Pulmonary alveoli are tiny air sacs located in the lungs where gas exchange occurs. They are the final destination for inhaled air, allowing oxygen to enter the bloodstream and carbon dioxide to be expelled. Each alveolus is surrounded by a network of capillaries, which facilitate this exchange of gases. The structure of pulmonary alveoli is crucial for their function. They are made up of thin walls, which help maximize the surface area for gas exchange. The total surface area of all alveoli in the lungs is roughly the size of a tennis court, making them essential for efficient respiration.