Single layer of cells attached to a basement membrane. Forms a smooth lining for the heart, blood and lymph vessels and alveoli of lungs.

• A. Columnar
• B. Cuboidal
• C. Squamous
• D. Transitional

Answer: (C)

This description points to simple squamous epithelium, a single layer of flat, thin cells that sit on a basement membrane. The flat shape minimizes the distance for substances to diffuse, making it ideal for areas where rapid exchange is needed. That’s why you find this thin lining lining the interior surfaces of blood and lymph vessels (endothelium) and the walls of the alveoli in the lungs, where gas exchange occurs. The basement membrane helps anchor the layer in place and provides support.

The other shapes fit different roles: cuboidal cells are cube-like and often line ducts and tubules where secretion and absorption occur; columnar cells are tall and are common in areas like the digestive tract where active absorption and secretion take place; transitional epithelium is specialized for stretching, such as in the urinary bladder.