The Mediastinum and Heart

Advertisements help pay for this website. Thank you for your support.

Main Anatomy Index | Blood vessels and nerves of the heart

Last updated 30 March 2006

The Mediastinum

The thoracic mediastinum (L. middle septum) is the space between the two pleura sacs.
It contains all the structures of the thorax except the lungs and the pleura.
It extends from the superior thoracic aperture to the diaphragm inferiorly; from the sternum and costal cartilages anteriorly to the bodies of the thoracic vertebrae posteriorly.
The structures in the mediastinum are surrounded by loose connective tissue, nerves, blood and lymphatic vessels, and fat.
In a living person, the looseness of the connective tissue and fat, and the elasticity of the lungs and pleura enable the mediastinum to accommodate movement and changes in volume in the thoracic cavity.

Subdivisions of the Mediastinum

The mediastinum is descriptively divided into inferior and superior parts by a plane passing though the sternal angle and the inferior border of T4 vertebra, the manubriosternal plane.

The superior mediastinum is from the superior thoracic aperture to this plane.
The inferior mediastinum is from the plane to the diaphragm and is further subdivided into three compartments by the pericardium (anterior, middle and posterior).

The Superior Mediastinum

This subdivision is superior to the manubriosternal plane.
Structures in the superior mediastinum are: the thymus (or its remains) anteriorly; the great vessels related to the heart in the middle; and the oesophagus, trachea and thoracic duct posteriorly.

The Anterior Mediastinum

This is the smallest subdivision of the mediastinum.
It is located anterior to the pericardium and posterior to the sternum and transversus thoracis muscles.
Although this mediastinum is small in adults, it is relatively large in childhood as the thymus extends into it from the superior mediastinum.

The Middle Mediastinum

This subdivision is of the highest clinical importance as it contains the pericardium and the heart and the immediately adjacent parts of the great arteries, phrenic nerves, main bronchi, and other structures in the root of the lungs.

The Posterior Mediastinum

This subdivision is located posterior to the pericardium and diaphragm, and anterior to the bodies of the inferior eight thoracic vertebrae.
Its main contents are the oesophagus and the descending thoracic aorta, which have descended into it from the superior mediastinum.

The Heart

The heart is a double self-adjusting muscular pump, the two parts of which normally work in unison.
The right side of the heart receives deoxygenated blood and pumps it to the lungs and the left side of the heart receives oxygenated from the lungs and pumps it into the aorta for systemic distribution.

The heart has four chambers. Each side consists of an atrium (L. antechamber), a receiving area that pumps blood into a ventricle (L. little belly), a discharging chamber.
The wall of each chamber consists of three layers: an internal layer or endocardium; a middle layer of cardiac muscle or myocardium; and an external layer or epicardium.
The myocardium forms the main mass of the muscle.

The heart and the roots of the great vessels occupy the pericardium, which is located in the middle mediastinum.

The heart is situated obliquely in the middle mediastinum. It is not in the median plane and is located about 2/3 to the left and 1/3 to the right of it.

The heart has a base (posterior aspect), apex (inferolateral aspect end), three surfaces (sternocostal, diaphragmatic, and pulmonary), and four borders (right, inferior, left, and superior).

The Base of the Heart

The base is located posteriorly and is formed mainly by the left atrium.
It lies opposite T5 to T8 (supine position) and T6 to T9 vertebrae (erect position) and faces superiorly, posteriorly and towards the right shoulder.

The base or posterior aspect of the heart is quadrilateral in shape and it is from its most superior part from which the ascending aorta and pulmonary trunk emerge, and into which the superior vena cava enters.
The base is separated from the diaphragmatic surface of the heart by the posterior part of the coronary groove (L. sulcus).
The heart does not rest on its base. The term refers to the somewhat conical shape of the heart with the base being opposite the apex.

The Apex of the Heart

This blunt apex is formed from by the left ventricle, which points inferolaterally.
The apex is located posterior to the left 5^th intercostal space in adults, 7 to 9 cm from the median plane, and just left of the midclavicular line.
The apex beat is an impulse imparted by the hear; it is its point of maximal pulsation or the lowest, most lateral point at which pulsation can be felt.

