The Heart: Anatomical Position, External Features, and Chambers

1. Anatomical Position

• It is positioned obliquely relative to the body's axis
• The apex points forward, downward, and to the left, lying deep to the left 5th intercostal space, approximately 8-9 cm from the midsternal line
• The base (posterior surface) is directed to the right, superiorly and posteriorly, opposite the bodies of vertebrae TV to TVIII (TVI-TIX when standing)
• One-third of the heart lies to the right of the midline; two-thirds lie to the left

2. External Features

Shape and Surfaces

Fig. 1 - Schematic illustration of heart orientation, surfaces, and margins (Gray's Anatomy for Students)

Anterior (Sternocostal) Surface

Fig. 2 - Anterior Surface of the Heart. LA = left atrium; LV = left ventricle; RA = right atrium; RV = right ventricle (Gray's Anatomy for Students)

• Formed predominantly by the right ventricle
• The right atrium with its three-cornered auricle and a narrow strip of the left ventricle are also visible
• The anterior interventricular sulcus (groove) marks the boundary between the ventricles and contains the anterior interventricular artery (branch of left coronary artery) and accompanying cardiac vein, embedded in adipose tissue
• The right side contour is formed by the right atrium and the superior vena cava
• The right auricle occupies the space between the superior vena cava and the root of the aorta
• The coronary sulcus (atrioventricular groove) separates the atria from the ventricles and is also filled with coronary vessels and adipose tissue
• The pulmonary trunk arises from the right ventricle, lying anterior to the aorta; they wind around each other

Diaphragmatic (Inferior) and Posterior Surfaces

Fig. 3 - Diaphragmatic Surface and Base (Posterior) of the Heart (Gray's Anatomy for Students)

• The diaphragmatic surface rests on the diaphragm, separated from the base by the coronary sinus
• The posterior interventricular groove runs on this surface, separating the left and right ventricles inferiorly
• The base is quadrilateral and directed posteriorly; it consists of the left atrium (predominantly), a small portion of the right atrium, and the proximal parts of the great veins (superior and inferior venae cavae and pulmonary veins)

Fig. 4 - Base of the Heart showing the left atrium, pulmonary veins, and coronary sinus (Gray's Anatomy for Students)

Margins and Borders

External Sulci (Grooves)

• Coronary sulcus (atrioventricular groove): Encircles the heart, separating atria from ventricles; contains the right coronary artery and coronary sinus
• Anterior interventricular sulcus: On the sternocostal surface; contains the anterior interventricular artery and great cardiac vein
• Posterior interventricular sulcus: On the diaphragmatic surface; contains the posterior interventricular artery (branch of right coronary artery) and middle cardiac vein
• Sulcus terminalis: A shallow groove on the external surface of the right atrium, corresponding internally to the crista terminalis

3. Chambers of the Heart

Fig. 5 - Internal chambers: Right atrium (right lateral view) and right ventricle (anterior view) opened (Color Atlas of Human Anatomy, Vol. 2)

A. Right Atrium

• Receives the superior vena cava (SVC) - opening directed downward and anteriorly, no valve
• Receives the inferior vena cava (IVC) at the lowest point - guarded by the crescent-shaped valve of the inferior vena cava (Eustachian valve)
• During fetal life, this valve directed blood through the foramen ovale into the left atrium; after birth, this becomes the oval fossa (fossa ovalis) bordered by the limbus fossae ovalis
• The coronary sinus opens between the IVC opening and the tricuspid valve, guarded by the valve of the coronary sinus (Thebesian valve)

• Contains pectinate muscles - ridges of cardiac muscle projecting into the cavity
• Continuous anteriorly with the right auricle
• Separated from the sinus of the venae cavae internally by the crista terminalis (a muscular ridge)

B. Right Ventricle

• Receives blood from the right atrium through the right atrioventricular (tricuspid) valve
• The tricuspid valve has three cusps (anterior, posterior, septal), connected by chordae tendineae to three papillary muscles (anterior, posterior, and septal)
• The walls are irregular with trabeculae carneae (muscular ridges)

• Funnel-shaped with smooth walls
• Leads to the pulmonary valve (3 semilunar cusps), through which blood enters the pulmonary trunk

• Predominantly muscular (~1.2 cm thick)
• Small membranous part (~1 mm thick) near the atrium, from which the septal leaflet of the tricuspid valve arises
• Bulges into the ventricular cavity

C. Left Atrium

• Receives four pulmonary veins (two from each lung) entering its upper portion - no valves at pulmonary vein openings
• Continuous anteriorly with the left auricle, which contains small pectinate muscles
• Near the interatrial septum is the valve of the foramen ovale - a remnant produced by the fossa ovalis of the right atrium
• No clear demarcation between smooth-walled and muscular portions (unlike the right atrium)
• Forms the majority of the base (posterior surface) of the heart

D. Left Ventricle

• Receives blood from the left atrium through the left atrioventricular (mitral/bicuspid) valve
• The mitral valve has two large cusps (anterior and posterior), connected by thick, strong chordae tendineae to two papillary muscles:
  • Anterior papillary muscle - arises from the sternocostal surface
  • Posterior papillary muscle - arises from the diaphragmatic surface
• Walls are irregular with trabeculae carneae

• Smooth-walled, passes along the interventricular septum toward the aorta
• Guarded by the aortic valve (3 strong semilunar cusps)
• The anterior cusp of the mitral valve divides the inflow and outflow tracts

• Mostly muscular with a small membranous part just caudal to the right and posterior aortic valve cusps

Summary Table: The Four Chambers