Given the task definition and input, reply with output. In this task, we ask you to rewrite a sentence in simple English without changing its general meaning. Essentially, you want to make the sentence easier to read by using simpler words, utilizing more straightforward sentence structures, and omitting non-essential information etc.

Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels , forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall .

Endothelium is a layer of cells that line the walls of the circulatory system which carries blood around the body .