The respiratory system (or ventilatory system) is the biological system that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs. Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians 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.
Famous quotes containing the word system:
“... the yearly expenses of the existing religious system ... exceed in these United States twenty millions of dollars. Twenty millions! For teaching what? Things unseen and causes unknown!... Twenty millions would more than suffice to make us wise; and alas! do they not more than suffice to make us foolish?”
—Frances Wright (1795–1852)