Complex sentence: The alveolar and pulmonary capillary gases equilibrate across the blood--air barrier, a membrane which forms the walls of the pulmonary alveoli, consisting of the alveolar epithelial cells, their basement membranes and the endothelial cells of the pulmonary capillaries.
Split into simpler sentences: The alveolar and pulmonary capillary gases equilibrate across the blood--air barrier, a membrane which forms the walls of the pulmonary alveoli. The pulmonary alveoli consist of the alveolar epithelial cells, their basement membranes and the endothelial cells of the pulmonary capillaries.