Which of the following best describes the walls of alveoli?

The walls of alveoli are best described as thin and moist due to their specific structure and function in the respiratory system. Alveoli are the tiny air sacs in the lungs where gas exchange occurs, and their thin walls facilitate the efficient diffusion of oxygen and carbon dioxide between the air in the alveoli and the blood in the surrounding capillaries.

The moisture on the surface of the alveolar walls is essential because it helps dissolve gases, enhancing the diffusion process. This moist environment also plays a role in maintaining lung surface tension, which prevents the alveoli from collapsing and allows for easier expansion during inhalation.

Thicker walls would hinder gas exchange and make breathing less efficient, while a dry environment would impede gas absorption. Therefore, the characteristics of being thin and moist are crucial for the optimal functioning of alveoli in the respiratory system.