Breathe Easy: Master the 2026 Respiratory System Anatomy Challenge!

Two main alveolar cell types carry out the essential tasks for respiration: gas exchange and surfactant function. The first, Type I pneumocytes, are extremely thin, squamous cells that line most of the alveolar surface. Their thin barrier creates the minimal diffusion distance between the air in the alveolus and the blood in the capillaries, making gas exchange—oxygen into the blood and carbon dioxide out of it—efficient.

The second type, Type II pneumocytes, are larger and cuboidal. Their primary role is to produce and secrete pulmonary surfactant, a surface-active phospholipid mixture that lowers surface tension at the air–water interface inside the alveoli. This prevents collapse during expiration and stabilizes the alveoli. In addition, Type II cells can proliferate and differentiate into Type I cells to repair the epithelium after injury.

Other options mix in immune cells or mislabel another cell type, but the classic alveolar epithelium is composed of Type I cells for gas exchange and Type II cells for surfactant production and repair.