Question

Briefly describe the mechanism of breathing

Answer 1

Mechanisms of breathing -- inspiration

When you breathe in:

intercostal muscles between the ribs contract, pulling the chest walls up and out

the diaphragm muscle 
below the lungs contracts and flattens, increasing the size of the chest

the lungs increase in size, so the pressure inside them falls. This causes air to rush in through the nose or mouth.

Mechanisms of breathing -- expiration

When you breathe out:

Intercostal muscles between the ribs relax so that the chest walls move in and down.

The diaphragm muscle below the lungs relaxes and bulges up, reducing the size of the chest.

The lungs decrease in size, so the pressure inside increases and air is pushed up the trachea and out through the nose or mouth.