Human Anatomy: Heart Location and Position

Location and Position of the Heart

• The heart is a conical muscular organ located in the middle mediastinum within the pericardium.
• The heart is situated superior to the diaphragm, left of the midline, anterior to the vertebral column, and posterior to the sternum.
• The weight of the heart is approximately 300g in males and 250g in females.
• The heart is about the size of a fist.

Position of the Heart

• The heart is positioned obliquely behind the body of the sternum and adjacent costal cartilages.
• One-third of the heart lies to the right, while two-thirds lie to the left of the median plane.

Surface Anatomy of the Heart

• The upper border of the heart is marked by a straight line joining a point at the lower border of the second left costal cartilage and a point at the upper border of the third right costal cartilage.
• The lower border of the heart is marked by a straight line joining a point at the lower border of the sixth right costal cartilage and the anterior interventricular groove.
• The anterior interventricular groove is situated nearer to the left margin of the heart and separates the apex from the rest of the inferior border.
• The posterior interventricular groove is found on the inferior surface of the heart.
• The two interventricular grooves meet at the inferior border of the heart near the apex.

Grooves/Sulci of the Heart

• The interatrial groove is faintly visible posteriorly while hidden by the aorta and pulmonary trunk anteriorly.

Apex of the Heart

• The apex is formed entirely by the left ventricle.
• It is directed downwards, forwards, and to the left.
• It is situated in the left 5th intercostal space, 9cm lateral to the mid-sternal line, just medial to the midclavicular line.

Base of the Heart

• The base, also known as the posterior surface, is mainly formed by the left atrium, with a small part of the right atrium.
• The left atrium has openings for four pulmonary veins.
• The right atrium receives the superior and inferior vena cava.

Borders of the Heart

• The heart has an upper, lower, right, and left border.
• The upper border, slightly oblique, is formed by both atria, primarily the left atrium.
• The right border is more or less vertical and formed by the right atrium.
• The lower border, nearly horizontal, is primarily formed by the right ventricle, with a small part near the apex formed by the left ventricle.
• The left border, oblique and curved, is mainly formed by the left ventricle, with a partial contribution from the left auricle.

Surfaces of the Heart

• The heart has an anterior/sternocostal, inferior/diaphragmatic, left, and posterior/base surface.
• The anterior/sternocostal surface is primarily formed by the right atrium and right ventricle, with partial contributions from the left ventricle and left auricle.
• The inferior/diaphragmatic surface rests on the central tendon of the diaphragm and is formed by the left ventricle on the left 2/3rd and the right ventricle on the right 1/3rd.
• The left surface is predominantly formed by the left ventricle, with a small part near the upper end formed by the left auricle.

Right Atrium

• The right atrium is the upper chamber of the heart.
• It receives venous blood from the entire body through the superior and inferior vena cava.
• It pumps venous blood into the right ventricle through the right atrioventricular or tricuspid opening.
• External features include a right auricle formed by the upper end prolongation and the sulcus terminalis, a shallow vertical groove on the right border that extends from the SVC to the IVC.
• The sulcus terminalis is produced by the crista terminalis, a muscular ridge in the interior.
• The upper part of the sulcus terminalis contains the SA (sinu atrial) node, the pacemaker of the heart.

Tributaries/Inlets of the Right Atrium

• The right atrium receives blood from the following:
  • Superior vena cava
  • Inferior vena cava
  • Coronary sinus
  • Anterior cardiac veins
  • Venae cordis minimi (Thebesian veins)

Interior of Right Atrium

• The interior of the right atrium is divided into three parts:
  • Smooth posterior part (sinus venarum)
  • Rough anterior part (pectinate part, including the auricle)
  • Interatrial septum

Smooth Posterior Part

• The smooth posterior part receives blood from the:
  • Superior vena cava at the upper end
  • Inferior vena cava at the lower end
    • The inferior vena cava opening is guarded by a valve (Eustachian valve).
    • During embryonic development, the Eustachian valve guides the inferior vena caval blood to the left atrium through the foramen ovale.
  • Coronary sinus
    • This opening is located between the opening of the inferior vena cava and the right atrioventricular orifice.
    • The coronary sinus opening is guarded by the valve of the coronary sinus.
  • Venae cordis minimi are small veins that open into all four chambers.

Rough Anterior Part

• The rough anterior part is characterized by transverse muscular ridges called Musculi pectinati (resembling teeth of a comb).

Interatrial Septum

• The interatrial septum has features like:
  • Fossa ovalis: A shallow saucer-shaped depression in the lower part that represents the site of the embryonic septum primum.
  • Annulus fossa ovalis/ limbus fossa ovalis: The margin of the fossa ovalis, representing the free edge of septum secundum.

Right Ventricle

• The interior of the right ventricle is divided into two parts:

  • Inflowing rough part: Contains muscular ridges called trabeculae carneae.
  • Outflowing smooth part/ Infundibulum: The upper conical part of the right ventricle, giving rise to the pulmonary trunk.

• These two parts are separated by a muscular ridge called the SUPRAVENTRICULAR CREST situated between the tricuspid (right atrioventricular orifice) and pulmonary orifice.

• The right ventricle has two orifices:

  • Right atrioventricular orifice: Guarded by the tricuspid valve.
  • Pulmonary orifice: Guarded by the pulmonary valve.

• The inflow rough part contains three types of trabeculae carneae or muscular ridges:

  • Ridges (septomarginal trabeculae/ moderator band)
  • Bridges
  • A pair of semilunar valves

• Important valvular notes:

  • The right atrioventricular valve is called the tricuspid valve due to its three cusps.
  • The left atrioventricular valve is called the bicuspid valve due to its two cusps.
  • Semilunar valves include the aortic and pulmonary valves, both having 3 semilunar cusps.
  • Cusps are folds of endocardium strengthened by fibrous tissues.

Fetal Circulation

• Before birth, blood from the placenta, about 80% saturated with oxygen, returns to the fetus through the umbilical vein.
• Most of the blood flows through the ductus venosus directly into the inferior vena cava, bypassing the liver.
• A smaller amount enters the liver sinusoids and mixes with blood from the portal circulation.
• The blood enters the right atrium, where it is guided towards the oval foramen by the valve of the inferior vena cava, and most of the blood passes directly into the left atrium.
• In the left atrium, blood mixes with a small amount of desaturated blood returning from the lungs and enters the left ventricle and ascending aorta.
• Desaturated blood from the superior vena cava flows through the right ventricle into the pulmonary trunk.
• During fetal life due to high resistance in the pulmonary vessels, most of the blood passes through the ductus arteriosus directly into the descending aorta, mixing with blood from the proximal aorta.
• Blood flows towards the placenta through two umbilical arteries.
• The oxygen saturation in these arteries is approximately 58%.

This quiz covers the anatomical location and positioning of the heart within the mediastinum. It examines key features such as its size, weight, and surface anatomy. Test your understanding of where the heart is situated relative to other body structures.