Final answer:
Type II alveolar cells are responsible for secreting surfactant, which is necessary for reducing surface tension in the alveoli and aiding in lung function.
Step-by-step explanation:
The structures that secrete surfactant are the type II alveolar cells. These cells are cuboidal epithelial cells that are interspersed among the type I alveolar cells, representing a minor portion of the cells in the alveolar wall. Type II alveolar cells produce pulmonary surfactant, a crucial substance made of phospholipids and proteins that significantly reduces the surface tension within the alveoli, facilitating easier lung expansion during inhalation and preventing alveolar collapse during exhalation.In contrast, type I alveolar cells are squamous epithelial cells that cover about 97 percent of the alveolar surface area and are highly permeable to gases, whereas alveolar macrophages play a role in the immune response by removing pathogens and debris. Understanding the function of type II alveolar cells is especially important when considering premature infants, who may be born before these cells have produced sufficient surfactant, leading to difficulties in inflating the lungs.