Physiology of How Breathing Works

The Breathing Muscles

When you inhale, the diaphragm and muscles between your ribs contract, creating a negative pressure—or vacuum—inside your chest cavity. The negative pressure draws the air that you breathe into your lungs.

Inflation of the Lungs

Lungs are not hollow like balloons but are made of spongy, flexible tissue that inflates when filled with air. So, how does the air get in there? Where does it go? Let’s follow a breath of air from start to finish.

Taking a Breath

When you take a breath, the air goes in through your nose and mouth and travels down your throat, through your voice box and into the trachea, which is also known as the windpipe.

Entering the Lungs

The end of your trachea splits into an upside down Y-shape and forms the bronchi. Air passes through either the right or left bronchus into both sides of the lungs.

Entering the Bronchial Tree

Inside of the lungs, the bronchi branch off into bronchioles, which look similar to branches of a tree.

Read what paradoxical breathing is and how to treat it.

Branching out Into Bronchioles

The air flows through the bronchioles, which keep getting smaller until the air reaches the ends of the branches.

Filling up Air Pockets

At the ends of the bronchioles are clusters of little pockets that collect the air, called alveoli.

Gas Exchange

When the air reaches the alveoli, oxygen diffuses through the membrane into small blood vessels called capillaries, and carbon dioxide diffuses from the blood in the capillaries into the alveoli.

Blowing It All Out

The second stage of breathing, blowing air out of the lungs, is called expiration or exhalation. After the oxygen and carbon dioxide trade places in the alveoli, the diaphragm relaxes and positive pressure is restored to the chest cavity. This forces the used air out of the lungs, following the reverse of the path that it used to get in the lungs. The entire breathing process is repeated 12 to 20 times per minute in a healthy adult.