Respiration, in a physiological context, refers to the complex metabolic process by which energy is released from nutrient molecules, primarily through the controlled breakdown of glucose. This process involves two main stages: external respiration, which is the gas exchange across respiratory membranes, and internal respiration, which is cellular respiration. Gas exchange involves taking in oxygen and expelling carbon dioxide, maintaining the necessary partial pressure gradients for efficient transport through the circulatory system. Cellular respiration utilizes oxygen to facilitate the conversion of chemical energy stored in food into adenosine triphosphate (ATP), the primary energy currency used to power cellular functions. The full biochemical pathway includes glycolysis, the citric acid cycle, and oxidative phosphorylation, making it fundamental to life processes in aerobic organisms.