Respiration in living organisms works by exchanging gases to support essential metabolic processes. Most animals inhale oxygen and exhale carbon dioxide, whereas plants consume carbon dioxide and expel oxygen. Gas exchange at the cellular wall is the entire process of respiration for unicellular organisms and other simple living things.
Respiration is a chemical process that involves the release of energy from glucose and other food substances. Higher animals typically have advanced physiologic respiratory systems for transporting oxygen-rich air to different body tissues. Terrestrial higher animals obtain air through the nose or mouth and carry the oxygen to a respiratory structure, such as the trachea. The alveoli in the lungs take in oxygen and expel waste carbon dioxide. The oxygen goes into the bloodstream through the hemoglobin.
All animals use biochemical respiration to deliver oxygen across the cell wall, and they use the gas to acquire energy. Oxygen reacts with glucose, resulting in the production of carbon dioxide, water and adenosine triphosphate. Higher organisms have a complex system that consists of the nose or mouth, trachea, lung, alveoli, pulmonary capillaries and the vascular system. In contrast, the cell walls of simple organisms are bared to atmospheric oxygen. In plants, respiration often occurs at the surface of leaves through stomatal holes, but it occurs on stems in some plant species.