9.1.1

Mammalian Gas Exchange

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Structure

Mammalian gas exchange takes place in specialised organs called the lungs. The lungs are specialised for the quick exchange of oxygen and carbon dioxide with the bloodstream.

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Structure

  • The structure of the lungs is designed for efficient gas exchange.
  • Air enters the body through the trachea and travels through a highly branched system where surface area is maximised for exchange of oxygen and carbon dioxide.
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Trachea

  • The trachea is the entrance to the mammalian gas exchange system.
  • When you breathe in, air flows through the trachea.
  • Ridges of cartilage surround the front of the trachea to provide protection and structure.
    • There is no cartilage at the back of the trachea so that the oesophagus (the tube where food travels) is not constricted.
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Bronchi

  • The trachea divides into two bronchi.
  • Air flows along each bronchus to a lung.
  • The bronchi are made from cartilage and smooth muscle.
  • The cartilage provides support. Smooth muscle can contract and dilate to control the size of the airway.
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Bronchioles

  • Each bronchus divides into many smaller bronchioles.
  • The many bronchioles branch throughout the lungs into small air-sacs called alveoli.
  • Bronchioles do not contain any cartilage. Patency is maintained via elastic fibers. Bronchioles are surrounded by a thin layer of smooth muscle.
    • Smooth muscle contracts to limit airflow to the alveoli. Smooth muscle dilates to increase airflow to the alveoli.
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Alveoli

  • The alveoli are sacs that fill with air when you breathe in.
  • Oxygen in the alveoli diffuses into the bloodstream and carbon dioxide in the bloodstream diffuses into the alveoli.
  • The alveoli provide a large surface area for gas exchange.
  • Alveoli have a very good blood supply, with alveolar sacs being right next to capillaries.
  • Alveoli do not have any cartilage or smooth muscle.
  • They contain elastic fibres which stretch during inhalation to allow the alveoli to fill with air. They recoil during exhalation to expel air.
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Ciliated epithelium

  • The trachea, bronchi, and large bronchioles are lined with ciliated epithelium.
  • Goblet cells are the mucus-secreting cells of ciliated epithelium. This mucus traps dust, particles, and pathogens.
  • Ciliated cells have multiple, thin projections from their surface ('cilia").
    • Cilia contract and "waft" the mucus produced by goblet cells up the trachea to the back of the throat, where it is swallowed.
  • Any swallowed pathogens are destroyed by stomach acid.

Alveoli

Alveoli are the millions of air sacs within the lungs where gases are exchanged with the bloodstream. They provide a large surface area for gas exchange.

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Capillaries

  • Each alveolus is surrounded by a network of capillaries.
  • The many capillaries provide a large surface area for gas exchange between the alveoli and the bloodstream.
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Alveolar epithelium

  • The alveolar epithelium is made up of a single layer of squamous epithelial cells that line the walls of the alveoli.
  • Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus.
  • In the alveoli, the squamous cells are arranged in a single layer to facilitate gas diffusion.
    • The squamous epithelium provides a very short diffusion distance from the alveoli to the capillaries which maximises the rate of gas exchange.
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Concentration gradient

  • The capillaries supply carbon dioxide to the alveoli and oxygen is rapidly carried away from the alveoli.
  • The quick transport of gases in the bloodstream maintains a steep concentration gradient of oxygen and carbon dioxide.
  • The steep concentration gradient allows quick diffusion of gases into and out of the bloodstream.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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