Diagram of the terminal region of the bronchial system in the lung. Bronchioles branch extensively into terminal then respiratory bronchioles and end in alveolar sacs and air-filled alveoli. Gas exchange between air and blood occurs across the alveolar membranes. It is estimated that the total surface area of the alveoli in a pair of lungs is about 100 sq metres. A singles tennis court is 196 sq metres.