Superior lobe of left lung

The lungs are the primary organs of the respiratory system in humans and most other animals including a few fish, and some snails.

In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart.

Their function in the respiratory system is to extract oxygen from the air and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange.

Respiration is driven by different muscular systems in different species.

Mammals, reptiles and birds use their different muscles to support and foster breathing.

In earlier tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians.

In humans, the main muscle of respiration that drives breathing is the diaphragm.

The lungs also provide airflow that makes vocal sounds including human speech possible. Humans have two lungs, a right lung, and a left lung.

They are situated within the thoracic cavity of the chest.

The right lung is bigger than the left, which shares space in the chest with the heart.

The lungs together weigh approximately 1.3 kilograms (2.9 lb), and the right is heavier.

The lungs are part of the lower respiratory tract that begins at the trachea and branches into the bronchi and bronchioles, and which receive air breathed in via the conducting zone.

The conducting zone ends at the terminal bronchioles.

These divide into the respiratory bronchioles of the respiratory zone which divide into alveolar ducts that give rise to the alveolar sacs that contain the alveoli, where gas exchange takes place.