7.2.4Unit 6: Practical Skills in Biology II
The Role of Muscle in the Cardiac Cycle — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 26 of 48 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Spread of Excitation
1
Cardiac muscle is myogenic, so it can contract and relax rhythmically without nervous stimulation.
2
The sinoatrial node in the wall of the right atrium sets the rhythm and starts a wave of depolarisation across both atria, causing atrial systole.
3
A layer of non-conducting tissue between atria and ventricles stops the wave passing directly down, so it can only reach the atrioventricular node.
4
The atrioventricular node delays the signal briefly, which allows the atria to empty before the ventricles contract.
5
The signal passes along the bundle of His in the septum, then through Purkyne fibres that spread upward from the apex, so the ventricles contract from the bottom upwards.
Section 2 of 4Stages of the Cardiac Cycle
Stages of the Cardiac Cycle
Atrial systole
- Atrial systole The atria contract and push the last blood into the ventricles through open atrioventricular valves.
Ventricular systole
- Ventricular systole The ventricles contract, the atrioventricular valves shut and blood is forced out through the aorta and pulmonary artery.
Diastole
- Diastole The whole heart relaxes, the semilunar valves close, and blood flows into the atria from the veins.
| Stage | Atria | Ventricles |
|---|---|---|
| Atrial systole | Contract | Relaxed and filling |
| Ventricular systole | Relaxed | Contract |
| Diastole | Relaxed | Relaxed |
Section 3 of 4Reading an ECG
P wave
- P wave Depolarisation of the atria, which leads to atrial systole.
QRS complex
- QRS complex Depolarisation of the ventricles leading to ventricular systole, and it is the largest wave because of the large ventricular muscle mass.
T wave
- T wave Repolarisation of the ventricles, which is followed by ventricular relaxation.
Tachycardia
- Tachycardia A resting heart rate above 100 beats per minute, seen as closely spaced complexes.
Bradycardia
- Bradycardia A resting rate below 60 beats per minute, which is common and harmless in very fit people.
Fibrillation
- Fibrillation Irregular, uncoordinated contraction of atria or ventricles, which may be fatal when the ventricles are affected.
Section 4 of 4Exam Checklist & Tips
7.2.4 – Key Points for the Exam
- Myogenic Able to initiate its own contraction without nervous stimulation.
- Sinoatrial node A group of cells in the right atrium that sets the heart rhythm.
- Atrioventricular node Conducting tissue that delays the impulse before it reaches the ventricles.
- Purkyne fibres Fibres that carry the impulse through the ventricle walls from the apex.
- Electrocardiogram A recording of the electrical activity of the heart.
Common Mistakes
- Writing that the nervous system starts each heartbeat; nerves only change the rate.
- Stating that the wave of excitation passes directly across the atria and ventricles without delay.
- Confusing the P wave with the QRS complex; P is atrial and QRS is ventricular.
- Saying ventricles contract from the top down; they contract from the apex upwards.
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