In animals and plants, the respiratory system (also known as the respiratory apparatus or ventilatory system) is a biological system that consists of specific organs and structures for gas exchange. Depending on the organism’s size, environment, and evolutionary history, the anatomy and physiology that allows this to happen vary drastically. Land animals’ respiratory surfaces are internalised as lungs linings. Millions of tiny air sacs in the lungs exchange gas; these are known as alveoli in mammals and reptiles and atria in birds. These little air sacs, which have a rich blood supply, bring the air into close contact with the blood.
The trachea, which branches into the two main bronchi in the middle of the chest, connects these air sacs to the outside world by a series of airways, or hollow tubes. These enter the lungs and branch into a series of smaller tubes known as bronchioles, which branch into progressively narrower secondary and tertiary bronchi. Parabronchi are the bronchioles found in birds. The bronchioles, or parabronchi, in mammals enter the microscopic alveoli, whereas the atria do so in birds. The muscles of respiration must pump air from the environment into the alveoli or atria during the breathing process.
Most fish and other aquatic species’ respiratory systems (including vertebrates and invertebrates) are made up of gills, which are partially or totally external organs that are bathed in the aqueous environment. Water flows through the gills in a variety of active and passive ways. Gas exchange occurs in the gills, which are made up of thin or extremely flat filaments, and the lammelae, which expose a large surface area of highly vascularized tissue to the water.
Other species, such as insects, have very simple anatomical components in their respiratory systems, and even the skin of amphibians plays a role in gas exchange. Plants have respiratory systems as well, albeit the directionality of gas exchange differs from that of animals.
Plants’ respiratory systems contain anatomical components like stomata, which can be found all over the plant.