Parts of the System

The lungs are two organs in the chest. A sac called Pleura surrounds and protects the lungs. They are divided into lobes that each have a bronchial branch. The right lung is always larger than the left.

Bronchi are tubes/airways that let air in and out. They divide repeatedly into even smaller airways. They make tiny branches called terminal bronchioles. The branching network in the lungs is called the bronchial tree.

The trachea is the windpipe that carries air in and out of the lungs. When air enters the trachea, elastic fiber in the walls of the trachea allow the airway to expand and contract to breathe. Cartilage rings in the trachea prevent them from collapsing. The trachea divides behind the sternum to make a left and right branch called bronchi, which each enter a lung.

Alveoli are cup shaped air sacs that provide a large surface area for the gas exchange. The average person has about 700 gas filled alveoli. Each alveoli has a network of capillaries (tiny blood vessels) for the gas exchange. Blood fills the vessels and air fills the alveoli for the gas exchange to begin.

The diaphragm is a dome shaped muscle that is located below the lungs. It helps with lung expansion. Nerves from the brain send impulses to the diaphragm making it contract or relax. When the diaphragm contracts, it moves down for the dome to be flattened and the chest cavity to be increased in size. During exhalation, it's the opposite so the diaphragm relaxes and its dome curves upwards into the chest cavity.