respirations

Respiration is a term that has multiple related meanings in biology. In humans and many other organisms, it commonly refers to breathing—the mechanical process of moving air into and out of the lungs—and to the gas exchange that occurs between the air in the lungs and the blood. The broader concept also includes cellular respiration, the metabolic reactions by which cells extract energy from nutrients, using oxygen and producing carbon dioxide as a waste product.

External respiration involves ventilation (inspiration and expiration) and the diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane. The lungs provide a large surface area, thin barriers, and a dense network of capillaries to enable efficient gas exchange. Ventilation is driven by the contraction and relaxation of diaphragmatic and intercostal muscles and is regulated to meet metabolic demand. Internal respiration is the tissue-level gas exchange: oxygen diffuses from the blood into tissues, and carbon dioxide diffuses from tissues into the blood.

Cellular respiration refers to the biochemical processes by which cells generate energy, primarily through aerobic metabolism. Glucose is oxidized via glycolysis, the citric acid cycle, and oxidative phosphorylation, yielding adenosine triphosphate (ATP), water, and carbon dioxide. In the presence of sufficient oxygen, aerobic respiration predominates; when oxygen is scarce, cells may rely on anaerobic glycolysis, producing lactate and less ATP.

Regulation and clinical aspects: breathing is controlled by respiratory centers in the brainstem (the medulla oblongata and pons) and modulated by chemoreceptors that sense carbon dioxide, oxygen, and pH. Voluntary control originates in the cerebral cortex. Measurements include respiratory rate, tidal volume, and minute ventilation, with clinical assessment often relying on blood gas analysis to gauge gas exchange (for example, PaO2, PaCO2, and pH). Abnormalities include tachypnea, bradypnea, dyspnea, hypoxemia, hypercapnia, and respiratory failure.