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The Respiratory System

Mechanics of inspiration and expiration

  • 3 things to know
  • A2 2

Overview

Boyle's law, inspiration and expiration at rest and in exercise, the extra muscles used, the respiratory centre and its nerves, chemoreceptors and stretch receptors, and recovery posture.

What you need to know

Answer each one before you reveal it.

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Key term

State Boyle's law and explain how it applies to breathing.

Key term

Describe the mechanics of inspiration at rest.

Key term

Why is expiration at rest described as passive?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State Boyle's law and explain how it applies to breathing.

    Model answer

    Boyle's law states that at a constant temperature the pressure of a gas is inversely proportional to its volume. A larger chest volume lowers the pressure in the lungs, so air flows in.

  2. 2

    Describe the mechanics of inspiration at rest.

    Model answer

    The diaphragm contracts and flattens while the external intercostals lift the ribs up and out. The thoracic cavity enlarges, lung pressure falls below atmospheric pressure and air flows in.

  3. 3

    Why is expiration at rest described as passive?

    Model answer

    Expiration at rest needs no muscle contraction. The diaphragm and external intercostals relax and the stretched lungs recoil elastically.

  4. 4

    Describe the pressure changes that cause expiration at rest.

    Model answer

    The thoracic cavity becomes smaller as the breathing muscles relax. Pressure in the lungs rises above atmospheric pressure, so air is pushed out.

  5. 5

    Name the additional muscles used for inspiration during exercise and state what each does.

    Model answer

    The sternocleidomastoid lifts the sternum. The scalenes and the pectoralis minor lift the ribs higher than at rest.

  6. 6

    How do the additional inspiratory muscles increase the volume of each breath?

    Model answer

    The extra muscles raise the ribs and sternum further than at rest. The thoracic cavity becomes larger, lung pressure falls lower and more air rushes in.

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Inspiration means breathing air into the lungs.
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