The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants.
In land animals, the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs. In mammals and reptiles, these are called alveoli, and in birds, they are known as atria. These microscopic air sacs have a rich blood supply, bringing the air into close contact with the blood. A system of airways, or hollow tubes, allow the air sacs to interface with the external environment; the largest of these is the trachea, which branches in the middle of the chest into the two main bronchi, which enter the lungs and branch into progressively narrower secondary and tertiary bronchi, which in turn branch into numerous smaller tubes known as the bronchioles in mammals and reptiles. In birds, the bronchioles are termed parabronchi. The bronchioles, or parabronchi, generally open into the microscopic alveoli (in mammals) and atria (in birds). Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.
