When the intercostal muscles contract, the rib cage moves up and out, and the chest cavity becomes larger, helping air to be drawn into the lungs.

What exactly happens step by step

  1. The external intercostal muscles (the main ones active during a normal breath in) shorten and pull each rib slightly upward and outward.
  1. Because the ribs are connected like a curved cage, this lifting makes the whole rib cage swing up and out, a bit like a bucket handle being raised.
  1. As the rib cage expands, the volume of the thoracic (chest) cavity increases, which lowers the air pressure inside the lungs compared with the outside air.
  1. Air then flows into the lungs down this pressure gradient – this is inspiration (inhalation).

Simple exam-style description

  • On contraction, the external intercostal muscles raise the ribs up and out.
  • This causes the rib cage to expand and the chest cavity volume to increase.
  • The pressure in the lungs falls, so air is drawn in.

That is the level of description usually expected when you are asked to “describe what happens to the rib cage when the intercostal muscles contract” in a school biology question.