Heart Borders

The borders of the heart can be projected onto the anterior thoracic wall using palpable landmarks such as the sternum, ribs, costal cartilages, and intercostal spaces. These surface projections provide an approximate external representation of the heart's position within the middle mediastinum.

The heart lies obliquely behind the sternum and adjacent costal cartilages, with approximately one-third of its mass to the right of the median plane and two-thirds to the left. Its external outline is produced by different cardiac chambers along its superior, right, inferior, and left borders.

Understanding these borders is useful for relating the heart to the chest wall, locating the cardiac apex, interpreting imaging, and understanding the positions used during cardiovascular examination.

Position of the Heart

The heart lies within the middle mediastinum, enclosed by the pericardium and resting superior to the diaphragm.

Its long axis runs obliquely from the right posterior-superior region toward the left anterior-inferior region.

General Surface Projection

The cardiac outline can be approximated on the anterior chest by identifying four principal points and connecting them to represent the superior, right, inferior, and left borders.

Exact projections vary with body habitus, respiration, posture, age, and cardiac size.

Four Principal Borders

Border	Main Cardiac Structure
Right border	Right atrium
Inferior border	Mainly right ventricle, with contribution from left ventricle near apex
Left border	Mainly left ventricle, with contribution from left auricle superiorly
Superior border	Atria and auricles, with roots of the great vessels

Right Border

The right border of the heart is formed mainly by the right atrium.

On the anterior chest wall, it can be projected as a gently curved line near the right margin of the sternum between the upper and lower right costal cartilage regions.

Right Atrium

The right atrium forms the prominent right-sided cardiac contour.

It receives systemic venous blood through the superior vena cava, inferior vena cava, and coronary sinus.

Surface Projection of the Right Border

A commonly used surface projection places the right border from approximately the right third costal cartilage to the right sixth costal cartilage, close to the right sternal margin.

This should be regarded as an anatomical approximation rather than a fixed measurement.

Superior Border

The superior border of the heart is related primarily to the atria and auricles and to the roots of the major vessels emerging from or entering the heart.

It lies deep to the upper sternum and adjacent costal cartilage region.

Great Vessels

Major vessels associated with the superior aspect of the heart include the ascending aorta, pulmonary trunk, and superior vena cava.

The pulmonary veins enter the left atrium more posteriorly and are not prominent components of the anterior cardiac outline.

Inferior Border

The inferior border is formed mainly by the right ventricle, with a smaller contribution from the left ventricle near the apex.

It runs from the lower right cardiac border toward the apex on the left.

Right Ventricle

The right ventricle forms much of the anterior surface of the heart and contributes most of the inferior border.

It lies immediately posterior to the lower sternum and adjacent costal cartilages.

Relationship to the Diaphragm

The inferior surface of the heart rests on the central tendon of the diaphragm through the fibrous pericardium.

This close relationship means diaphragmatic movement during respiration influences the position of the heart.

Left Border

The left border of the heart is formed predominantly by the left ventricle.

The left auricle contributes to the superior portion of the left cardiac contour.

Left Ventricle

The left ventricle forms much of the left border and produces the apex of the heart.

Its thick muscular wall generates the pressure required for systemic circulation.

Left Auricle

The left auricle is a small muscular appendage of the left atrium.

It contributes to the superior part of the left border near the pulmonary trunk.

Cardiac Apex

The apex of the heart is formed by the left ventricle.

It points anteriorly, inferiorly, and to the left and provides one of the most important surface landmarks of the heart.

Surface Projection of the Apex

In an adult, the apex is commonly projected to the left fifth intercostal space near the midclavicular line.

The precise position varies with body habitus, posture, respiration, and cardiac anatomy.

Apex Beat

The cardiac apex may produce a palpable movement of the chest wall known clinically as the apical impulse or point of maximal impulse.

It is commonly sought in the left fifth intercostal space near the midclavicular line.

Surface Projection Points

Approximate Point	Surface Landmark
Upper right	Near right third costal cartilage at sternal margin
Lower right	Near right sixth costal cartilage at sternal margin
Apex	Left fifth intercostal space near midclavicular line
Upper left	Near left second costal cartilage at sternal margin

Anterior Surface

The sternocostal surface of the heart faces anteriorly and is formed predominantly by the right ventricle.

The right atrium and left ventricle also contribute to the visible anterior cardiac contour.

Base of the Heart

The base of the heart faces mainly posteriorly and is formed predominantly by the left atrium, with a smaller contribution from the right atrium.

Because it is posterior, the anatomical base should not be confused with the superior border seen in an anterior surface projection.

Relationship to the Sternum

Much of the heart lies posterior to the body of the sternum and adjacent costal cartilages.

The right ventricle is particularly prominent immediately behind the lower sternum.

Relationship to the Ribs

The ribs and intercostal spaces provide a practical coordinate system for describing the cardiac projection.

The second through sixth costal regions are especially useful when outlining the heart on the anterior thoracic wall.

Relationship to the Lungs

The heart is flanked by the lungs and pleural cavities.

The medial surfaces of the lungs conform partly to the mediastinal structures, including the pericardium and heart.

Cardiac Notch of the Left Lung

The anterior border of the left lung contains the cardiac notch, which accommodates the leftward projection of the heart.

