The tunica intima is the innermost layer of blood vessels, including arteries and veins. It consists of a thin layer of endothelial cells that line the lumen, or interior space, of the vessel. This layer plays a crucial role in reducing friction as blood flows through the vessel and helps regulate blood flow and pressure. In addition to endothelial cells, the tunica intima contains a subendothelial layer made of connective tissue. This layer provides structural support and can influence the function of the vessel. The health of the tunica intima is vital for overall cardiovascular health, as damage can lead to conditions like atherosclerosis.