Email. Print. The endothelium is a thin membrane that lines the inside of the heart and blood vessels. Endothelial cells release substances that control vascular relaxation and contraction as well as enzymes that control blood clotting, immune function and platelet (a colorless substance in the blood) adhesion.hat Tests Are Used to Determine Endothelial Function. Acetylcholine endothelial function and adenosine coronary flow reserve tests enhance a physician's ability to diagnose and treat patients with endothelial dysfunction.