How the Lungs Work - The Respiratory System - The Respiratory System

Your lungs lie on each side of your heart inside your chest cavity. The right lung is divided into three lobes (sections), and the left lung is divided into two lobes. Your left lung is slightly smaller than your right lung, since your heart takes up some space on the left side. When you breathe in, air enters your airways and travels down into the alveoli (air sacs) in your lungs. This is where gas exchange takes place.

An animation walks through the structures of the lungs. Medical Animation Copyright © 2022 Nucleus Medical Media Inc. All rights reserved.

The circulatory system, which is made up of the heart and blood vessels, supports the respiratory system by bringing blood to and from the lungs. The circulatory system helps  to deliver nutrients and oxygen from the lungs to tissues and organs throughout the body and removes carbon dioxide and waste products. Other body systems that work with the respiratory system include the nervous system, lymphatic system, and immune system.

The Respiratory System
The Respiratory System. The image shows an enlarged view of the airways and lungs, as well as the trachea; bronchial tubes, or bronchi; and bronchioles. The image also shows a close-up view of gas exchange at the alveoli. The blue arrows show the oxygen in the air you inhale passing into the bloodstream, and the green arrows show the carbon dioxide from your body passing out of the bloodstream. Medical Illustration Copyright © 2020 Nucleus Medical Media Inc. All rights reserved.


The airways are pipes that carry oxygen-rich air to the alveoli in your lungs. They also carry the waste gas carbon dioxide out of your lungs. The airways include these parts of your body:

  • Mouth
  • Nose and linked air passages called the nasal cavity and sinuses
  • Larynx (voice box)
  • Trachea (windpipe)
  • Tubes called bronchial tubes, or bronchi, and their branches
  • Small tubes called bronchioles that branch off of the bronchial tubes

Learn what happens in your lungs when you breathe in and breathe out. Medical Animation Copyright © 2022 Nucleus Medical Media Inc. All rights reserved.


Air comes into your body

Air first enters your body through your nose or mouth, which moistens and warms the air since cold, dry air can irritate your lungs. The air then travels past your voice box and down your windpipe. Rings of tough tissue, called cartilage, acts as a support to keep the bronchial tubes open.

Inside your lungs, the bronchial tubes branch into thousands of thinner tubes called bronchioles. The bronchioles end in clusters of tiny air sacs called alveoli.

Air fills your lung’s air sacs

Your lungs have about 150 million alveoli. Normally, your alveoli are elastic, meaning that their size and shape can change easily. Alveoli are able to easily expand and contract, because their insides are coated with a substance called surfactant. Surfactant reduces the work it takes to breathe by helping the lungs inflate more easily when you breathe in and preventing the lungs from collapsing when you breath out.

Each of these alveoli is made up of a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through your body.

Blood low in oxygen flows through the lungs

The pulmonary artery and its branches deliver blood to the capillaries that surround the alveoli. This blood is rich in carbon dioxide and low in oxygen.

Oxygen flows into your blood

Carbon dioxide moves from the blood into the air inside the alveoli. At the same time, oxygen moves from the air into the blood in the capillaries.

How does my body protect the airways from food or bacteria?