A term applied to several activities and processes occurring in all animals and plants: e.g., the breathing movements associated with the lungs, the uptake of oxygen and the release of carbon dioxide, and the metabolic process in which complex organic substances are broken down into simpler products with the release of energy. This energy is incorporated into the energy-carrying substance ATP and subsequently used for other metabolic processes. The metabolic aspect of respiration is carried out within the cells of organisms by the citric acid cycle (Kreb's cycle) and oxidative phosphorylation.
Breathing movements, if any, and the exchange of oxygen and carbon dioxide are called ventilation or external respiration, while the energy-releasing processes which utilize the oxygen and produce carbon dioxide (or the reverse in plants) are called internal respiration or cellular respiration.
In humans, external respiration is the process whereby air is breathed from the environment into the lungs to provide oxygen for internal respiration. Air, which contains about 20% oxygen, is drawn into the lungs via the nose or mouth, the pharynx, trachea, and bronchi. This is achieved by muscle contraction of the intercostal muscles in the chest wall and of the diaphragm; their coordinated movement, controlled by a respiratory center in the brainstem, causes expansion of the chest, and thus of the lung tissue, so that air is drawn in (inspiration). Expiration is usually a passive process of relaxation of the chest wall and diaphragm, allowing the release of the air, which is by now depleted of oxygen and enriched with carbon dioxide. Exchange of gases with the blood circulating in the pulmonary capillaries occurs across the lung alveoli and follows simple diffusion gradients.
Disorders of respiration
These include lung disease (e.g., emphysema, pneumonia, and pneumoconiosis); muscle and nerve disease (e.g., brainstem stroke, poliomyelitis, myasthenia gravis, and muscular dystrophy; skeletal deformity; asphyxia, and disorders secondary to metabolic and heart disease. See also respiratory distress syndrome and respiratory failure.