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Lungs: seventy square metres, folded into a chest questions

The route from trachea to alveolus and the cartilage, ciliated epithelium, goblet cells, smooth muscle and elastic fibres found at each level, the structure of an alveolus and the thickness of the barrier between air and blood, the mechanism of inspiration and expiration in terms of muscles, volume and pressure, and pulmonary ventilation rate with the lung volumes read from a spirometer trace.

5 original questions · 18 marks · the lungs: seventy square metres, folded into a chest notes · Exchange surfaces and gas exchange

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• 5 marks · Calculate

  A spirometer trace shows 21 complete breaths in 84 seconds, each moving 0.55 dm³ of air. Calculate the pulmonary ventilation rate in dm³ min⁻¹. About 0.15 dm³ of every breath stays in the dead space; calculate the volume of air reaching the alveoli each minute.

  Mark scheme

  1. M1 ventilation rate = 21 × (60 ÷ 84), converting the count to a number of breaths per minute
  2. A1 15 breaths per minute
  3. M1 pulmonary ventilation rate = tidal volume × ventilation rate = 0.55 × 15
  4. A1 8.25 dm³ min⁻¹, with the unit given
  5. A1 alveolar volume = (0.55 − 0.15) × 15 = 6.0 dm³ min⁻¹, because the dead space is paid for on every breath

• 4 marks · Describe

  Describe how air is drawn into the lungs during inspiration, giving the events in the order in which they happen.

  Mark scheme

  1. B1 the external intercostal muscles contract, pulling the ribcage upwards and outwards
  2. B1 the diaphragm contracts, flattens and moves downwards
  3. B1 the volume of the thoracic cavity increases, and the lungs are stretched with it
  4. A1 the pressure inside the lungs falls below atmospheric pressure, so air moves in down the pressure gradient

• 4 marks · Explain

  Explain why inspiration always costs the body energy while quiet expiration costs almost none.

  Mark scheme

  1. B1 inspiration depends on the diaphragm and external intercostal muscles contracting, and muscle contraction requires ATP
  2. B1 elastic fibres in the alveolar walls and airways are stretched as the lungs inflate, storing energy as they do so
  3. B1 in quiet expiration those muscles simply relax, and the stretched elastic tissue recoils, which needs no contraction of any muscle
  4. A1 the recoil reduces the volume of the thorax, so pressure in the lungs rises above atmospheric and air moves out

• 3 marks · Suggest

  In emphysema the elastic fibres of the alveolar walls are destroyed and the walls themselves break down, so that many small alveoli are replaced by fewer, larger air spaces. Suggest why a person with emphysema becomes breathless on mild exertion.

• 2 marks · Name

  A section through an airway shows irregular blocks of cartilage in the wall, together with a ciliated epithelium containing goblet cells. Name this airway, and name the tissue that holds a bronchiole open instead of cartilage.

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