Respiration is the process by which oxygen reaches the body's cells and is used by them to produce energy. During breathing, lungs take in oxygen when the person inhales, and lets out carbon dioxide when the person exhales. The channel that carries air in and out is called the respiratory tract and includes the sinuses, nasal cavity, throat (pharynx), voice box (larynx), windpipe (trachea), lungs and a system of arteries, veins and vessels that carry blood to and from the lungs.
In the chest the windpipe divides into 2 main tubes called bronchi. Each bronchus leads to a lung where it divides into small air passages called bronchioles.