Which lining forms the capillary walls to enable diffusion?

• A. Simple cuboidal epithelium.
• B. Stratified squamous epithelium.
• C. Simple squamous endothelium lining capillaries for diffusion.
• D. Pseudostratified epithelium.

Answer: (C)

Diffusion across capillary walls depends on a very thin, permeable barrier. That barrier is the simple squamous endothelium lining the capillaries—a single layer of flat cells with a minimal cytoplasmic thickness. This thin, smooth surface places oxygen, carbon dioxide, nutrients, and waste only a short distance from blood to tissue, allowing rapid exchange between the bloodstream and surrounding cells. The endothelium forms the inner lining of all capillaries, so it’s perfectly suited for diffusion and exchange rather than protection or secretion. Other epithelial types are thicker or multi-layered and thus less conducive to rapid transfer of substances.