This creates an area where the pericardium is less extensively covered by lung anteriorly.

Bare Area of Pericardium

Near the lower left sternal region, a small portion of the pericardium is relatively uncovered by lung because of the cardiac notch.

This relationship has anatomical and procedural significance.

Relationship to the Pericardium

The heart is enclosed within the fibrous and serous pericardium.

The surface borders of the heart therefore lie deep to the pericardial sac rather than directly against the chest wall.

Cardiac Borders and Chambers

Cardiac Region	Principal Chamber or Structure
Right border	Right atrium
Anterior surface	Right ventricle
Inferior border	Mainly right ventricle
Left border	Mainly left ventricle
Apex	Left ventricle
Base	Mainly left atrium

Midclavicular Line

The midclavicular line is a vertical surface reference drawn through the midpoint of the clavicle.

It is commonly used with the fifth intercostal space to describe the approximate position of the cardiac apex.

Cardiac Auscultation and Heart Borders

The anatomical positions of the cardiac valves do not correspond exactly to the sites where their sounds are best heard.

Auscultation areas are positioned where sound is transmitted along the direction of blood flow and through the thoracic tissues.

Aortic Area

The traditional aortic auscultation area lies in the right second intercostal space near the sternal border.

This is superior to most of the projected cardiac outline.

Pulmonary Area

The traditional pulmonary auscultation area lies in the left second intercostal space near the sternal border.

Tricuspid Area

Tricuspid valve sounds are commonly assessed along the lower left sternal region.

This area corresponds broadly to the anterior location of the right ventricle rather than directly to the surface position of the valve itself.

Mitral Area

The traditional mitral auscultation area lies near the cardiac apex in the left fifth intercostal space around the midclavicular line.

This relationship makes the apex an especially important cardiac surface landmark.

Clinical Significance

Cardiac Examination

Knowledge of the projected heart borders provides a framework for inspection, palpation, percussion, and auscultation of the precordium.

It helps relate visible or palpable findings to the underlying cardiac chambers.

Apical Impulse

The position and character of the apical impulse can provide information about the relationship of the heart to the chest wall.

A displaced impulse may occur with changes in cardiac size, thoracic anatomy, diaphragmatic position, or other conditions.

Cardiomegaly

Enlargement of cardiac chambers can alter the apparent cardiac silhouette on imaging and may change the position of palpable precordial impulses.

The direction of enlargement depends partly on which chambers are affected.

Chest Radiography

The cardiac silhouette on a frontal chest radiograph reflects the contours of the heart and great vessels.

Radiographic borders do not correspond perfectly to simple surface projection lines because imaging represents three-dimensional structures in projection.

Echocardiography

Surface landmarks of the ribs, sternum, and cardiac apex help guide placement of the ultrasound transducer during transthoracic echocardiography.

Different acoustic windows provide views of specific chambers, valves, and great vessels.

Pericardiocentesis

Knowledge of the relationship between the heart, pericardium, pleura, sternum, and costal margins is essential when planning access to the pericardial space.

Modern procedures commonly use imaging guidance to improve localization and safety.

Precordial Examination

The term precordium refers to the region of the anterior chest wall overlying the heart and lower mediastinum.

Inspection and palpation of this region can reveal impulses produced by cardiac movement.

Respiratory Variation

The heart moves slightly with respiration because of its relationship to the diaphragm and mediastinal structures.

Surface projections should therefore be regarded as approximate rather than fixed boundaries.

Postural Variation

Cardiac position changes somewhat with posture.

Standing, lying supine, and lying in the left lateral position can alter the relationship of the heart and apex to the chest wall.

Body Habitus

Thoracic shape and body habitus influence cardiac orientation.

A relatively vertical heart may be seen in a long, narrow thorax, while a more transverse orientation can occur with a broader thoracic configuration or elevated diaphragm.

Heart Borders Versus Cardiac Silhouette

The surface borders of the heart are anatomical projections onto the chest wall.

The cardiac silhouette is an imaging appearance produced by the heart and associated great vessels, so the two concepts should not be treated as identical.

Key Surface Anatomy Features

Feature	Key Point
Right border	Mainly right atrium
Inferior border	Mainly right ventricle
Left border	Mainly left ventricle
Superior border	Atria, auricles, and great vessel roots
Apex	Left ventricle
Apex surface landmark	Left fifth intercostal space near midclavicular line
Main anterior chamber	Right ventricle
Base	Mainly left atrium and faces posteriorly
Main surface references	Sternum, costal cartilages, ribs and intercostal spaces
Clinical use	Precordial examination, imaging orientation and cardiac localization

Anatomical Importance

The projected borders of the heart provide a practical connection between the anatomy of the middle mediastinum and landmarks visible or palpable on the anterior chest wall. The right atrium forms most of the right border, the right ventricle forms most of the inferior border, and the left ventricle forms most of the left border and the cardiac apex.

The sternum, costal cartilages, ribs, intercostal spaces, and midclavicular line allow these borders to be approximated on the body surface. Of particular importance is the cardiac apex, which is usually projected to the left fifth intercostal space near the midclavicular line.

These relationships provide the anatomical framework for examination of the precordium, localization of the apical impulse, cardiac auscultation, interpretation of cardiac imaging, and understanding how enlargement or displacement of the heart can alter its relationship to the thoracic wall.