CIE AS/A Level Biology -8.3 The heart- Study Notes- New Syllabus
CIE AS/A Level Biology -8.3 The heart- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -8.3 The heart- Study Notes- New Syllabus
Key Concepts:
- describe the external and internal structure of the mammalian heart
- explain the differences in the thickness of the walls of the:
• atria and ventricles
• left ventricle and right ventricle - describe the cardiac cycle, with reference to the relationship between blood pressure changes during systole and diastole and the opening and closing of valves
- explain the roles of the sinoatrial node, the atrioventricular node and the Purkyne tissue in the cardiac cycle (knowledge of nervous and hormonal control is not expected)
External and Internal Structure of the Mammalian Heart
🌱 Overview
The mammalian heart is a muscular, hollow organ that pumps blood through the double circulatory system. It has four chambers, valves, and is surrounded by protective structures.
1. External Structure of the Heart
| Feature | Description / Function |
|---|---|
| Shape & Size | Cone-shaped, roughly the size of a fist. Apex points downward, forward, and left. |
| Pericardium | Double-layered sac surrounding the heart; protects and reduces friction. |
| Atria (Left & Right) | Upper chambers; receive blood from veins. Thin muscular walls. |
| Ventricles (Left & Right) | Lower chambers; pump blood into arteries. Thick muscular walls, especially the left ventricle. |
| Coronary Blood Vessels | Supply heart muscle with oxygen and nutrients. |
| Major Blood Vessels | – Aorta: carries oxygenated blood to body. – Pulmonary artery: carries deoxygenated blood to lungs. – Pulmonary veins: return oxygenated blood from lungs. – Vena cava (superior & inferior): return deoxygenated blood from body. |
2. Internal Structure of the Heart
| Feature | Description / Function |
|---|---|
| Chambers | – Right atrium: receives deoxygenated blood from body. – Right ventricle: pumps blood to lungs via pulmonary artery. – Left atrium: receives oxygenated blood from lungs. – Left ventricle: pumps oxygenated blood to body via aorta. |
| Atrioventricular (AV) valves | – Tricuspid valve: between right atrium and ventricle. – Bicuspid / mitral valve: between left atrium and ventricle. – Prevent backflow of blood into atria. |
| Semilunar valves | – Pulmonary valve: at pulmonary artery exit. – Aortic valve: at aorta exit. – Prevent backflow into ventricles. |
| Interventricular septum | Muscular wall separating left and right ventricles; prevents mixing of oxygenated and deoxygenated blood. |
| Chordae tendineae & Papillary muscles | Connect AV valves to ventricular walls; prevent valve prolapse during contraction. |
| Endocardium | Smooth inner lining of chambers; reduces friction for blood flow. |
| Myocardium | Thick muscular layer; strongest in left ventricle for systemic circulation. |
| Epicardium | Outer layer of heart wall; forms part of pericardium. |
🧠 Key Points:
– Double circulation: Right side pumps to lungs, left side pumps to body.
– Valves ensure unidirectional blood flow.
– Left ventricle is thicker than right due to higher pressure required for systemic circulation.
– Coronary vessels provide oxygen and nutrients to heart muscle itself.
– Double circulation: Right side pumps to lungs, left side pumps to body.
– Valves ensure unidirectional blood flow.
– Left ventricle is thicker than right due to higher pressure required for systemic circulation.
– Coronary vessels provide oxygen and nutrients to heart muscle itself.
Differences in Thickness of the Heart Walls
🌱 Overview
The heart walls vary in thickness according to their function and the pressure they must generate. Key comparisons: atria vs ventricles and left ventricle vs right ventricle.
1. Atria vs Ventricles
| Feature | Atria | Ventricles | Reason for Difference |
|---|---|---|---|
| Wall thickness | Thin | Thick | Atria pump blood only to ventricles (short distance, low pressure) vs ventricles pump blood to lungs/body (longer distance, higher pressure) |
| Function | Receive blood from veins | Pump blood into arteries | Requires stronger contraction in ventricles |
Key Point: Ventricles have thicker muscular walls to generate higher pressure needed for systemic and pulmonary circulation.
2. Left Ventricle vs Right Ventricle
| Feature | Left Ventricle | Right Ventricle | Reason for Difference |
|---|---|---|---|
| Wall thickness | Thickest of all chambers | Thinner than left ventricle | Left ventricle pumps blood to whole body (high resistance, high pressure); right ventricle pumps blood to lungs (low resistance, low pressure) |
| Shape of cavity | Circular/oval lumen | Crescent-shaped lumen | Helps generate higher pressure efficiently in left ventricle |
Key Point: Left ventricular walls are much thicker than right to overcome systemic vascular resistance.
🧠 Summary:
– Atria < Ventricles in wall thickness → due to distance and pressure of blood pumped.
– Right Ventricle < Left Ventricle → due to pulmonary vs systemic circulation pressures.
