the walls of the alveoli are mostly made of
Pulmonary alveolus The alveoli consist of an epithelial layer and an extracellular matrix surrounded by small blood vessels called capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn. The alveoli contain some collagen and elastic fibres. The elastic fibres allow the alveoli to stretch as they are filled with air during inhalation. They then spring back during exhalation in order to expel the carbon dioxide-rich air.
Walls of Jerusalem The Walls of Jerusalem (Arabic: أسوار القدس‎; Hebrew: חומות ירושלים‬) surround the Old City of Jerusalem (approx. 1 km²). In 1535, when Jerusalem was part of the Ottoman Empire, Sultan Suleiman I ordered the ruined city walls to be rebuilt. The work took some four years, between 1537 and 1541.[1][2]