The endothelium

The endothelium, which forms the inner lining of the blood vessels, is a truly expansive cell layer, weighing approximately 1 kg in an average-sized human and covering a total surface area of 4000–7000 m² . Endothelial cells from a single human, when lined end-to-end, would wrap more than four times around the circumference of the earth.

The endothelium forms the inner cellular lining of blood vessels. Endothelial cells (ECs) are not inert but, rather, are highly metabolically active. The endothelium plays an important role in many physiological functions, including the control of vasomotor tone, blood cell trafficking, hemostatic balance, permeability, proliferation, survival, and innate and adaptive immunity.