A Level Biology Practice Exam 2025 – All-in-One Study Guide to Excel in Your A Levels!

The main function of alveoli in the lungs is to increase the surface area for gas exchange. Alveoli are tiny air sacs that provide a large surface area where oxygen and carbon dioxide can be exchanged between the air and the blood. The extensive network of alveoli maximizes the area available for this crucial process, optimizing the efficiency of oxygen uptake and carbon dioxide removal.

The other options do not accurately represent the primary role of alveoli. While transport of oxygen to tissues is essential, this function occurs in the blood after gas exchange has taken place in the alveoli. Storing carbon dioxide is not a function of alveoli; rather, they facilitate its expulsion from the body during exhalation. Producing mucus is a role carried out by other structures in the respiratory system, such as goblet cells, which help trap particles and pathogens but are not a function of the alveoli themselves. Thus, the correct answer reflects the critical role that alveoli play in respiratory physiology.