Your chest expands and then relaxes during breathing because your muscles are changing the size of your chest cavity, which changes the air pressure in your lungs and makes air flow in and out.

The basic idea

When you breathe in:

  • Certain muscles pull your ribcage up and out and your diaphragm moves down.
  • This makes your chest cavity larger, so the pressure inside your lungs drops.
  • Air flows in to fill that lower‑pressure space, and your chest looks like it expands.

When you breathe out:

  • Those muscles relax and your ribcage and lungs recoil back to their resting size.
  • The chest cavity gets smaller, pressure inside the lungs rises a bit.
  • Air is pushed out, and your chest seems to relax or fall.

What’s moving inside

Key players:

  • Diaphragm: Dome‑shaped muscle under your lungs; it flattens downward when you inhale, increasing chest space.
  • Intercostal muscles: Muscles between your ribs; they lift and spread the ribs during inhalation, widening the ribcage.
  • Elastic lungs and chest wall: These tissues naturally spring back when muscles relax, helping your chest fall during exhalation.

A simple way to picture it: imagine your chest as a flexible box and your lungs as balloons glued to its inside. When the box gets bigger (muscles pulling ribs and diaphragm), the balloons are stretched and air rushes in; when the box relaxes smaller, the balloons are squeezed and air goes out.

Information gathered from public forums or data available on the internet and portrayed here.