Key ideas for breathing and gas exchange

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  • Diaphragm contracts and becomes flatter
  • External intercostal muscles contract and pull ribs upwards
  • Volume of chest cavity increases
  • Pressure in chest cavity drops below atmospheric pressure
  • Air moves into lung


  • Diaphragm relaxes and is pushed upwards by digestive organs
  • External intercostal muscles relax and ribs drop
  • Volume of chest cavity decreases
  • Pressure in lungs increases above atmospheric pressure
  • Air moves out of the lungs

(Internal intercostal muscles only contract during forced expiration)

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Measuring Lung Capacity

A spirometer measures lung capacity and breathing rate - it can measure the following

  • Tidal volume - the volume of air moved in and out of the lungs during each breath at rest. It is normally around 0.5dm3
  • Vital Capacity - The largest volume of air that can be moved into and out of the lungs in one breath. It is normally around 5dm3
  • Residual Volume - the volume of air that remains in the lungs even after forced expiration. normally around 1.5dm3
  • Inspiratory Reserve Volume - the volume of extra air that can be taken in above the amount of normal tidal volume
  • Expiratory Reserve Volume - the volume of extra air that can be moved out above the amount of normal tidal volume
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The main airways are the Trachea, Bronchi and Bronchioles

Airways should have the following features

  • Large enough to allow sufficient air flow without obstruction
  • Must divide into smaller airways to deliver air to all the alveoli
  • Must be strong enough to not collapse when air pressure is low
  • Must be flexible to allow movement
  • Must be able to stretch and recoil

Airways are adapted for their function in the following ways

  • Incomplete rings of cartilage in the trachea and bronchi offer structural support while allowing movement
  • Goblet cells produce mucus to trap particles such as pollen or bacteria
  • Ciliated epithelium have cilia which move to waft the mucus to the top of the throat
  • Smooth muscle in all of the airways allow it to constrict the lumen, restricting airflow
  • Elastic fibres cause recoil that will dilate the airways to their original size
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Adaptations for gas exchange

The lungs are adapted in several ways for efficient gas exchange. Some adaptions increase the rate of diffusion by maintaining a diffusion gradient, some shorten the diffusion pathway

  • Large surface areas (more space for diffusion) provided by the alveoli
  • Barrier that is permeable to oxygen and carbon dioxide (plasma membranes are the only barriers)
  • The walls of the alveoli which are only one cell thick (reduced diffusion distance)
  • The walls of the cappilaries which are only one cell thick (reduced diffusion distance).
  • The cells in the walls of the alveoli and capillaries are flattened squamous epithelial cells (reduced diffusion distance).
  • The capillaries are in close contact with alveoli (reduced diffusion distance).
  • The capillaries are narrow, squeezing blood cells against their walls (reduced diffusion distance).
  • Ventilating the lungs keeps levels of oxygen high and carbon dioxide low in the lungs (maintains gradient).
  • Constant blood flow keeps levels of carbon dioxide high and oxygen low in the capillaries (maintains gradient)
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