7.1.1

Lung Structure & Alveoli

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Structure

Human 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.

Trachea

Trachea

  • The trachea is the entrance to the human 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.
Bronchi

Bronchi

  • The trachea divides into two bronchi.
  • Air flows along each bronchus to a lung.
  • The bronchi are made from cartilage and smooth muscle.
Bronchioles

Bronchioles

  • Each bronchus divides into many smaller bronchioles.
  • The many bronchioles branch throughout the lungs into small air-sacs called alveoli.
Alveoli

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.
  • There are millions of alveoli in the lungs
  • The alveoli provide a large surface area for gas exchange.
Control of ventilation

Control of ventilation

  • Ventilation is controlled by the ribcage, intercostal muscles and the diaphragm.
  • When you breathe in, these structures move to allow the lungs to fill with air.
  • When you breathe out, these structures move to allow air to leave the lungs.

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.

Capillaries

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.
Alveolar epithelium

Alveolar epithelium

  • The epithelium is made up of a single layer of epithelial cells that line the walls of the alveoli.
  • The epithelium provides a very short diffusion distance from the alveoli to the capillaries which maximises the rate of gas exchange.
Concentration gradient

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.
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Practice questions on Lung Structure & Alveoli

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Which of these is NOT a structure in the lungs?Multiple choice
  2. 2
    Ventilation is controlled by:Fill in the list
  3. 3
    Which of the following statements about the lungs are true?True / false
  4. 4
    Which of the following statements about alveoli are true?True / false
  5. 5
    What is the benefit of the single layer of epithelial cells in the alveoli for gas exchange?Multiple choice
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Cell Specialization

Ventilation