AP BIOLOGY

Detailed explanation-1: -Type I pneumocytes cover 70% of the internal surface of each alveolus. These cells are thin and squamous, ideal for gas exchange. They share a basement membrane with pulmonary capillary endothelium, forming the air-blood barrier where gas exchange occurs.

Detailed explanation-2: -The alveoli contain two cell types, epithelial (pulmonary) cells and endothelial (vascular) cells. It is this tight two-celled structure that facilitates gas exchange.

Detailed explanation-3: -Type 1 pneumocytes are alveolar cells that line the alveolar surface. Type 2 pneumocytes are alveolar cells that secrete surfactant proteins to reduce surface tension.

Detailed explanation-4: -The main function of the type I cell is the maintenance of a barrier to prevent the leakage of fluid and proteins across the alveolar wall into the air spaces, while allowing gases to freely cross the air-blood barrier.

Detailed explanation-5: -The type I cell is responsible for gas exchange and the type II cell synthesizes and secretes surfactant. At birth the lung has no mature alveoli but instead contains approximately 20 million primitive terminal sacs. These sacs are lined by mature alveolar epithelium; they resemble large shallow cups.