This document describes the structure and function of blood vessels. It discusses the three layers - tunica intima, media, and adventitia - that make up arteries and veins. It also explains the types of capillaries and their roles in peripheral circulation and transport of substances to blood. Capillaries are the narrowest blood vessels and come in three varieties: continuous, fenestrated, and sinusoidal, which differ in their endothelial cell structure and locations in organs.