HEART

                                                        INTRODUCTION

Mediastinum(plural-mediastina)(Latin intermediate) is the middle space left in the thoracic cavity in between the lungs. Its most important content is the heart, enclosed in the pericardium in the middle part of the inferior mediastinum or the middle mediastinum.

SUPERIOR AND INFERIOR MEDIASTINA

For descriptive purpose, the mediastina is divided into superior mediastinum and the inferior mediastinum.

                                                                          Fig.: Subdivisions of the mediastinum

SUPERIOR MEDIASTINUM

Boundaries

Anteriorly: Manubrium sterni

Posteriorly: Upper four thoracic vertebrae

Superiorly: Plane of the thoracic inlet

Inferiorly: An imaginary plane passing through the sternal angle in front, and the lower border of the body of the fourth thoracic vertebra behind.

On each side: Mediastinal pleura

Contents

1.Trachea and esophagus.

2.Muscles: Origins of (i)sternohyoid, (ii) sternothyroid, (iii) lower ends of longus colli.

3.Arteries: (i)Arch of aorta, (ii) brachiocephalic artery, (iii) left common carotid artery, (iv) left subclavian artery.

4.Veins: (i)Right and left brachiocephalic veins, (ii)Upper half of the superior vena cava, (iii)left superior intercostal vein.

5.Nerves: (i)Vagus, (ii)phrenic, (iii)cardiac nerves of both sides, (iv)left recurrent laryngeal nerve.

6.Thymus

7.Thoracic duct

8.Lymph nodes: Paratracheal, brachiocephalic, and tracheobronchial.

                                          Fig:. Arrangement of the large structures in the superior mediastinum  

INFERIOR MEDIASTINUM

The inferior mediastinum is divided into- anterior, middle and posterior mediastina.

Anterior Mediastinum

Anterior mediastinum is a very narrow space in front of the pericardium, overlapped by the thin anterior borders of both lungs. It is continuous through the superior mediastinum with the pretracheal space of the neck. It contains areolar tissue and part of thymus gland.

Boundaries

Anteriorly: Body of sternum

Posteriorly: Pericardium

Superiorly: Imaginary line separating the superior mediastinum from the inferior mediastinum. 

Inferiorly: Superior surface of diagram.

On each side: Mediastinal pleura.

Contents

1. Sternopericardial ligaments.

2. Lymph nodes with lymphatics

3. Small mediastinal branches of the internal thoracic artery.

4.The lowest part of of the thymus

5. Areolar tissue.

Middle mediastinum

Middle mediastinum is occupied by the pericardium and its contents, along with the phrenic nerves and the pericardicophrenic vessels.

PERICARDIUM AND HEART

Pericardium, comprising fibrous and serous layer, encloses the heart pulsating from 'womb to tomb'.
Heart is a vital organ, pumping blood to the entire body. Its pulsations are governed by the brain through various nerves.

PERICARDIUM

FEATURES

The pericardium (Greek around heart) is a fibroserous sac which encloses the heart and the roots of the great vessels. It is situated in the middle mediastinum. It consists of the fibrous pericardium and the serous pericardium. 

Fibrous pericardium encloses the heart and fuses with the vessels which enter/leave the heart. Heart is situated within the fibrous and serous pericardial sacs. As heart develops, it invaginates itself into the serous sac, without causing any breach into its continuity. The last part to enter the region of atria, from where the visceral pericardium is reflected as the parietal pericardium.

                                                                                    Fig.: Layers of the pericardium


FIBROUS PERICARDIUM
Fibrous pericardium is a conical sac made up of fibrous tissue. The parietal layer of serous pericardium is attached to its deep surface. The following features of the fibrous pericardium are noteworthy.
1. The apex is blunt and lies at the level of the sternal angle. It is fused with the roots of the great vessels and with the pretracheal fascia.
2. The base is broad and inseperably blended with the central tendon of the diaphragm.
3. Anteriorly, it is connected to the upper and lower ends of body of the sternum by weak superior and inferior sternopericardial ligaments.
4. Posteriorly, its related to the principal bronchi, the oesophagus with the nerve plexus around it and the descending thoracic aorta.
5. On each side, it is related to the mediastinal pleura, the mediastinal surface of the lung, the phrenic nerve, and the pericardiacophrenic vessels.
6. It protects the heart against sudden overfilling and prevents over expansion of the heart.

