Final answer:
The innermost layer of tissue in an artery is the tunica intima, which includes the endothelium and reduces friction inside the vessel.
Step-by-step explanation:
The innermost layer of tissue in an artery is called the tunica intima. This layer consists of a single layer of endothelial cells, which is known as the endothelium, surrounded by a thin layer of connective tissue. The tunica intima plays an important role in the cardiovascular system: it reduces friction between the blood and the inside of the blood vessel walls, helps regulate capillary exchange, and alters blood flow by releasing local chemicals called endothelins. The tunica intima, along with the tunica media and the tunica externa, are the three tissue layers that make up the walls of arteries and veins.