What is the epithelium that lines the interior surface of blood vessels and the heart?

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

Answer: (C)

Endothelium is the lining you’re looking for. It’s a specialized simple squamous epithelium that covers the inside of all blood vessels (the tunica intima) and also lines the heart (endocardium). This single, flat layer provides a very smooth, low-friction surface that allows nutrients, gases, and waste to diffuse efficiently between blood and tissues. It also plays active roles in regulating blood flow, vessel permeability, coagulation, and inflammation by releasing substances like nitric oxide and other mediators.

Other options describe different membranes: mesothelium lines serous membranes (like the lining of the peritoneum, pleura, and pericardium); stratified squamous epithelium protects surfaces subject to abrasion (skin, mouth, esophagus, etc.); pseudostratified columnar epithelium lines much of the respiratory tract and has cilia. These do not line the interior surfaces of vessels and the heart.