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The heart is a hollow, roughly pyramidal shaped, fibromuscular organ lying obliquely behind the sternum, in the middle mediastinum. Approximately one-third of the heart lies to the right of midline and two-third to the left. Males have slightly heavier heart (300 gm.) than females (250 gm.). Heart has 4 chambers: - 2 posterosuperior: atrial chambers and 2 anteroinferior: ventricular chambers.


External features

The heart has following borders and surfaces:-

  1. Borders
    1. Right border:-Formed by right atrium.
    2. Left border (obtuse margin):-Formed mainly by left ventricle and partly by left auricle (in its upper most part).
    3. Inferior border (acute margin):-Formed mainly by right ventricle and partly by left ventricle near apex.
    4. Upper border: - Mainly by left atrium and partly by right atrium where SVC enters.
    5. Apex:-Formed by left ventricle.
  2. Surfaces
  • Anterior (sternocostal) surface: - Formed mostly by right ventricle (major) and right auricle and partly by left ventricle and left auricle.
  • Inferior (diaphragmatic) surface: - It is formed by left ventricle (left 2/3) and right ventricle (right 1/3). It is traversed by posterior interventricular groove (PIV) containing PIV branch ofRCA.
  • Base (posterior surface):- Formed mainly by left atrium and partly by right atrium. It is separated from T5 to T8 vertebrae by pericardium, oblique pericardial sinus, esophagusand descending thoracic aorta.
  • Right surface:-Mainly by right atrium.
  • Left surface:-Mainly by left ventricle and partly by left auricle.

Internal features

  1. Right atrium
    It has thinnest walls of the four chambers. It is divided by crista terminalis into two parts:-
    1. Rough anterior part or atrium proper (pectinate part), including auricle.
      It is derived from right half of primitive atrial chamber. There is a series of transverse muscular ridges calledmusculipectinatiwhich are attached to crista terminalis and gives appearance like "teeth of a comb". SA node is situated in the upper part of crista terminates. Auricle lies in the superomedialportion.
    2. Posterior smooth part or sinus venorum.
      It is derived from absorption of right horn of sinus venosus. It has openings of:-
      1. Superior vena cava: -Has no valve.
      2. Inferior vena cava: Orifice is guarded by a rudimentary valve of IVC (Eustachian valve) derived from embryonic right venous valve, which directs the blood from IVC towardsforamenovalein fetal live. This valve is non-functional in adults.
      3. Coronary sinus: -Orifice lies between tricuspid orifice and IVC orifice. It is guarded by a functional semilunar valve (thebesian valve). Thebesian valve develops from right venous valve.
      4. Venae cardisminimae (thebesian veins):-Open through foramina venorumminimarum.
      5. Anterior cardiac vein
      6. Right marginal vein:-Sometimes it may open into coronary sinus (not in right atrium).
    3. Interatrial septal region
      1. Interatrial septum develops from approximation of embryonic septum primum and septumsecundum. Features on right atrial side are:
        • Fossa ovalis:-Oval shaped depression lying above the level of IVC opening in the interatrialseptum. It is the embryologic remnant offoramenovale, which connects right atrium to left atrium in embryonic life. Floor of the fossa ova is formed by septum primum.
        • Limbus fossa ovalis (Annulus ovalis):-It is thickened rim present above the fossa ovalis. It represents the lower free margin of septum secundum.
        • Triangle of Koch:-It is a triangular region in the lower part of interatrial septum which contains AV node. It is bounded above by tendon of Todarobelow by base of septal leaflet of tricuspid valveand anteriorly (base) by orifices of coronary sinus.
      2. Immediately above the membranous septum, the septal region shows a slight bulge known as torus aorticus, produced by right posterior (non-coronary) aortic sinus (of valsalva).
      3. Crista terminalis (divides the right atrium into anterior and posterior part is a ridge of smooth muscle fibres extending from the SVC to valve of IVC. It is developed from embryonic right venous valve and is represented on the surface by sulcus terminalis.



