Pericardium and Heart

Fibrous pericardium:

• Tough, outer, non-distensible fibrous sac enclosing the heart and roots of the great vessels.
• Attaches inferiorly to the central tendon of the diaphragm (blending with it), anteriorly to the sternum via sternopericardial ligaments, and superiorly/posteriorly blends with the adventitia of the great vessels as they pierce it.
• Limits excessive/acute distension of the heart, providing a protective mechanical constraint.

Serous pericardium:

• Thin, double-layered serous membrane lining the interior of the fibrous pericardium and reflecting onto the heart's surface, analogous in organisation to the pleura around the lungs.
• Parietal layer: Lines the internal surface of the fibrous pericardium.
• Visceral layer (epicardium): Closely invests the heart's surface itself, forming the outermost layer of the heart wall.
• Between the two serous layers is the pericardial cavity, a potential space containing a thin film of serous fluid, allowing frictionless cardiac movement within the fibrous sac.

Pericardial cavity:

• Normally contains only 15–50 mL of serous fluid; pathological accumulation (pericardial effusion) can compress the heart if it develops rapidly, given the fibrous pericardium's non-distensible nature (see Clinical Anatomy in Section 12).

• Formed where the serous pericardium reflects around the great vessels, creating two clinically important recesses within the pericardial cavity.

Transverse sinus:

• Passage behind the ascending aorta/pulmonary trunk and in front of the superior vena cava/left atrium.
• Surgically important: allows a surgeon's finger or clamp to be passed behind the great arterial vessels (aorta and pulmonary trunk together), useful for temporarily occluding them during cardiac surgery (e.g., cardiopulmonary bypass cannulation).

Oblique sinus:

• A blind-ending recess posterior to the left atrium, bounded by the reflections of the pericardium around the pulmonary veins (laterally/superiorly) and the inferior vena cava.
• Allows the posterior surface of the left atrium some freedom of movement within the pericardial sac; does not communicate with the transverse sinus.

• Attachments: Diaphragm (inferiorly), sternum (sternopericardial ligaments, anteriorly), great vessel adventitia (superiorly/posteriorly).
• Innervation: Phrenic nerves (C3–C5) provide the principal sensory innervation to the fibrous pericardium and parietal serous pericardium, explaining referred pain patterns in pericarditis (which can mimic cardiac ischaemic pain, but classically may also involve shoulder-tip referral); the visceral (epicardial) layer receives autonomic innervation from the cardiac plexus, largely insensitive to pain in the same way as visceral pleura.
• Blood supply: Pericardiacophrenic arteries (branches of the internal thoracic artery, accompanying the phrenic nerves) are the main supply, with contributions from the musculophrenic and bronchial arteries, and branches of the descending thoracic aorta.

• Base: Posterior surface of the heart, formed mainly by the left atrium (with a smaller contribution from the right atrium); faces posteriorly toward the vertebral column (T5–T8 level), related to the oesophagus.
• Apex: Formed by the tip of the left ventricle, points anteriorly, inferiorly, and to the left; classically located in the 5th intercostal space, midclavicular line: the surface landmark for the palpable apex beat.
• Surfaces:
• Sternocostal (anterior) surface: mainly right ventricle, with parts of the right atrium and left ventricle.
• Diaphragmatic (inferior) surface: mainly left ventricle, with part of the right ventricle; rests on the central tendon of the diaphragm.
• Left (pulmonary) surface: mainly left ventricle, related to the left lung's cardiac impression.
• Borders:
• Right border: right atrium (roughly vertical, corresponding to the right side of the heart shadow on chest X-ray).
• Inferior border: mainly right ventricle, with the apex of the left ventricle.
• Left border: mainly left ventricle, with a small contribution from the left auricle superiorly.
• Superior border: formed by the roots of the great vessels and both atria/auricles.
• Grooves:
• Coronary (atrioventricular) sulcus: encircles the heart, marking the external division between the atria and ventricles; contains the right coronary artery, circumflex artery, and coronary sinus.
• Anterior and posterior interventricular sulci: mark the external division between the right and left ventricles; contain the anterior and posterior interventricular arteries respectively, along with their accompanying veins.

