1. 1
   A student is studying a resting adult's lungs. At rest, the adult breathes 14 times per minute, and each breath moves 500 cm3 of air into and out of the lungs. The student examines the structure of the alveoli, the tiny air sacs where gas exchange takes place between air and the blood in the surrounding capillaries.

   (a)

   Which structural feature of alveoli increases their total surface area for gas exchange in the lungs?

   [1 mark]

   • AA single layer of flattened epithelial cells lining each alveolus
   • BThe presence of millions of tiny alveoli clustered at the end of each bronchiole
   • CA dense network of capillaries surrounding the alveoli
   • DA thin film of moisture coating the inner alveolar surface

   (b)

   During normal breathing at rest, in which direction does oxygen diffuse between the alveolus and the capillary blood?

   [1 mark]

   • AFrom the alveolus into the capillary blood, down a concentration gradient
   • BFrom the capillary blood into the alveolus, down a concentration gradient
   • CFrom the alveolus into the capillary blood, by active transport
   • DOxygen does not move because the concentrations are equal

   (c)

   Explain two ways in which the structure of an alveolus is adapted to allow efficient diffusion of gases.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A patient with a lung condition called fibrosis has alveoli walls that have thickened due to scarring, increasing the average diffusion distance from 0.5 micrometres to 2.0 micrometres. Doctors compare this patient's gas exchange to a healthy person's, using Fick's law: rate of diffusion is proportional to (surface area times concentration difference) divided by membrane thickness.

   (a)

   According to Fick's law, if the surface area available for diffusion is doubled while the concentration difference and membrane thickness stay constant, what happens to the rate of diffusion?

   [1 mark]

   • AIt halves
   • BIt stays the same
   • CIt doubles
   • DIt quarters

   (b)

   Based on Fick's law, what effect would the thickened alveolar walls in the fibrosis patient have on the rate of diffusion of oxygen into the blood, compared with a healthy person?

   [1 mark]

   • AThe rate of diffusion would increase
   • BDiffusion would stop being dependent on a concentration gradient
   • CThe rate of diffusion would be unaffected
   • DThe rate of diffusion would decrease

   (c)

   The fibrosis patient often feels breathless during light exercise. Using Fick's law and your knowledge of gas exchange, explain why the thickened alveolar walls cause this symptom.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A group of factory workers who have been exposed to certain industrial dusts for many years are found to have a lung disease that destroys the walls between adjacent alveoli, causing many small alveoli to merge into fewer, larger air sacs. A doctor investigates why these workers experience breathlessness and low blood oxygen levels.

   (a)

   A group of factory workers develop a lung disease that destroys the walls between adjacent alveoli, causing many small alveoli to merge into fewer, larger air sacs. Explain why this reduces the efficiency of gas exchange in the workers' lungs.

   [3 marks]

   (b)

   The doctor also finds that the affected workers have a much higher concentration of carbon dioxide in their blood than healthy individuals. Explain, using ideas about surface area, concentration gradients and diffusion distance, how the disease could cause this build-up of carbon dioxide.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A mountaineer climbs to a high altitude where the concentration of oxygen in the air is much lower than at sea level, reducing the concentration gradient available to drive oxygen diffusion into the blood across the alveoli.

   (a)

   Explain how the structure of the alveoli and the properties of diffusion described by Fick's law together allow some gas exchange to continue at high altitude, and evaluate why the mountaineer is still likely to experience symptoms of oxygen shortage despite these adaptations.

   [6 marks]

   (b)

   The mountaineer's expedition team decides to use portable oxygen canisters above a certain altitude to breathe air enriched with extra oxygen. Using Fick's law, explain how breathing oxygen-enriched air would help restore more effective gas exchange in the mountaineer's lungs, and discuss one limitation of relying on this method during a long expedition.

   [6 marks]

   Total for question 4: 12 marks

End of questions