SEROUS PERICARFIUM
Serous pericardium is thin, double-layered serous membrane lined by mesothelium. The outer layer or parietal pericardium is fused with the fibrous pericardium. The inner layer or the visceral pericardium, or epicardium is fused to the heart, except along the cardiac grooves, where it is separated from the heart by blood vessels. The two layers are continuous with each other at the roots of the great vessels, i.e. ascending aorta, pulmonary trunk, two venae cavae, and four pulmonary veins.

                       
                                                      Fig.: Development of layers of serous pericardium

Sinuses of Pericardium
The epicardium at the roots of the great vessels is arranged in form of two tubes. The arterial tube encloses the ascending aorta and the pulmonary trunk at the arterial end of the heart tube, and the venous tube encloses the vena cava and pulmonary veins at the venous end of the heart tube. The passage between the two tubes is known as the transverse sinus of pericardium.

Contents of Pericardium
1.Heart with cardiac vessels and nerves.
2.Ascending aorta.
3.Pulmonary trunk.
4.Lower half of the superior vena cava.
5.Terminal part of the inferior vena cava.
6.The terminal parts of the pulmonary veins.

                                                            Fig.: The pericardial activity seen after the removal of the heart

Blood Supply
The fibrous and parietal pericardia are supplied by branches from:
1. Internal thoracic
2. Musculophrenic arteries
3. The descending thoracic aorta
4. Veins drain into corresponding veins.

Nerve Supply

The fibrous and parietal pericardia are supplied by the phrenic nerves. They are sensitive to pain. The epicardium is supplied by autonomic nerves of the heart and is not sensitive to pain. Pain of pericarditis originates in the parietal pericardium alone.

                                                             HEART

The heart is a conical hollow muscular organ situated in the middle mediastinum. It is enclosed within the pericardium. It pumps blood to various parts of the body to meet their nutritive requirements. The Greek name for the heart is cardia from which we have the adjective cardia.

The heart is placed obliquely behind the body of the sternum and adjoining parts of the costal cartilages, so that one-third of it lies to the right and two-thirds to the left of the median plane. 

The heart measures about 12 x 9 cm and weighs about 300g in males and 250g in females.

EXTERNAL FEATURES

The Human heart has four chambers. These are the right and left atria and the right and left ventricles. The atria(Latin chambers) lie above and behind the ventricles. On the surface of the heart, they are not separated from the ventricles by an atrioventricular groove.The Heart has:

> An apex directed downwards, forwards and to the left.

> A base(posterior surface) directed backwards.

> Three surfaces- anterior/sternocostal, inferior and left lateral

> Borders: The surfaces are demarcated by upper, inferior, right and left borders.

RIGHT ATRIUM

Position
The right atrium is the right upper chamber of the heart. It receives venous blood from the whole body, pumps it to the right ventricle through the atrioventricular or tricuspid opening. 

External features
1. The chamber is elongated vertically, receiving the superior vena cava at the upper and the inferior vena cava at the lower end.2. The upper end is prolonged to the left to form the right auricle (Latin little ear). The auricle covers the root of the ascending aorta and partly overlaps the infundibulum of the right ventricle.

3. Along the right border of the atrium, there is a shallow vertical groove which passes from the superior vena cava to the inferior vena cava. This groove is called the sulcus terminaslis. The upper part of the sulcus terminalis contains the sinuatrial or SA node which acts as the pacemaker of teh heart.

4. The right atrioventricular groove separates the right atrium from the right ventricle.

Tributes or Inlets of the Right Atrium

1.  Superior vena cava

2. Inferior vena cava 

3. Coronary sinus

4. Anterior cardiac veins

5. Venae cordis minimae(thebesian veins)

6. Sometimes the right marginal vein

Internal Features

The interior of the right atrium can be broadly divided into following three parts:

Smooth posterior part or Sinus venarum

1. Developmentally, it is derived from the right horn of the sinus venosus

2. Most of the tributaries except the anterior cardiac veins open into it

3. The intervenous tubercle of lower is a very small projection, scarcely visible, on the posterior wall of the atrium just below the opening of superior vena cava.
Rough anterior part or Pectinate part, including the auricle
1. Developmentally, it is derived from the primitive atrial chamber.
2. It presents a series of transverse muscular ridges called musculi pectinati.

Interatrial Septum
1. Developmentally, it is derived from the septum primum and septum secundum.
2. It presents the fossa ovalis, a shallow saucer-shaped depression, in the lower part. The fossa represents the site of the embryonic septum primum.
3. The annulus ovalis or limbus (Latin a border) fossa ovalis is the prominent margin of the fossa ovalis. It represents the lower free edge of the septum secundum.
4. The remains of the foramen ovale are occasionaly present. This is a small slit-like valvular opening between the upper part of the fossa and the limbus. It is normally occluded after birth, but may sometimes persist.

