rough inflow part by the prominent supraventricular crest.

Interventricular septum

• The interventricular septum is positioned obliquely and bulges toward the right ventricle.
• It consists of a large muscular part and a small membranous part.
• The muscular part lies mainly anteroinferiorly.
• The membranous part forms the small posterosuperior portion of the septum.
• The septal leaflet of the tricuspid valve is attached near the membranous septum.
• The portion of the membranous septum above the attachment of the tricuspid valve forms the atrioventricular part and separates the right atrium from the left ventricle.
• The lower part of the membranous septum, together with the muscular part, separates the right and left ventricles.

Left Atrium

The left atrium is a thin-walled, quadrangular chamber located mainly on the posterior aspect of the heart.

External Features

• The left atrium forms the major part, approximately the left two-thirds, of the base of the heart.
• The left auricle is an ear-shaped projection of the left atrium that extends anteriorly. It overlaps part of the infundibulum of the right ventricle.
• The left auricle contributes to a small part of the sternocostal surface, left surface, and left border of the heart.
• The atrioventricular groove separates the left atrium from the left ventricle.
• The interatrial groove is a shallow posterior groove that separates the left atrium from the right atrium externally.

Internal Features

• The interior of the left atrium is mainly smooth.
• The left auricle contains muscular ridges called musculi pectinati.
• The left atrium receives oxygenated blood through four pulmonary veins, with two veins opening on each side.
• The septal wall separates the left atrium from the right atrium and forms the interatrial septum.
• The interatrial septum shows a shallow depression called the fossa lunata, which corresponds to the fossa ovalis of the right atrium.
• Posteriorly, the left atrium is related to the oblique pericardial sinus.

Opening in the Left Atrium

1. Four pulmonary veins open into the left atrium: the right superior, right inferior, left superior, and left inferior pulmonary veins.
2. The pulmonary vein openings are not guarded by valves.
3. Numerous venae cordis minimae also open into the left atrium.
4. The left atrioventricular orifice connects the left atrium with the left ventricle.
5. The left atrioventricular orifice is guarded by the mitral valve.

Left Ventricle

• The left ventricle is a conical, thick-walled chamber of the heart.
• It receives oxygenated blood from the left atrium through the left atrioventricular orifice, which is guarded by the mitral valve. It pumps blood into the ascending aorta through the aortic orifice.

External Features

• The left ventricle forms the apex of the heart.
• It contributes approximately one-third of the sternocostal surface.
• It forms about two-thirds of the diaphragmatic surface.
• It forms most of the left border and left surface of the heart.
• The left ventricle has anterior, inferior, and left surfaces.

Internal Features

• The cavity of the left ventricle is approximately circular in cross-section.
• Internally, it is divided into an inflow part and an outflow part.

Inflowing part

• The inflow part forms the main cavity of the left ventricle.
• Its inner surface is rough because of the presence of trabeculae carneae.
• It develops from the left part of the primitive ventricle.
• The left ventricle contains two papillary muscles.
• The anterior papillary muscle arises from the sternocostal wall.
• The posterior, or inferior, papillary muscle arises from the diaphragmatic wall.
• The tips of both papillary muscles are connected to the cusps of the mitral valve by chordae tendineae.

Outflowing part

• The outflow part is called the aortic vestibule.
• It is a smooth, conical region located immediately below the aortic opening.
• It develops from the bulbus cordis.
• Blood passes from the aortic vestibule into the ascending aorta through the aortic orifice.
• The aortic orifice is guarded by the semilunar aortic valve.

Openings in Left Ventricle

• The left atrioventricular orifice is guarded by the bicuspid, or mitral, valve.
• The aortic orifice is guarded by the semilunar aortic valve.

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