The Sternocostal (Anterior) Surface of the Heart

This surface of the heart is mainly formed by the right ventricle and is visible in PA radiographs of the thorax.

The Diaphragmatic (Inferior) Surface of the Heart

The surface of the heart is usually horizontal or slightly concave.
It is formed by both ventricles, mainly the left one.
The posterior interventricular sulcus divides this surface into a right 1/3 and a left 2/3.

The Pulmonary (Left) Surface of the Heart

This surface of the heart is mainly formed by the left ventricle and occupies the cardiac notch of the left lung.

The Borders of the Heart

The heart has four borders: right, inferior, left, and superior.
These borders are actually the borders of its sternocostal surface.

The right border is formed by the right atrium. It is slightly convex and is almost in line with the superior and inferior vena cavae.

The inferior border is sharp and thin, and nearly horizontal.
It is formed mainly by the right ventricle and only slightly by the left ventricle.

The left border is formed mainly by the left ventricle and only slightly by the left auricle.

The superior border is where the great vessels enter and leave the heart. It is formed by the right and left auricles and the superior conical portion of the right ventricle, the conus arteriosus (infundibulum), between them.
The pulmonary trunk arises from the conus arteriosus.

Surface Anatomy of the Heart

The cardiac apex corresponds approximately to the apex beat. The true cardiac apex is actually further inferolaterally and does not contact the thoracic wall in systole.

The right border of the heart corresponds to a line from the right 3^rd costal cartilage's superior border (1.2 cm from the sternal margin) to the 6^th costal cartilage's junction with the sternum.

The left border of the heart is marked by a line from the apex beat to the lower border of the left 2^nd costal cartilage 1.2 cm from the sternal margin.

Areas for auscultation:
Mitral valve: at the apex.
Tricuspid valve: left sternal edge, 4^th intercostal space.
Aortic valve: 2^nd right intercostal space.
Pulmonary valve: 2^nd left intercostal space.

Radiological Anatomy of the Heart

In PA radiographs of the thorax, the right border of the cardiovascular silhouette is formed by (superior to inferior): (1) the superior vena cava; (2) the right atrium; (3) the inferior vena cava.

The left border of the cardiovascular silhouette is formed by: (1) the arch of the aorta, which produces a characteristic aortic knob; (2) the pulmonary trunk; (3) the left auricle; (4) the left ventricle.

Chambers of the Heart

The heart has 4 chambers, two atria and two ventricles.
The coronary sulcus encircles most of the superior part of the heart and separates the atria from the ventricles.
The division of the ventricles is by the anterior and posterior interventricular sulci.

The Right Atrium

This chamber forms the right border of the heart between the SVC and IVC.
It receives blood from these large vessels and the coronary sinus.

The internal wall of the right atrium consists of: (1) a smooth posterior part, called the sinus venarum (sinus of the venae cavae), which receives the venae cavae and coronary sinus, and (2) a rough anterior part, which has internal muscular ridges (musculi pectinati) that resembles the teeth of a comb (L. pectin).

The right auricle (atrial appendage) is a small, conical muscular pouch that projects to the left from the right atrium and overlaps the ascending aorta.

The two distinct parts of the right atrium are separated externally by a shallow vertical groove on the anterolateral aspect of the right atrium, called the sulcus terminalis, and internally by a vertical crest or ridge called the crista terminalis.
The crista extends between the two vena caval orifices.

The interatrial septum forms the thin posteromedial wall of right atrium.
A prominent feature of this thin dividing septum is the thumbprint sized fossa ovalis, a large, shallow translucent oval depression.
The fossa ovalis has an incomplete sharp margin known as the limbus fossae ovalis.

The small opening of the coronary sinus is located between the right atrioventricular orifice and the orifice of the IVC.

Atrioseptal Defect (ASD)

A probe-sized atrial septal defect (ASD) appears in the superior part of the fossa ovalis in up to 25% of people.
Before birth, there is such a communication (the foramen ovale) between the right and left atria, but it normally closes before birth.
A small ASD is usually of no clinical significance, but a large ASD allows recently oxygenated blood from the lungs in the left atrium to be shunted back to the right atrium (due to the higher pressure in the LA).
Consequently, the RA and right side of the heart is overworked.
The RA, SVC and IVC dilate to accommodate the excess volume of blood.
An elevated jugular pulse is seen in the neck and due to the dilation of the RA, the right cardiac margin enlarges to the right.

