HSC Exam Practice

Sample response. Partial pressure is the pressure exerted by one particular gas in a mixture, directly proportional to its concentration in that mixture. Gases diffuse from areas of higher partial pressure to areas of lower partial pressure, the direction of net diffusion is always down the partial pressure gradient.

Marking notes. 1 mark for defining partial pressure (pressure of one gas in a mixture, proportional to its concentration). 1 mark for explaining that diffusion occurs from high to low partial pressure.

Sample response. External gas exchange occurs at the alveolar surface (between alveolar air and pulmonary capillary blood): O₂ diffuses from alveolar air into blood; CO₂ diffuses from blood into alveolar air. Internal gas exchange occurs at systemic capillaries (between blood and tissue cells): O₂ diffuses from blood into tissue cells; CO₂ diffuses from tissue cells into blood.

Marking notes. 1 mark per exchange site (2); 1 mark per correct directional pair for O₂ and CO₂ (2). Total 4 marks.

Sample response. Fick’s law: Rate of diffusion ∝ (surface area × concentration gradient) / membrane thickness. The three variables are: (1) surface area, (2) concentration gradient, (3) membrane thickness.

Marking notes. 1 mark for the correct relationship (rate proportional to SA and gradient; inversely proportional to thickness). 1 mark for naming all three variables correctly.

Sample response. Ventilation (breathing) continuously refreshes alveolar air with O₂-rich air and removes CO₂-enriched air, maintaining alveolar pO₂ at ~100 mmHg and pCO₂ at ~40 mmHg. If ventilation stopped, alveolar O₂ would be consumed by diffusion into blood but never replaced; alveolar pO₂ would fall until it equalled blood pO₂, and the gradient would collapse, stopping diffusion. Blood flow continuously delivers deoxygenated blood (pO₂ ~40 mmHg) to the pulmonary capillaries and removes oxygenated blood, keeping the blood-side pO₂ lower than alveolar pO₂. If blood flow stopped, blood in pulmonary capillaries would equilibrate with alveolar air and the gradient would collapse, stopping diffusion.

Marking notes. 1 mark, ventilation refreshes alveolar air / maintains alveolar pO₂. 1 mark, what happens if ventilation stops (gradient collapses). 1 mark, blood flow delivers deoxygenated blood / maintains blood-side low pO₂. 1 mark, what happens if blood flow stops (gradient collapses). Total 4 marks.

Sample response. Alveolar membrane gradient: alveolar air pO₂ (100) − arterial blood arriving pO₂ (~40) = 60 mmHg. Tissue capillary gradient: arterial blood pO₂ (95) − tissue cell pO₂ (~25, using midpoint 20–30) = ~70 mmHg.

Marking notes. 1 mark per correct calculation with correct locations identified. Accept tissue gradient values using 20 (75 mmHg) or 30 (65 mmHg). Total 2 marks.

Sample response. According to Fick’s law, rate of diffusion is proportional to surface area × concentration gradient / membrane thickness. Although the CO₂ partial pressure gradient is small (5 mmHg), the alveolus compensates with an enormous surface area (~250 m² across ~500 million alveoli) and an extremely thin membrane (~0.5 μm). These two Fick variables maximise diffusion rate even when the gradient is modest. Additionally, continuous ventilation refreshes alveolar air (keeping alveolar pCO₂ low at 40 mmHg) and continuous blood flow delivers CO₂-rich venous blood (pCO₂ ~45 mmHg) to the capillaries, maintaining the gradient so exchange does not stop.

Marking notes. 1 mark, identifies that large SA and/or thin membrane compensate for the small gradient (Fick’s law applied). 1 mark, explains that maintained gradient via ventilation and/or blood flow ensures continuous exchange. Total 2 marks.

Sample response. The claim is incorrect. The table shows that arterial blood (having just left the alveoli) has a pO₂ of 95 mmHg, this is only 5 mmHg below alveolar pO₂ (100 mmHg). Blood is therefore nearly fully oxygenated, not “low in oxygen.” Diffusion is driven by partial pressure gradients, not by total oxygen content. The 5 mmHg gradient is sufficient to drive diffusion across the 0.5 μm membrane because of the alveolus’s enormous surface area. Even fully oxygenated blood has slightly lower pO₂ than alveolar air, so the gradient always favours diffusion into blood.

Marking notes. 1 mark, uses table data to show blood is nearly fully oxygenated (pO₂ 95 vs alveolar 100). 1 mark, correctly states diffusion is driven by partial pressure gradient, not total content. 1 mark, explains that the small 5 mmHg gradient is still sufficient given large SA and thin membrane. Total 3 marks.

Sample response. During intense exercise, cells increase their rate of aerobic respiration, consuming O₂ faster and producing more CO₂. As a result: tissue cell pO₂ drops further (possibly to ~10–15 mmHg) and pCO₂ rises higher (60+ mmHg). Venous blood pO₂ also falls (to ~15 mmHg) and pCO₂ rises. By Fick’s law, these changes increase the concentration gradient driving O₂ from blood into tissue cells (gradient rises from ~65 mmHg to ~80 mmHg+), which accelerates O₂ delivery to meet increased demand. The larger gradient also drives faster CO₂ removal from tissues.

Marking notes. 1 mark, predicts tissue / venous pO₂ decreases during exercise (faster O₂ consumption). 1 mark, predicts tissue / venous pCO₂ increases (faster CO₂ production). 1 mark, applies Fick’s law: steeper gradient → faster O₂ delivery to tissues. Total 3 marks.

Sample response. The alveolus is adapted to maximise gas exchange efficiency through three structural features, each linked to a variable in Fick’s law (rate ∝ SA × concentration gradient / membrane thickness). First, the lung contains approximately 500 million alveoli providing a total surface area of ~250 m². By Fick’s law, rate is proportional to surface area, so this enormous area allows billions of O₂ and CO₂ molecules to cross simultaneously, meeting the body’s full metabolic demand. Second, the combined thickness of the alveolar epithelium (type I cells) and capillary endothelium is only ~0.5 μm. By Fick’s law, rate is inversely proportional to membrane thickness, so this minimal distance allows gases to diffuse in milliseconds. Third, the alveolar surface is coated with a thin fluid layer containing surfactant, gases must dissolve in this moisture layer before diffusing, so a dry surface would prevent exchange entirely. Beyond structure, the concentration gradient is maintained by two active mechanisms: ventilation continuously refreshes alveolar air (keeps alveolar pO₂ at ~100 mmHg, preventing it from falling as O₂ diffuses into blood); blood flow continuously removes oxygenated blood from pulmonary capillaries and replaces it with deoxygenated blood (keeps blood pO₂ low on the blood side, maintaining the gradient). Both are essential, if either stops, the gradient collapses and gas exchange ceases.

Marking notes.