Respiratory System Anatomy Practice Test

Gas exchange happens across a very thin barrier formed by the cells lining the alveolar surface. The cells that cover most of that surface are the Type I pneumocytes. They are flat, squamous cells that constitute about the vast majority of the alveolar surface area, creating the diffusion pathway for oxygen and carbon dioxide. Their extreme thinness minimizes the distance gases must travel to reach the blood in nearby capillaries. In contrast, Type II pneumocytes are cuboidal and fewer in surface area; they produce surfactant to lower surface tension and can replenish Type I cells, but they are not the primary surface for gas exchange. Alveolar macrophages sit in the air space for immune defense, while endothelial cells line the capillaries and contribute to the diffusion barrier as well, but the alveolar surface itself is dominated by Type I pneumocytes.