The Right Ventricle

This chamber forms the largest part of the sternocostal surface of the heart, a small part of the diaphragmatic surface, and almost the entire inferior border of the heart.

Its superior left angle tapers into a cone-shaped pouch, called the infundibulum or the conus arteriosus (L. infundibulum, funnel), which leads to the pulmonary trunk.
Its internal wall is smooth, whereas the rest of the right ventricular wall is roughened by a number of irregular muscle bundles, and muscular ridges and bridge known as trabeculae carneae.

The septomarginal trabecula crosses the cavity of the ventricle from the interventricular septum to the base of the anterior papillary muscle.
The carries the right branch of the atrioventricular bundle, which is part of the conducting system of the heart.

A thick, muscular ridge, the supraventricular crest, arches toward and over the anterior cusp of the right atrioventricular (tricuspid) valve, and separates the ridged muscular wall of the ventricle from the smooth-walled conus arteriosus.

The papillary muscles are conical projects with their bases attached to the wall of the ventricle.
A number of slender fibrous bands, called chordae tendineae, arise from the apices of these muscles and are attached to the free edges and ventricular surfaces of the cusps of the right atrioventricular (tricuspid) valve.
The chordae tendineae prevent the cusps of the tricuspid valve from being driven into the right atrium when ventricular pressure rises.
There are usually three papillary muscles in the right ventricle (anterior, posterior and septal).

The Left Atrium

The left atrium extends anteriorly left of the pulmonary trunk.
Four pulmonary veins (two on each side) enter the left atrium.
The valvule of the foramen ovale lies on the interatrial septal wall corresponding to the floor of the fossa ovalis on the right atrial side.

The left atrium opens into the left atrioventricular orifice to the left ventricle. The left atrioventricular valve or mitral valve is bicuspid.
The musculi pectinati are fewer and smaller than in the right atrium.

The Left Ventricle

The left atrioventricular orifice with the mitral valve opens into the left ventricle.
It is similar to the right ventricle in that it has trabeculae carneae, chordae tendineae and papillary muscles, except that it only has two papillary muscles (anterior and posterior) for the corresponding mitral valve leaflets.

The aortic vestibule leads to the semilunar aortic valve.
The aortic valve has three cusps (left, right and posterior), each with lunules and nodules as for the pulmonary valve.

The interventricular septum has muscular (inferior) and membranous (superior) parts; the latter part lies towards the aortic valve and is partly confluent with the fibrous support of the right and posterior cusps.

The left ventricular wall is 2-3 times thicker than the right ventricular wall.

General Structure of the Cardiac Wall

It consists of three layers:
The endocardium, the internal layer. This layer is continuous with the endothelium of the great vessels.
The myocardium, the muscular layer.
The epicardium, the subepicardial fat and connective tissue beneath the visceral layer of the serous pericardium.

The intercellular spaces between the conducting and contractile elements of the heart are filled with connective tissue.

The Cardiac Fibrous Skeleton

This is a complex framework of dense collagen, with membranous, tendinous and fibro-areolar extensions, approximately along the plane of the coronary sinus, and intimately related to the valve orifices of the atrioventricular and semilunar valves.
Its functions include:

To ensure electrophysiological discontinuity between the atria and ventricles (only connection is via the atrioventricular bundle).
To provide mechanical attachment for the atrial and ventricular muscles.
To maintain the cardiac position within the pericardium.
To provide a stable but deformable base for the valvular fibrous cores.

The aortic and two atrioventricular valves are intimately interconnected through their basal collagenous frameworks.
The pulmonary valve is remote from the others, being connected by the long deformable tendon of the conus arteriosus.

Myocardial Architecture

Atrial fibres form two layers: superficial, common to both atria, and deep, confined to each.
Ventricular fibres also consist of superficial and deep layers, where the deep layers also contribute to the papillary muscles.

The ventricular myocardium is regarded as a series of nested spiral laminae.
These fibre-pathways have varying obliquity so that the myocardium is capable of reducing all the dimensions of the ventricles simultaneously.