Respiraatio

Respiraatio, or respiration, is a broad biological process in which living organisms exchange gases and extract energy from nutrients. In common usage the term covers two related processes: ventilation and gas exchange (external respiration) and cellular respiration (intracellular energy production).

In animals with lungs, respiration involves moving air into and out of the lungs (ventilation) and exchanging oxygen and carbon dioxide across the alveolar membrane. Oxygen diffuses from the air into the blood, where it binds to hemoglobin; carbon dioxide diffuses in the opposite direction and is expelled.

Cellular respiration takes place inside cells and consists of glycolysis, the citric acid cycle, and oxidative phosphorylation. It converts glucose and other nutrients into ATP, using oxygen to drive the electron transport chain. The typical aerobic yield in humans is about 30–32 ATP per glucose; anaerobic pathways can operate without oxygen but produce less energy and lactic acid or ethanol.

Regulation of breathing is controlled by nervous and chemical signals. In most mammals the brainstem (medulla oblongata and pons) sets the pace and depth of respiration, adjusted by chemoreceptors that sense carbon dioxide, hydrogen ions, and oxygen levels, as well as higher brain centers for voluntary control.

Respiration is also a fundamental process in plants and many microorganisms. In plants, cellular respiration occurs in mitochondria and provides energy for metabolic needs, while photosynthesis stores energy during daylight. Overall, respiration supports energy production and maintenance of cellular and systemic homeostasis.