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 much thinner and less developed. With the exception of pulmonary and umbilical veins and arteries, arteries carry oxygenated blood from the heart, while veins returnmycontentbreak deoxygenated blood to the heart.
The thicker and more muscular walls of arteries help them to withstand and absorb the pressure waves which begin in the heart and are transmitted by the blood. The arterial wall expands and swells with the force of each contraction of the heart, then snaps back to push the blood forward as the heart rests. From the arteries, blood enters smaller branches of arteries called arterioles and then the capillary network.
Just as arterioles are smaller branches of arteries, so venules are smaller branches of veins. Venules receive blood from the capillaries and branch into veins that return blood to the heart. They do not have the need 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. One-way valves ensure flow in the right direction.