The Cardiovascular System: Structure and Function

Q1, Sample Band 5 response (8 marks), annotated

Starting at the right atrium (deoxygenated), blood passes through the tricuspid valve into the right ventricle. The right ventricle contracts, pushing blood through the pulmonary valve into the pulmonary arterythe one artery that carries deoxygenated blood, an exception to the usual artery rule. [1, naming exception: pulmonary artery]

In the lung capillaries, gas exchange occurs: O₂ diffuses from alveolar air into the blood down its concentration gradient and binds to haemoglobin forming oxyhaemoglobin; CO₂ diffuses in the reverse direction from blood into the alveolar air and is exhaled. Blood is now oxygenated. [1, composition change at lungs, mechanism described]

Oxygenated blood returns via the pulmonary veinsthe one vein carrying oxygenated blood, an exception to the usual vein rule, into the left atrium. [1, naming exception: pulmonary vein]

Blood passes through the bicuspid (mitral) valve into the left ventricle, which generates ~120 mmHg pressure and ejects blood through the aortic valve into the aorta. Blood travels through systemic arteries and arterioles to reach capillary beds throughout the body. [1, all systemic vessels named in order]

At the capillaries, O₂ and glucose diffuse out of blood into tissue cells; CO₂ and metabolic waste diffuse from tissue cells into blood. Blood becomes deoxygenated. [1, composition change at capillaries, mechanism described]

Deoxygenated blood drains through venules and systemic veins into the superior and inferior vena cava, returning to the right atrium to complete the circuit. [1, correct venous return structures]

Marking criteria:

• 1 markAll four chambers named in correct order with oxy/deoxy status at each.
• 1 markAll four valves named in correct sequence (tricuspid → pulmonary → bicuspid → aortic).
• 1 markPulmonary artery and pulmonary vein named as exceptions with correct explanation.
• 1 markGas exchange at lung capillaries described with at least one mechanism (diffusion, O₂ binds haemoglobin, or CO₂ exhaled).
• 1 markExchange at systemic capillaries described: O₂/glucose out, CO₂/waste in.
• 1 markVena cava named as return vessel; circuit correctly closed back to right atrium.
• 2 marksOverall coherence: sequence is entirely correct with no transpositions of vessels/valves (award both if all structures are in the right order; 1 if 1–2 minor sequence errors).

Q2, Sample Band 5 response (7 marks), annotated

Structural differences between arteries and veins:

Wall thickness: Arteries have a thick wall with a prominent smooth muscle layer and abundant elastic fibres; veins have a thinner wall with less smooth muscle. Arteries must withstand high pressure (~120 mmHg) generated by ventricular contraction, so thick walls prevent rupture and elastic fibres smooth pulsatile flow into continuous flow. Veins carry blood at low pressure (~5–10 mmHg) and do not require the same structural integrity. [1, structure difference 1 + functional link]

Lumen size: Arteries have a relatively narrow lumen compared to wall thickness; veins have a wide lumen. The narrow arterial lumen helps sustain high pressure over long distances. The wide venous lumen reduces resistance to slow, low-pressure return flow. [1, structure difference 2 + functional link]

Why capillaries are the only exchange site: Arteries and veins have walls that are too thick (multiple cell layers including muscle and connective tissue) for diffusion to be efficient, the diffusion distance is too large. Capillary walls are only one cell thick (~0.5–1 μm), minimising diffusion distance and allowing O₂, CO₂, glucose and metabolic waste to cross rapidly by diffusion. The narrow lumen also forces red blood cells to pass single file, maximising contact time with the exchange surface. [2, diffusion distance + single-file RBCs]

Why veins have valves and arteries do not: Arteries carry blood at sustained high forward pressure from the heart, this pressure itself prevents backflow, so valves are structurally unnecessary and would only impede blood flow. Veins carry blood at very low residual pressure and often must return blood against gravity (e.g. from the legs). Without pocket valves, gravity would cause blood to pool in the lower limbs. Valves open when skeletal muscle contractions squeeze the vein, then snap shut on pressure reversal to prevent backflow, enabling upward venous return. [2, arterial pressure prevents backflow + venous valves explained with gravity]

Marking criteria:

• 2 marksTwo structural artery–vein differences, each with a functional reason (1 mark per correct pair).
• 2 marksCapillary as sole exchange site: 1 for diffusion distance, 1 for single-file RBCs or another structural reason (e.g. no muscle/connective tissue layer).
• 2 marksValve explanation: 1 for arteries (sustained pressure prevents backflow), 1 for veins (low pressure + gravity requires valves).
• 1 markAll three vessel types correctly placed in the context of the circuit (delivery, exchange, return) in a coherent overall response.

Q3, Sample Band 6 response (6 marks), annotated

The claim is entirely incorrect. [1, clear evaluative judgement]

What the claim gets wrong:

At the lungs: Blood composition changes dramatically. Entering the pulmonary capillaries, blood has low O₂ (~14 mL/100 mL) and high CO₂ (~53 mL/100 mL). After gas exchange, it leaves with high O₂ (~19 mL/100 mL) and low CO₂ (~48 mL/100 mL). The heart does not return the same blood; it returns oxygenated blood on the left side after the lungs have changed it. [1, lungs change O₂ and CO₂ with values]

At active tissues: At every capillary bed, O₂ and glucose leave the blood by diffusion into respiring tissue cells; CO₂ and metabolic waste enter the blood. Blood leaving active muscle has lower O₂ and glucose and higher CO₂ than blood entering. The composition changes proportionally with the metabolic activity of the organ. [1, tissue capillaries: O₂/glucose fall, CO₂ rises]

At the liver: Urea concentration rises across the liver (e.g. 4.0 to 6.1 mmol/L) because the liver is the sole site of urea synthesis from deamination of amino acids. Glucose may also be removed (glycogenesis) or released (glycogenolysis) depending on blood glucose levels. Blood leaving the liver is metabolically processed. [1, liver: urea rises, glucose regulated]

At the kidneys: Urea concentration falls dramatically across the kidneys (e.g. 6.0 to 1.8 mmol/L) as it is filtered from blood into the renal tubules and excreted in urine. The kidneys actively regulate the composition of blood plasma. [1, kidneys: urea falls sharply]

Accurate reformulation: Blood composition changes continuously as it circulates around the body. At the lungs, O₂ is loaded and CO₂ is unloaded; at all active tissue capillaries, O₂ and glucose fall while CO₂ rises; at the liver, urea rises and glucose is regulated; at the kidneys, urea falls sharply as it is excreted. The heart pumps blood that is in a constant state of compositional flux, the circuit is a dynamic exchange system, not a static loop. [1, defensible reformulation covering all four organ types]

Marking criteria:

• 1 markStates an overall evaluative judgement (e.g. “the claim is entirely incorrect” or “the claim misrepresents cardiovascular function”).
• 1 markCorrectly describes composition change at lungs (O₂ rises, CO₂ falls; accept with supporting values).
• 1 markCorrectly describes composition change at tissue capillaries (O₂ and glucose fall, CO₂ rises).
• 1 markCorrectly describes liver changes (urea rises; glucose regulated).
• 1 markCorrectly describes kidney changes (urea falls sharply; glucose unchanged due to reabsorption).
• 1 markReformulates the claim accurately, covering all major organ categories and framing blood composition as continuously changing around the circuit.