(int'ĭ-mă )

[L. intima, inmost parts]

The innermost layer of the wall of an artery or vein. It consists of a layer of endothelial cells that normally form a semipermeable barrier that regulates the entry or exit of substances from the lumen. Materials may cross this barrier by transport systems. The endothelial cells are very smooth, which prevents abnormal clotting; they secrete chemicals that are important for normal blood coagulation and for controlling relaxation and contraction of the smooth muscle tissue in the middle layer of the vessel. As the normal artery ages, the intima thickens due to an increase in lipid material.