Respiratory System

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In this chapter you will be introduced to the structures of the respiratory system. How they enable "gas exchange" will be explained.


After completing this section, you should know:

  • why animals need energy and how they make it in cells
  • why animals require oxygen and need to get rid of carbon dioxide
  • what the term gas exchange means
  • the structure of alveoli and how oxygen and carbon dioxide pass across their walls
  • how oxygen and carbon dioxide are carried in the blood
  • the route air takes in the respiratory system (i.e. the nose, pharynx, larynx, trachea, bronchus, bronchioles, alveoli)
  • the movements of the ribs and diaphragm to bring about inspiration
  • what tidal volume, minute volume and vital capacity are
  • how the rate of breathing is controlled and how this helps regulate the acid-base balance of the blood


  • Animals need to breathe to supply the cells with oxygen and remove the waste product carbon dioxide.
  • The lungs are situated in the pleural cavities of the thorax.
  • Gas exchange occurs in the alveoli of the lungs that provide a large surface area. Here oxygen diffuses from the alveoli into the red blood cells in the capillaries that surround the alveoli. Carbon dioxide, at high concentration in the blood, diffuses into the alveoli to be breathed out.
  • Inspiration occurs when muscle contraction causes the ribs to move up and out and the diaphragm to flatten. These movements increase the volume of the pleural cavity and draw air down the respiratory system into the lungs.
  • The air enters the nasal cavity and passes to the pharynx and larynx where the epiglottis closes the opening to the lungs during swallowing. the air passes down the trachea kept open by rings of cartilage to the bronchi and bronchioles and then to the alveoli.
  • Expiration is a passive process requiring no energy as it relies on the relaxation of the muscles and recoil of the elastic tissue of the lungs.
  • The rate of breathing is determined by the concentration of carbon dioxide in the blood. As carbon dioxide makes blood acidic, the rate of breathing helps control the acid/base balance of the blood.
  • The cells lining the respiratory passages produce mucus which traps dust particles, which are wafted into the nose by cilia.

Learning Activities

Library of Resources

Presentations and Blackboard quizzes can be accessed by students of Otago Polytechnic.