The primary structure involved in gas exchange in the lungs is the alveoli. These small, sac-like structures are the functional units of the respiratory system, facilitating the exchange of oxygen and carbon dioxide between the air and blood. The alveoli have a large surface area and are surrounded by a network of capillaries, allowing for efficient diffusion of gases. When inhaled air reaches the alveoli, oxygen diffuses into the blood while carbon dioxide is expelled from the blood into the alveoli to be exhaled. This process is vital for maintaining respiratory health and overall metabolic function.