Composed of just a single layer of cells, the endothelium acts as a semipermeable barrier between the vessel lumen (i.e., the street, where the blood flows) and the arterial wall proper - outlive
{max-width: 500px, display: block, margin: 0 auto, border: 5px solid black}
{max-width: 500px, display: block, margin: 0 auto, border: 5px solid black}