The lungs are paired, vital organs within the respiratory system, primarily responsible for gas exchange between the organism and the external environment. Their structure includes a complex branching network of airways, starting from the trachea and descending into increasingly smaller passages called bronchi and bronchioles. Gas exchange occurs at the alveolar level, where thin-walled sacs provide an enormous surface area for the diffusion of oxygen into the bloodstream and carbon dioxide into the inhaled air. The process of breathing, or ventilation, involves the physical mechanics of expanding and contracting the chest cavity, which creates pressure gradients necessary for moving air into and out of these gas-exchange surfaces, thereby sustaining metabolic functions throughout the body.