describe the external and internal structure of the mammalian heart

Overview of the Mammalian Heart

The mammalian heart is a muscular, cone‑shaped organ that forms the centre of a closed double‑circulation system:

• Systemic circulation – carries oxygen‑rich blood from the left side of the heart to the body and returns de‑oxygenated blood to the right atrium via the superior and inferior venae cavae.
• Pulmonary circulation – carries de‑oxygenated blood from the right side of the heart to the lungs (pulmonary artery) and returns oxygen‑rich blood to the left atrium via the pulmonary veins.

Four major vessels are directly attached to the heart and define the direction of blood flow:

External Structure of the Heart

1. Coverings (Pericardium)

• Fibrous pericardium – tough outer layer that anchors the heart to the diaphragm and sternum.
• Serous pericardium – consists of two layers:
  • Parietal layer – lines the inner surface of the fibrous pericardium.
  • Visceral layer (epicardium) – covers the heart surface; also regarded as the outermost layer of the heart wall.
  The parietal and visceral layers enclose the pericardial cavity, a thin fluid‑filled space that reduces friction during each beat.

2. Surface Markings

• Coronary (atrioventricular) sulcus – encircles the heart, separating atria from ventricles; contains the coronary arteries and coronary sinus.
• Anterolateral (sternocostal) surface – faces the sternum; shows the right and left ventricles and the left anterior descending (LAD) artery.
• Posterior (diaphragmatic) surface – rests on the diaphragm; displays the right and left atria and the openings of the IVC and SVC.

3. Major Vessels Attached to the Heart

• Superior vena cava (SVC)
• Inferior vena cava (IVC)
• Pulmonary trunk – divides into right and left pulmonary arteries that carry de‑oxygenated blood to the lungs.
• Pulmonary artery (branches of the trunk)
• Aorta
• Pulmonary veins (usually four)
• Coronary arteries – right coronary artery (RCA) and left coronary artery (LCA) arise from the aortic sinuses and run in the coronary sulcus.
• Coronary veins – drain myocardial blood into the coronary sinus, which empties into the right atrium.

Internal Structure of the Heart

1. Layers of the Heart Wall (outside → inside)

1. Epicardium – visceral serous layer (also the outermost myocardial layer).
2. Myocardium – thick band of cardiac muscle; the contractile engine.
3. Endocardium – thin, smooth lining of chambers and valves; continuous with the endothelium of the great vessels.
4. Fibrous skeleton (cardiac skeleton) – dense connective tissue that:
   • Provides attachment points for all cardiac valves.
   • Electrically insulates atria from ventricles, forcing the impulse to travel through the specialised conduction system.

2. Chambers and Their Characteristics

3. Valves and Valvular Apparatus

Atrioventricular (AV) Valves

• Tricuspid valve – between RA and RV; three leaflets, each attached to papillary muscles by chordae tendineae.
• Mitral (bicuspid) valve – between LA and LV; two leaflets, also supported by chordae tendineae and papillary muscles.

Semilunar (SL) Valves

• Pulmonary valve – between RV and pulmonary trunk; three cusps.
• Aortic valve – between LV and aorta; three cusps.

Supporting Structures

• Chordae tendineae – strong fibrous cords that anchor AV‑valve leaflets to papillary muscles.
• Papillary muscles – contract during ventricular systole, pulling on the chordae to prevent valve prolapse.

4. Septa

• Interventricular septum – separates RV and LV; has a muscular portion and a small membranous portion (common site of congenital defects).
• Atrial septum – separates RA and LA; contains the fossa ovalis, a remnant of the fetal foramen ovale.

5. Conduction System (Electrical Control)

6. Cardiac Cycle & Pressure Changes

The cardiac cycle consists of a repeating sequence of atrial and ventricular systole and diastole. Pressure differences drive valve opening and closing.

Phases (with typical pressures)

• AV valves close when ventricular pressure exceeds atrial pressure (end of ventricular filling).
• Semilunar valves close when aortic or pulmonary pressure exceeds ventricular pressure (end of ejection).

7. Heart Sounds

• S₁ (“lub”) – closure of the AV valves (tricuspid & mitral) at the start of systole.
• S₂ (“dub”) – closure of the semilunar valves (pulmonary & aortic) at the end of systole.
• Additional sounds (S₃, S₄, murmurs) indicate abnormal flow or pathology.

8. Coronary Circulation (Blood Supply to the Myocardium)

• Right coronary artery (RCA) – arises from the right aortic sinus; supplies the right atrium, right ventricle, part of the interventricular septum and, in ~60 % of people, the SA node.
• Left coronary artery (LCA) – arises from the left aortic sinus and quickly divides into:
  • Left anterior descending (LAD) artery – runs down the anterior interventricular sulcus; supplies the anterior LV wall and most of the septum.
  • Circumflex artery – follows the atrioventricular groove; supplies the lateral LV wall and left atrium.
• Coronary veins – collect de‑oxygenated myocardial blood and drain into the coronary sinus, which opens into the right atrium.

9. Clinical Relevance

10. Comparison of Wall Thickness (Relative)

Key Points for Revision (Cambridge AS/A‑Level)

1. Identify the four layers of the pericardium and their protective roles.
2. Recall the three wall layers (epicardium, myocardium, endocardium) plus the fibrous skeleton and its mechanical/electrical functions.
3. Describe the four chambers, their positions, wall‑thickness differences and the direction of blood flow in systemic and pulmonary circuits.
4. Distinguish atrioventricular valves from semilunar valves; remember leaflet numbers and the chordae‑papillary‑muscle apparatus.
5. Explain the structure and significance of the interventricular and atrial septa, including the fossa ovalis.
6. Outline the complete cardiac cycle, the associated pressure changes, and how these drive valve opening/closing.
7. Identify each component of the cardiac conduction system (SA node, AV node, bundle of His, bundle branches, Purkinje fibres) and its role in coordinating the heartbeat.
8. Link heart sounds S₁ and S₂ to the closure of specific valves.
9. Summarise the origin, main branches, and drainage of the coronary circulation.
10. Recall common clinical conditions (myocardial infarction, valve stenosis/regurgitation, arrhythmias) and how they relate to cardiac structure.