Artery/Vein Tissues

Arteries and veins have the same layers of tissues in their walls, but the proportions of these layers differ. Lining the core of each is a thin layer of endothelium, and covering each is a sheath of connective tissue, but an artery has thick intermediate layers of elastic and muscular fiber, while in the vein, these are less developed. The arterial wall helps to withstand and absorb the pressure waves which begin in the heart and are transmitted by the blood. The wall expands with the force of a swell, then snaps back to push the blood forward as the heart rests. Valves in the arteries prevent the blood from flowing backwards at this time, so pressure peaks are slowly flattened out as the fluid proceeds through the arterial vessels. As blood enters the capillary network (the smallest vessels in the body), the pressure falls off. When the capillaries meet and form the veins, the blood is oozing rather than surging. There is no need now for the strength and elasticity of the arteries, so the walls of the veins are thin and almost floppy. To make up for this, many veins are located in the skeletal muscles, and the least movement of a limb squeezes the vein and drives the blood toward the heart. Valves are again used to ensure flow in the right direction.