The illustrations show five types of tightly packed epithelial tissues, which form the thin membranes lining most internal and external surfaces of an animal’s body. Thinner tissues allow for the exchange of particles through the membrane, whereas thicker tissues serve largely as a barrier.
Which type of epithelial tissue lines a lung’s air sacs, which must freely pass oxygen to blood vessels in exchange for carbon dioxide?

simple squamous


An air sac within a lung functions by taking in oxygen, which it passes through its lining to blood vessels, which pass carbon dioxide back through the lining to be expelled into the air. To accomplish the exchange, the lining must be single layered, thin, and leaky (diffuse). The tissue shown as simple squamous best serves this purpose.

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