Which epithelium allows passage of materials by diffusion and filtration in sites where protection is not important?

• A. Pseudostratified columnar epithelium
• B. Stratified squamous epithelium
• C. Simple cuboidal epithelium
• D. Simple squamous epithelium

Answer: (D)

Thin, flat epithelial cells form a single, smooth layer that minimizes the distance substances must travel, making diffusion and filtration efficient. Simple squamous epithelium is exactly this: a single layer of flattened cells with flattened nuclei, creating a very thin barrier. This arrangement is ideal for sites where protection is not the priority and rapid exchange is needed, such as the walls of air sacs in the lungs where gases diffuse, and the inner lining of blood vessels where nutrients and wastes pass by diffusion. Other epithelial types are thicker or specialized for protection, secretion, or absorption: stratified squamous provides many layers for abrasion resistance; pseudostratified columnar often has cilia and supports movement and protection; simple cuboidal is a single layer but is thicker than squamous and better suited to secretion and absorption rather than rapid diffusion.