Right Atrium

• Receives venous return from the superior vena cava (superiorly), inferior vena cava (inferiorly), and coronary sinus (via its own ostium, guarded by the thebesian valve).
• Smooth-walled part (sinus venarum): posterior, derived embryologically from the sinus venosus.
• Rough-walled part (with pectinate muscles): anterior, including the right auricle; separated from the smooth part by the crista terminalis (a muscular ridge, internal correlate of the external sulcus terminalis, and the origin site of the SA node).
• Fossa ovalis: oval depression in the interatrial septum, the remnant of the fetal foramen ovale.
• Inlet: Tricuspid valve (into the right ventricle); Outlet: none other than the tricuspid valve (blood flows onward only into the right ventricle).

Right Ventricle

• Inflow tract: Smooth-walled, receives blood through the tricuspid valve.
• Trabeculated part: Contains coarse muscular ridges (trabeculae carneae) and the characteristic septomarginal trabeculation (moderator band): carries the right bundle branch of the conduction system directly to the anterior papillary muscle.
• Outflow tract (conus arteriosus/infundibulum): Smooth-walled, funnel-shaped, leading to the pulmonary valve.
• Inlet: Tricuspid valve; Outlet: Pulmonary valve (into the pulmonary trunk).
• Papillary muscles: Typically three (anterior, posterior, septal), giving rise to chordae tendineae supporting the tricuspid valve cusps.

Left Atrium

• Receives the four pulmonary veins (two from each lung), typically without valves at their ostia.
• Forms the base (posterior surface) of the heart; smaller and thicker-walled than the right atrium.
• Left auricle: the rough, pectinate-muscle-lined appendage, a common site of thrombus formation in atrial fibrillation given its irregular shape and relatively stagnant flow.
• Inlet: Pulmonary veins (×4); Outlet: Mitral valve (into the left ventricle).

Left Ventricle

• Thickest-walled chamber (highest pressure workload, systemic circulation), with a roughly conical/ellipsoid shape and fine trabeculae carneae.
• Inflow tract: Receives blood through the mitral valve.
• Outflow tract (aortic vestibule): Smooth-walled, leads to the aortic valve.
• Inlet: Mitral valve; Outlet: Aortic valve (into the ascending aorta).
• Papillary muscles: Typically two (anterolateral, posteromedial), larger than the right ventricle's, supporting the mitral valve's two cusps via chordae tendineae.

Valve anatomy (general principles):

• The two atrioventricular valves (tricuspid, mitral) are supported by chordae tendineae attaching to papillary muscles, preventing prolapse of the cusps into the atria during ventricular systole.
• The two semilunar valves (pulmonary, aortic) have no chordae/papillary muscle attachments; they close purely passively by the pressure gradient reversal at the end of systole, with each cusp forming a small pocket (sinus) that fills and coapts the cusps shut.
• The aortic sinuses (of Valsalva), behind the right and left aortic cusps specifically, give rise to the right and left coronary arteries: the only two of the three aortic sinuses with a vessel origin (the posterior/non-coronary sinus does not).

Fibrous skeleton:

• A dense connective tissue framework (four fibrous rings, or annuli fibrosi, around each valve orifice, plus the right and left fibrous trigones connecting them) that provides structural support for all four valve orifices.
• Serves as the origin/insertion point for the atrial and ventricular myocardium and electrically insulates the atria from the ventricles, except at the single normal point of conduction, the AV node/bundle (see Coronary Circulation and Cardiac Conduction section), a critical functional role preventing aberrant/re-entrant conduction pathways in the normal heart.

Papillary muscles:

• Right ventricle: typically three (anterior, posterior, septal); left ventricle, typically two (anterolateral, posteromedial).
• Contract just before/during ventricular systole, tensioning the chordae tendineae to hold the AV valve cusps in a closed, coapted position and prevent them from prolapsing backward under the high systolic pressure.

Chordae tendineae:

• Fine, fibrous cords connecting the free edges and ventricular surfaces of the AV valve cusps to the tips of the papillary muscles.
• Their integrity is essential for competent AV valve closure: rupture (e.g., from infective endocarditis, ischaemic papillary muscle dysfunction/rupture post-myocardial infarction) causes acute valvular regurgitation.