Answer :-

ALVEOLI are designed in the shape balloon/sac like structures, forming the terminals of the "Bronchioles".

Such a structure of the alveoli provides a large surface area (like villi in intestine) for gaseous exchange through diffusion.

Extra Information :-

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☆ The wall of alveoli consists of 2 types of ALVEOLAR epithelial tissues :-

• TYPE I alveolar cells
• Type II alveolar cells

☆ Type I alveolar cells are predominant SQUAMOUS epithelial cells.

☆ Type I alveolar cells are more in number.

☆ Type II alveolar cells are large and are scattered among the squamous cells.

☆ Type II alveolar cells are fewer in number.

☆ Type II alveolar cells produce "Surfactant", an oily secretion containing the mixture of "Phospholipids" and "Protiens".

☆ Type II alveolar cells are also known as "SEPTAL cells.

☆ The alveolar wall possesses "Alveolar Macrophages" (dust cells).

☆ The alveolar wall possesses wandering "Phagocytes", ingesting fine dust particles and other debris.

☆ The number of alveoli in and average adult human being is estimated to be approximately around "300 million".

☆ In an average adult human being, the alveoli are estimated to expose a surface area of about "70 sq.metres".