Respiration

Respiration, term applied to several activities and processes involving the exchange of gases with the environment, occurring in all animals and plants. Breathing movements, if any, and the exchange of oxygen and carbon dioxide, may be called external respiration, while energy-releasing processes at the cellular level are termed “internal respiration,” or tissue respiration. Air, which contains about 20% oxygen, is drawn into the lungs (inspiration) via the nose or mouth, the pharynx, trachea, and bronchi. Expiration is usually a passive process of relaxation of the chest wall and diaphragms allowing the release of the air, which is depleted of oxygen and enriched with carbon dioxide. Exchange of gases with the blood circulating in the pulmonary capillaries occurs by diffusion across the lung alveoli. Disorders of respiration include lung disease (e.g., emphysema, pneumonia and pneumoconiosis), muscle and nerve disease (e.g., brain-stem stroke, poliomyelitis, myasthenia gravis, and muscular dystrophy, skeletal deformity, asphyxias, and disorders secondary to metabolic and heart disease. Tissue respiration involves the combination of oxygen with glucose or other nutrients to form high-energy compounds. This reaction also produces carbon dioxide and water.