RIGHT VENTRICLE

Position

The right ventricle is a triangular chamber which receives blood from the right atrium and pumps it to the lungs through the pulmonary trunk and pulmonary arteries.

External features

1. Externally, the right ventricle has two surfaces- anterior or sternocostal and inferior or diaphragmatic.

2. The interior has two parts:

    a. The inflowing part is rough due to the presence of muscular ridge called trabeculae carneae.

    b. The outflowing part or infundibulum is smooth and forms the upper conical part of the right ventricle which gives rise to the pulmonary trunk.

Internal features
1. The interior has two orifices:
    a. The right atrioventricular or tricuspid orifice, guarded by the tricuspid valve.
    b. The pulmonary orifice guarded by the pulmonary valve.
2. The interior of the inflowing part shows trabeculae carneae or muscular ridges of three types:
    a. Ridges or fixed elevations
    b. Bridges
    c. Pillars or papillary muscles

                                                                             Fig.: Interior of the right ventricle

3. The septomarginal trabecula or moderator band is a muscular ridge extending from the ventricular ventricular septum to the base of the anterior papillary muscle. It contains the right branch of the AV bundle.
4. The cavity of the right ventricle is crescentic in section because of the forward bulge of the intraventricular septum.
5. The wall of the right ventricle is thinner than that of the left ventricle in a ratio of 1:3.

LEFT ATRIUM

Position

The left atrium is a quadrangular chamber situated posteriorly. Its appendage, the left auricle projects anteriorly to overlapthe infundibulum of the right ventricle. The left atrium forms the left two-thirds of the base of the heart, the greater part of the upper border, parts of the sternocostal and left surfaces and the left border.

Features

1. The posterior surface of the atrium forms the anterior wall of oblique sinus of pericardium.

2. The anterior wall of the atrium is formed by the interatrial septum.

3. Two pulmonary veins open into the atrium on each side of the posterior wall.

LEFT VENTRICLE

Position

The left ventricle receives oxygenated  blood from the left atrium and pumps it into the aorta. It forms the apex of the heart, a part of the sternocostal surface, most of the left border and left surface, and the left two-thirds of the diaphragmatic surface.

Features

1. Externally, the left ventricle has three surfaces- anterior or sternocostal, inferior or diaphragmatic, and left.

2. The cavity of the left ventricle is circular in cross section.

                                   Table: Comparison of right atrium and left atrium

Right Atrium	Left Atrium
Receives venous blood of the body	Receives oxygenated blood from the lungs
Pushes blood to right ventricle through tricuspid valve	Pushes blood to left ventricle through bicuspid valve
Forms right border, part of sternocostal and small part of base of the heart	Forms major part of base of the heart
Enlarged in tricuspid stenosis	Enlarged in mitral stenosis

 

                                             
          
                                                                               Fig.: Internal view of left atrium

STRUCTURE OF HEART

VALVES

The valves of the heart maintain unidirectional flow of the blood and prevent its regurgitation in the opposite direction. There are two parts of valves in the heart, a pair of atrioventricular valves and a pair of semilunar valves. The right atrioventricular valve is known as the tricuspid valve because it has three cusps. The left atrioventricular valve is known as the bicuspid valve because it has two casps. It is also called the mitral valve. 

Atrioventricular Valves

1. Both valves are made up of the following components.

    a. A fibrous ring

    b. The cusps

    c. The chordae tendinae

    d. Papillary muscles

    

2. Blood vessels are present only in the fibrous ring and in the basal one-third of the cusps. Nutrition to the central two-thirds of the cusps is derived directly from the blood in the cavity of the heart.

3. The tricuspid valve has three cusps and can admit the tips of three fingers. The three cusps: the anterior, posterior or inferior, and septal.

4. The mitral or bicuspid valve has two cusps- a large anterior or aortic cusp, and a small posterior cusp.

                                                                Fig.: Interior of Heart diagram

Semilunar Valves

1.The aortic and pulmonary valves are called semilunar valves because their cusps are semilunar in shape. Both valves are similar to each other.

2. Each valves has three cusps which are attached directly to the vessel wall, there being no fibrous ring. These valves are closed during ventricular diastole when each cusp bulges towards the ventricular activity.

3. Opposite the cusps the vessel walls are slightly dilated to form the aortic and pulmonary sinuses.