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A-Level Biology October/November 2024 Q1(b): There is a regular and efficient supply of blood from the heart to the lungs. Describe…
A-Level Biology · Paper 9700/23 · October/November 2024 · Question 1(b) · [5 marks]
There is a regular and efficient supply of blood from the heart to the lungs. Describe the sequence of events that occurs in the heart to make sure that there is a regular and efficient supply of blood to the lungs.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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The sequence begins in the wall of the right atrium. The sinoatrial node (SAN), acting as the pacemaker, generates a wave of excitation. This impulse spreads across the walls of the right atrium, causing it to contract (atrial systole). This contraction, along with the opening of the tricuspid valve, pushes blood into the right ventricle.
The wave of excitation is collected by the atrioventricular node (AVN), which introduces a brief time delay. This delay ensures the atria have finished contracting before the ventricles begin. The AVN then sends the impulse down the Purkyne tissue in the septum to the apex of the heart.
This triggers the contraction of the right ventricle from the apex upwards. As the pressure in the right ventricle exceeds the pressure in the right atrium, the tricuspid valve closes to prevent backflow. When ventricular pressure exceeds the pressure in the pulmonary artery, the pulmonary semi-lunar valve opens and blood is forced into the pulmonary artery to be transported to the lungs.
How the marks are awarded
- B1 — Stating that the sinoatrial node (SAN) generates impulses or waves of excitation.
- B1 — Describing the impulse spreading across the walls of the right atrium, causing it to contract.
- B1 — Identifying the role of the atrioventricular node (AVN) in causing a time delay.
- B1 — Stating that the right ventricle contracts.
- B1 — Stating that the semi-lunar valve opens, allowing blood to flow into the pulmonary artery.
Common mistakes
- Confusing the left and right sides of the heart, for example, mentioning the bicuspid valve or the aorta, which are involved in the systemic circuit, not the pulmonary circuit.
- Describing the sequence of events incorrectly, such as stating that the ventricles contract before the atria.
- Forgetting to mention the electrical control system (SAN, AVN, Purkyne tissue) and only describing the mechanical pumping action.
- Failing to name the specific valves (tricuspid/right atrioventricular and pulmonary/semi-lunar) or describing their actions incorrectly.
Examiner tip: For processes with a specific sequence, such as the cardiac cycle, create a flowchart that links each electrical event to its corresponding mechanical event (contraction and valve action).
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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