– Wall thickness reflects the functional demands of each chamber.
– Atria < Ventricles in wall thickness → due to distance and pressure of blood pumped.
– Right Ventricle < Left Ventricle → due to pulmonary vs systemic circulation pressures.
– Wall thickness reflects the functional demands of each chamber.
The Cardiac Cycle
🌱 Overview
The cardiac cycle is the sequence of events in one heartbeat: contraction (systole) and relaxation (diastole) of the atria and ventricles. Includes changes in blood pressure, blood flow, and valve movements.
🔬 Phases of the Cardiac Cycle
1. Atrial Systole
- What happens: Atria contract, pushing blood into ventricles.
- Valves:
- Atrioventricular valves (tricuspid & mitral) → open
- Semilunar valves (pulmonary & aortic) → closed
- Blood pressure: Slight increase in atrial pressure; ventricular pressure rises slightly as ventricles fill.
2. Ventricular Systole
- What happens: Ventricles contract, pumping blood into arteries.
- Valves:
- Atrioventricular valves → close (prevent backflow, causes first heart sound – “lub”)
- Semilunar valves → open when ventricular pressure > arterial pressure
- Blood pressure: Ventricular pressure rises sharply; atrial pressure low, filling from veins.
3. Diastole (Ventricular and Atrial Relaxation)
- What happens: Heart relaxes, chambers refill with blood.
- Valves:
- Semilunar valves → close (prevent backflow, causes second heart sound – “dub”)
- Atrioventricular valves → open when ventricular pressure < atrial pressure
- Blood pressure: Ventricular pressure falls; blood flows passively from atria to ventricles.
📊 Summary Table: Cardiac Cycle
| Phase | Heart Action | Valves | Blood Pressure Changes |
|---|---|---|---|
| Atrial systole | Atria contract | AV open, SL closed | Atrial pressure ↑, ventricular pressure ↑ slightly |
| Ventricular systole | Ventricles contract | AV closed, SL open | Ventricular pressure ↑ sharply, atrial pressure low |
| Diastole | Heart relaxes, chambers refill | AV open, SL closed | Ventricular pressure ↓, atrial pressure ↑ slightly |
🧠 Key Points:
– Valve movements prevent backflow and maintain unidirectional blood flow.
– Systole = contraction, Diastole = relaxation.
– Blood pressure rises during systole and falls during diastole.
– Heart sounds (“lub-dub”) correspond to closure of AV and semilunar valves.
– Valve movements prevent backflow and maintain unidirectional blood flow.
– Systole = contraction, Diastole = relaxation.
– Blood pressure rises during systole and falls during diastole.
– Heart sounds (“lub-dub”) correspond to closure of AV and semilunar valves.
Role of the Sinoatrial Node, Atrioventricular Node, and Purkyne Tissue in the Cardiac Cycle
🌱 Overview
The heart beats rhythmically due to specialized pacemaker and conduction tissues. Key structures: Sinoatrial (SA) node, Atrioventricular (AV) node, Purkyne tissue. This intrinsic system coordinates contraction without nervous or hormonal input.
1. Sinoatrial (SA) Node
- Location: Wall of the right atrium near the entrance of the superior vena cava.
- Role:
- Acts as the natural pacemaker of the heart.
- Initiates electrical impulses that spread across atria → atrial systole.
- Sets the rate of heartbeat.
2. Atrioventricular (AV) Node
- Location: Between atria and ventricles, near the tricuspid valve.
- Role:
- Receives impulses from SA node.
- Delays transmission to ventricles slightly → allows atria to empty completely before ventricles contract.
3. Purkyne Tissue (Bundle of His and Purkinje Fibres)
- Location:
- Bundle of His runs down interventricular septum.
- Divides into Purkinje fibres spreading into ventricular walls.
- Role:
- Conducts impulses rapidly to ventricular muscle.
- Ensures ventricles contract simultaneously from apex upwards, efficiently pumping blood into arteries.
📊 Summary Table: Pacemaker and Conduction System
| Structure | Location | Function |
|---|---|---|
| SA node | Right atrium wall | Initiates heartbeat; spreads impulse to atria |
| AV node | Between atria and ventricles | Delays impulse; ensures ventricles fill |
| Purkyne tissue | Ventricular walls (via Bundle of His) | Rapid conduction; ventricles contract apex → base |
🧠 Key Points:
– SA node = “pacemaker” → sets heartbeat.
– AV node = delays impulse → coordinated atrial and ventricular contraction.
– Purkyne fibres = rapid conduction → synchronized ventricular contraction.
– Ensures efficient pumping of blood during the cardiac cycle.
– SA node = “pacemaker” → sets heartbeat.
– AV node = delays impulse → coordinated atrial and ventricular contraction.
– Purkyne fibres = rapid conduction → synchronized ventricular contraction.
– Ensures efficient pumping of blood during the cardiac cycle.