explain the differences in the thickness of the walls of the: atria and ventricles, left ventricle and right ventricle

Heart – Structure, Function and Wall Thickness (Cambridge AS/A‑Level Biology 9700)

Learning Objectives

• Describe the external and internal anatomy of the heart (8.3.1).
• Explain why the atrial walls differ from the ventricular walls and why the left ventricle is thicker than the right (8.3.2).
• Outline the main phases of the cardiac cycle and the associated pressure changes (8.3.3).
• Summarise the electrical conduction pathway that coordinates the cardiac cycle (8.3.4).
• Link structural features of the heart to their physiological roles (8.3.5).

1. External & Internal Anatomy (8.3.1)

The heart is a muscular, four‑chambered organ enclosed in the pericardial sac.

• Pericardium – fibrous outer layer (protects & anchors) + serous pericardium (parietal & visceral layers) with lubricating fluid.
• Chambers – right atrium (RA), left atrium (LA), right ventricle (RV), left ventricle (LV).
• Septa – inter‑atrial septum, inter‑ventricular septum (muscular & membranous portions).
• Valves – atrioventricular (tricuspid, mitral) and semilunar (pulmonary, aortic) valves.
• Wall layers (outside → inside) – epicardium (visceral pericardium), myocardium (contractile muscle), endocardium (smooth lining of chambers & valves).

Key anatomical terms required by the syllabus:

• Epicardium = visceral pericardium
• Myocardium = contractile cardiac muscle
• Endocardium = inner lining of chambers & valves
• Fibrous pericardium = tough outer sac
• Serous pericardium = parietal & visceral layers
• Tricuspid valve = right AV valve
• Mitral (bicuspid) valve = left AV valve
• Pulmonary valve = right semilunar valve
• Aortic valve = left semilunar valve
• Inter‑atrial septum, muscular & membranous IV septum

2. Wall‑Thickness Differences (8.3.2)

The thickness of a cardiac wall is an adaptation to the pressure it must generate. Higher downstream resistance → greater systolic pressure → more myocardial mass → thicker wall.

2.1 Atria vs. Ventricles

• Atria act as low‑pressure reservoirs; they receive blood from the veins (≈ 5 mm Hg) and push it into the ventricles, which are already at a relatively low pressure.
• Ventricles must eject blood into high‑resistance arterial systems (pulmonary artery & aorta); therefore they contain a much larger amount of contractile muscle.

2.2 Left Ventricle vs. Right Ventricle

• Left ventricle (LV) pumps oxygen‑rich blood into the systemic circulation (aorta). Systemic vascular resistance is high, requiring a systolic pressure of ≈ 120 mm Hg.
• Right ventricle (RV) pumps deoxygenated blood into the low‑resistance pulmonary circuit, needing only ≈ 25 mm Hg systolic pressure.
• Consequently the LV wall is about twice as thick as the RV wall.

Source: average adult values, Gray’s Anatomy (41st ed., 2015).

3. Cardiac Cycle (8.3.3)

The cardiac cycle is one complete heartbeat and consists of five mechanical phases, each with characteristic pressure changes.

1. Atrial systole – atria contract, raising atrial pressure and delivering the final 20–30 % of ventricular filling.
2. Isovolumetric ventricular contraction – ventricles begin to contract; all four valves are closed, so volume is constant while pressure rises sharply.
3. Ventricular ejection – when ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is expelled (LV ≈ 120 mm Hg, RV ≈ 25 mm Hg).
4. Isovolumetric ventricular relaxation – after ejection the ventricles relax, semilunar valves close, and pressure falls with no change in volume.
5. Ventricular filling – AV valves open as ventricular pressure falls below atrial pressure; rapid filling is followed by a slower diastasis phase until the next atrial systole.

4. Electrical Conduction System (8.3.4)

• SA node (sino‑atrial node) – primary pacemaker in the right atrial wall; initiates impulse at 60–100 bpm.
• Internodal pathways – conduct the impulse from the SA node to the AV node.
• AV node (atrioventricular node) – delays the impulse (~0.1 s) to allow complete ventricular filling, then passes it to the bundle of His.
• Bundle of His & bundle branches – transmit the impulse down the interventricular septum to both ventricles.
• Purkinje fibres – spread the impulse rapidly throughout the ventricular myocardium, producing a synchronous contraction.

5. Structure–Function Relationships (8.3.5)

• Wall thickness – directly proportional to the pressure each chamber must generate (LV > RV > ventricular walls > atrial walls).
• Valve design – AV valves are thin, flexible leaflets for low‑pressure flow; semilunar valves are thick, fibrous cusps that withstand high arterial pressure.
• Coronary supply – the thick left‑ventricular myocardium receives a richer coronary network to meet its high metabolic demand.
• Pericardial sac – fibrous pericardium limits over‑expansion; serous fluid reduces friction during each beat.

Key Points to Remember

1. Wall thickness is a functional adaptation to the pressure each chamber must generate.
2. Atria have thin walls because they operate at low venous pressures.
3. The left ventricle has the thickest wall to overcome high systemic resistance; the right ventricle is thinner because pulmonary resistance is low.
4. The cardiac cycle comprises atrial systole, isovolumetric contraction, ventricular ejection, isovolumetric relaxation and ventricular filling.
5. The SA node initiates the heartbeat; the AV node provides a crucial delay; Purkinje fibres ensure rapid, coordinated ventricular contraction.
6. Structural features such as valve composition, coronary circulation and the pericardium are closely linked to their specific physiological roles.