Which epithelium lines the interior surface of blood vessels and the heart?

• A. Endothelium (simple squamous epithelium)
• B. Mesothelium
• C. Stratified squamous epithelium
• D. Pseudostratified columnar epithelium

Answer: (A)

The interior lining of blood vessels and the heart is formed by a specialized single layer of flat cells called the endothelium. This simple squamous epithelium lines the lumen of all blood vessels (the tunica intima) and the inner surface of the heart (the endocardium). Its thin, smooth surface minimizes friction to keep blood flowing smoothly and it acts as a selective barrier that regulates exchange between blood and surrounding tissues. It also has active roles in vascular tone, coagulation, inflammation, and immune signaling through substances like nitric oxide and other mediators. Other options describe epithelia that line different regions—mesothelium lines serous cavities, stratified squamous lines surfaces subject to abrasion, and pseudostratified columnar lines portions of the respiratory tract and some ducts—so they don’t line the interior surfaces of vessels and the heart.