The respiratory system (called also respiratory apparatus, ventilatory system) is abiological system consisting of specific organs and structures used for the process ofrespiration in an organism. The respiratory system is involved in the intake and exchange ofoxygen and carbon dioxide between an organism and the environment.
In air-breathing vertebrates like human beings, respiration takes place in the respiratory organs called lungs. The passage of air into the lungs to supply the body with oxygen is known as inhalation, and the passage of air out of the lungs to expel carbon dioxide is known as exhalation; this process is collectively called breathing or ventilation. In humansand other mammals, the anatomical features of the respiratory system include trachea,bronchi, bronchioles, lungs, and diaphragm. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and theblood. This exchange process occurs in the alveoli (air sacs) in the lungs.
In fish and many invertebrates, respiration takes place through the gills. Other animals, such as insects, have respiratory systems with very simple anatomical features, and inamphibians even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leaves known as stomata.