  1. Right Ventricle
    1. It is anteroinferiorchamberand projects to the left of right atrium. It is divided into-
      1. Rough inflowing part (ventricle proper)
        It is developed from right half of primitive ventricle.
      2. Smooth outflowing part (infundibulum or conusarteriosus)
        Its interior is rough due to the presence of muscular ridges known as trabeculae carneae, which are more prominent in apical region. Trabeculae carneae are of3 types.
        • Ridges - Linear elevations.
          Supraventricular crest - a ridge present between the pulmonary and atrioventricular orifices, extends downwards in the posterior wall of the infundibulum.
        • Bridges - Muscular elevations with fixed ends on ventricular walls, the centre being free.Septomarginaltrabeculae: It is a specialized bridge which extends from the right of ventricular septum to the base of anterior papillary muscle. It contains the right branch of atrioventricularbundle.
        • Papillary muscles are conical projections of muscle fiber bundles. Their base is attached to the ventricular-wall and the apex is attached to the chordae tendinae, which are further attached to the cusps of atrioventricular (A V) valves. There are three papillary muscles in the right and two in the left ventricle. They regulate closure of atrioventricular valves.
    2. Smooth outflowing part develops from mid portion of bulbuscordisand surrounds pulmonary valve. Supraventricular crest (crista supraventricularis) or infundibuloventricular crest separates tricuspid (A V) orifice and pulmonary orifice, i.e. inlet and outlet parts.
  2. Left Atrium
    1. It has slightly thicker walls than right atrium. It is divided into :-
      1. Posteriorly smooth walled part receiving 4 pulmonary veins which develops from absorption of embryonic pulmonary veins.
      2. Rough walled part (left auricle) having network of musculipectinatidevelopingfrom left half of primitiveatrium. Left auricle projects anteriorly to overlap infundibulum of right ventricle.
    2. Left atrium forms the anterior wall of oblique sinus of pericardium. It lies in front of esophagus.
  3. Left Ventricle
    1. It has three times thicker walls than right ventricle. It is divided into two parts :-
      1. Rough walled in flowing part (left ventricle proper) developing from left of primitive ventricle.
      2. Smooth walled outflowing part (aortic vestibule) developingfrombulbuscordis. Aortic vestibule leads to ascending aorta.

Arterial supply of the heart

The arterial supply of the heart is provided by the right and the left coronary arteries, which arise from the ascending aorta above the aortic valve.


  1. Right coronary artery
    It is a branch of anterior aortic sinusof ascending aorta. It runs in the right anterior coronary sulcus (right atrioventricular groove) and then winds round to inferior border to run backwards in the right posterior coronary sulcus and reach the posterior interventricular groove where it ends by anastomosing with left coronary artery. Branches of right coronary artery are:-
    1. Acute marginal
    2. Posterior interventricular (descending) artery in 85-90% of cases.
    3. Right conus (infundibular) artery (sometimes it arises directly from anterior aortic sinus and is called third coronary artery.)
    4. Nodal branch to SA node (in 65% case).
    5. Atrial, anterior ventricular and posterior ventricular.

Right coronary artery supplies:-

  1. Right atriumand a part of left atrium.
  2. Most of the right ventricle (except the area adjoining the anterior interventricular groove) and small partofleft ventricle adjoining posterior interventricular groove.
  3. Posterior 1/3 of interventricular septum.
  4. Whole conducting system (except right bundle branch and a part ofleft branch of AV bundle).
  5. SA node (in 65%).
  1. Left coronary artery
    Left coronary artery is larger than right coronary artery, and supplies major part of heart, it arises from left posterior aortic sinus of ascending aorta. It enters the atrioventricular groove and gives anterior interventricularbranch. Further continuation of LCA is known as circumflex artery which runs in left anterior coronary (interventricular) sulcus, and then continues in left posterior coronary sulcus. Near the posterior interventriculargroove it terminates by anastomosing with RCA.

Branches ofLCA are -

  1. Anterior interventricular (descending) artery.
  2. Circumflex artery.
  3. Left diagonal.
  4. Obtuse marginal (left marginal).
  5. Left conus artery.
  6. Atrial branch, anterior ventricular branch and posterior ventricular branch.
  7. Nodal (in 35%) for SA node.

Left coronary artery supplies:-

  1. Most of left atrium.
  2. Most of the left ventricle- (except the area adjoining posterior interventricular groove) and small part of right ventricle adjoining anterior interventricular groove.
  3. Anterior 2/3 of ventricular septum.
  4. Right bundle branch, left bundle branch and in 35% of cases SA node.
  • SA node is supplied by right coronary artery in65% of population and by left coronary artery in 35% of population
  1. Cardiac dominance
    The artery which gives the posterior interventricular branch is the dominant artery. In 85-90% cases right coronary artery gives posterior interventricular artery, i.e. in 90% population, there is right dominance. In 10-15% population circumflex artery (a branch of LCA) gives posterior interventricularartery, i.e. left dominance. In codominance or balanced pattern, branches of both RCA and LCA run in the posterior interventricular groove. Dominance should not be confused with the amount of tissue supplied; left coronary artery supplies major part of heart, whether right dominance or left dominance.  
  2. Involvement of coronary circulation in thrombosis
    1. Left anterior descending artery (anterior interventricular) artery is most commonly involved in thrombotic occlusion. This results in infarction of anterior wall of left ventricle apex of heart and anterior 2/3 of ventricular septum.
    2. Right coronary artery or its posterior interventricularbranchis involved 2nd most commonly. This results in infarction of inferior/posterior wall of left and right ventricle, posterior 1/3 of ventricular septum.
    3. 3rd most commonly involved vessel is circumflex coronary artery (branch ofLCA). This results in infarction of lateral wall of left ventricle except apex.
    4. Left main coronary artery, diagonal branches of LAD artery and marginal branches of circumflex artery are uncommon to be involved.

