You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "the walls of the alveoli are mostly made of?" whose answer is collagen and elastic fibres.
Each document should contain about 30 words, and these documents should contain useful information.

Query: primary composition of alveoli walls
Useful Output:

Doc 1: This diagram shows the main constituents of alveolus, and the interalveolar wall. The thickness of the alveolar-capillary barrier varies from 0.2 to 2.5 µm. The
Doc 2: Actually, the “wall” of alveolar ducts consists of nothing more than a network of alveolar openings. In the human lung there are around 30,000
Doc 3: The alveolar walls are made up of collagen and elastic fibres, which expand during inspiration and return to their former shape during expiration. There are
Doc 4: Largely polyhedral shape; Open at one end, like a cup; Walls of the alveoli are composed of the pulmonary capillary sheet; Alveolar surfaces are
Doc 5: Alveoli are thin-walled, sac-like structures lined by a single layer of flattened squamous epithelial cells: type I pneumocytes. This extremely thin nature of