At the end of the bronchioles, the air enters one of the many millions of alveoli where gaseous exchange takes place.

Each bronchus branches out into smaller tubes called bronchioles. Air travels through these bronchioles.

The trachea divides into two bronchi. One bronchus enters each lung.

Air enters the body and is warmed as it travels through the mouth and nose.

Breathing

Breathing is the term given to the process of taking air into and out of the lungs.

Two important structures for breathing are the diaphragm and intercostal muscles.

The diaphragm is a sheet of muscle that separates the chest (or thoracic) cavity from the rest of the body.

The intercostal muscles are found between the ribs and they control rib movement.

Inspiration (breathing in)

The diaphragm contracts and moves downwards. The intercostal muscles contract and move the ribs upwards and outwards. This increases the size of the chest and decreases the air pressure inside it which sucks air into the lungs.

Expiration (breathing out)

The diaphragm relaxes and moves back to its domed shape. The intercostal muscles relax so the ribs move inwards and downwards under their own weight. This decreases the size of the chest and increases the air pressure in the chest so air is forced out of the lungs.