Endothelium
The endothelium is a thin, smooth layer found in blood vessels, lymphatic vessels and the heart. It is made up of many endothelial cells [1]. Considering its function, it acts as a semi-selective barrier between the vessel lumen and surrounding tissue, controlling the passage of materials and the transit of white blood cells into and out of the bloodstream (1). It also repairs damaged or diseased organs via an injection of blood vessel cells(2).
References
(1) Eskin, S.G.; Ives, C.L.; McIntire, L.V.; Navarro, L.T. "Response of cultured endothelial cells to steady flow". Microvascular Research. 28 (1): 87–94. doi:10.1016/0026-2862(84)90031-1.
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(2) Roberts OL, Holmes K, Müller J, Cross DA, Cross MJ (Dec 2009). "ERK5 and the regulation of endothelial cell function". Biochem Soc Trans. 37 (6): 1254–9. doi:10.1042/BST0371254. PMID 19909257.