Veins of the heart

Three systems of veins drain the heart;-

  • Coronary sinus and its tributaries;
  • anterior cardiac vein;
  • Venae cordisminimae.


  1. Coronary sinus
    It opens in the posterior wall of right atrium, in the posterior part of coronary sulcus. It opens in the right atrium between IYC and tricuspid orifices. The opening is guarded by semilunar valve of coronary sinus (Thebesian valve) which prevents regurgitation of blood into coronary sinus during atrial systole. Coronary sinus develops from body and left horn of sinus venosus. Tributaries of coronary sinus are;
    1. Great cardiac vein: Lies in the anterior interventriculargroove. Left marginal vein drains into it.
    2. Middle cardiac vein: -Lies in the posterior interventricular groove.
    3. Posterior vein of left ventricle.
    4. Small cardiac vein:-It lies in the posterior part of coronary sulcus with RCA. Right marginalvein may sometimes open into small cardiac vein, more often; however, right marginal vein opens directly into right atrium.
    5. Oblique vein of left atrium (vein of marshal):- It is continuous above with ligament of IVC. These two structures are embryological remnants ofleft common cardinal vein (duct of Cuvier).
  2. Anterior cardiac vein
    1. It drains into right atrium piercing the anterior surface of the right atrium close to sulcus terminalis.
  3. Venae cordis minimi (smallest cardiac veins or Thebesian vein)
    1. These are multiple small veins present in all four chambers of heart, more numerous on right side.

All the veins mentioned above drain into coronary sinus except anterior cardiac vein, venae cordis minimi and often right marginal vein. These three drain directly into right atrium.


Nerve supply of heart

  1. Motor component
    Motor component includes parasympathetic and sympathetic systems. Parasympatheticfibres reach the heart through vagus and are cardioinhibitory. Sympathetic fibers are derived from T1 to T5 segment of spinal cordand are cardiostimulatory (excitatory) .Both parasympathetic and sympathetic nerves form the superficial and deep cardiac plexus, the branches of which run along the coronary arteries to reach the myocardium.
    1. Superficial cardiac plexus: - Lies in front of right pulmonary artery below the arch of aorta. It is formed by :-
      1. Superior cervical cardiac branch of the left sympathetic chain.
      2. Inferior cervical cardiac branch of left vagus.
    2. Deep cardiac plexus: - Lies in from of the bifurcation of trachea, behind the arch of aorta. It is formed by all sympathetic and parasympathetic cardiac branches except those forming the superficial plexus.
  2. Sensory component
    Pain sensation arising due to ischemia is conveyed by afferents which pass through sympathetic cardiac fibers and reach the T1 to T 5 cord segments on left side. Since these dorsal root ganglia also receive sensory impulses from the medial side of arm, forearm and upper part of front of chest, the pain gets referred to these areas through these thoracic splanchnic nerves (T1-T4). Though the pain is usually referred to the left side, it may even be referred to right arm, jaw, epigastrium, neck, shoulder or back.

Conducting system of heart


The conducting system is made ofspecialized myocardium (cardiac muscIe), that is capable for initiationand conduction of cardiac impulse. It has following parts: -

  1. SA node: - SA node is located in the upper part of crista terminalis at the junction of SVC and the rightatrium. SAnode is supplied by right vagus/parasympathetic (inhibitory) and right Sympathetic (excitatorysystem as it develops from structures on the right side of embryo.
  2. AV node:-It lies in the right atrial floor near the interatrial septum in the 'triangle of Koch'. It is supplied by AV nodal artery, a branch of RCA. AV node develops from left side of heart, thus is supplied by left vagus and left sympathetic fibers.
  3. Atrioventricular bundle or bundle of His:-It arises fromAV node and crosses theAVring (annulus fibrosus). In the muscular septum it divides into right and left branches. It has a dual bloodsupply from AV nodal artery (branch ofRCA) and anterior descending (interventricular) branch ofLCA.
  4. Right bundle branch (RBB) and left bundle branch (LBB):- These bundle branches consist of modified muscle fibers (Purkinjefibers). Both RBB and LBB are supplied by,except a small part of theLBB which is supplied by RCA.

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