K327 / K328 / 2027

Respiration in humans

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Which route correctly takes inhaled air towards its main gas-exchange surface?

  1. Larynx -> trachea -> bronchi -> bronchioles -> alveoli
  2. Alveoli -> larynx -> bronchioles -> trachea
  3. Trachea -> bronchioles -> capillaries -> bronchi -> alveoli
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Larynx -> trachea -> bronchi -> bronchioles -> alveoli

Gas exchange occurs across the thin alveolar and capillary walls at the end of this route.

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Why does a continuous capillary blood supply help alveolar gas exchange?

  1. It makes the alveolar wall thicker.
  2. It carries inhaled air directly to the heart, which pumps the oxygen round the body.
  3. It brings oxygen-poor blood and removes oxygen-rich blood, maintaining a gradient.
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It brings oxygen-poor blood and removes oxygen-rich blood, maintaining a gradient.

Maintaining the difference between alveolar air and incoming blood supports continued net oxygen diffusion.

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Cilia are damaged and mucus accumulates in the airways. Which explanation best links cause and effect?

  1. Tar damages cilia, reducing movement of mucus towards the throat.
  2. Carbon monoxide makes cilia carry more oxygen.
  3. Nicotine thickens the mucus into fibrin, which sticks the cilia together.
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Tar damages cilia, reducing movement of mucus towards the throat.

Poorer clearance leaves mucus and trapped material in the airways.

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Which statement correctly describes aerobic respiration in a human cell?

  1. Glucose is broken down to lactic acid, and oxygen is released as a product.
  2. Breathing takes in oxygen, which the lungs turn into glucose for the cells.
  3. Glucose and oxygen are used; carbon dioxide and water form; energy is released.
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Glucose and oxygen are used; carbon dioxide and water form; energy is released.

This is the aerobic word equation and its energy consequence.

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Which is a correct word equation for anaerobic respiration in human muscle?

  1. glucose + oxygen -> lactic acid + energy released
  2. glucose -> lactic acid + energy released
  3. glucose -> ethanol + carbon dioxide + energy released
  4. lactic acid + oxygen -> glucose + energy released
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glucose -> lactic acid + energy released

Human muscle without enough oxygen breaks glucose down to lactic acid, releasing a little energy.

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Why does a runner keep breathing rapidly and deeply after vigorous exercise?

  1. To breathe the lactic acid made in the muscles directly out of the lungs
  2. To supply extra oxygen needed for recovery and removal of the oxygen debt
  3. Because the cells no longer need any oxygen at rest
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To supply extra oxygen needed for recovery and removal of the oxygen debt

The additional oxygen supports recovery after anaerobic activity, so ventilation remains elevated for a time.

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Invented data: a runner's breathing rate is 14 breaths/min at rest, 40 at the end of a sprint and 20 after three minutes. Why is it still above 14 after three minutes?

  1. The heart has stopped pumping, so breathing must make up for it
  2. The muscles are still respiring anaerobically to make carbon dioxide
  3. The lungs are still breathing out the lactic acid made in the sprint
  4. Extra oxygen is still needed to break down lactic acid
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Extra oxygen is still needed to break down lactic acid

The oxygen debt is repaid after exercise by oxidising lactic acid, so breathing stays fast.

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Question 1 of 8

Connect air and blood

4 marks

Trace inhaled air from the larynx to an alveolus. Explain how three features of the alveolus and its capillary supply support gas exchange, including the net directions of oxygen and carbon dioxide. [4]

Airway order first; then structure -> effect -> diffusion.

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