Lungs

Structure

The right lung has three lobes; the left has two and is slightly smaller to accommodate the heart. Each lung is enclosed by a double-layered pleural membrane and is divided into segments served by their own bronchi and arteries.

Function

With each breath, air flows through the trachea and branching bronchi into roughly 300 million alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out.

Mechanics

Breathing is driven mainly by the diaphragm and intercostal muscles. At rest, an adult moves about half a liter of air per breath, roughly 12 to 20 times per minute.