Differences and Shared Features of Arteries and Veins

Arteries and veins differ in structure because they operate under different pressures. 

• Arteries have thick, muscular walls and smaller lumens to withstand the high pressure produced by the heart. 

• Veins have thinner walls and larger lumens because blood pressure is much lower after passing through capillaries. Many veins, especially those in the limbs, contain valves that prevent backflow and help move blood toward the heart against gravity.

Despite their differences, arteries and veins share several features.

• All vessels have a lumen, the hollow space through which blood flows. 

• Their walls are made of living tissue that requires nutrients and oxygen. 

Because blood moves too quickly through large vessels to nourish these cells, larger arteries and veins contain vasa vasorum, small blood vessels that supply the vessel walls. Blood vessels also contain nervi vasorum, small nerves that regulate contraction and relaxation of the vessel walls.

Tunica Media

The tunica media is the middle layer and is usually the thickest layer in arteries. It contains smooth muscle and elastic fibers arranged mainly in circular layers.

Contraction of this muscle causes vasoconstriction, narrowing the vessel and increasing blood pressure. Relaxation causes vasodilation, widening the vessel and decreasing blood pressure. These changes are controlled by nerves, hormones, and local chemical signals.

Tunica Externa

The tunica externa is the outer layer made mostly of connective tissue. It helps anchor the vessel in place and contains vasa vasorum and nervi vasorum. In veins, this layer is often the thickest and may contain some smooth muscle fibers.

Elastic (Conducting) Arteries

Large arteries close to the heart are called elastic arteries. 

Their high elastic fiber content allows them to expand when blood enters and recoil between heartbeats. This recoil helps maintain steady blood pressure and continuous blood flow.

Muscular (Distributing) Arteries

As arteries branch farther from the heart, they become muscular arteries. These vessels contain more smooth muscle and fewer elastic fibers. 

Their main role is to distribute blood to specific organs by adjusting blood flow through vasoconstriction and vasodilation.

Capillaries

Capillaries are the smallest blood vessels and are the main sites of gas and nutrient exchange. Their thin walls allow oxygen, carbon dioxide, nutrients, and wastes to move between blood and surrounding tissues. Blood flow through capillaries is slow, allowing time for exchange.

There are three types of capillaries based on their permeability:

• Continuous capillaries have small gaps that allow water and small molecules to pass and are found in most tissues.

• Fenestrated capillaries contain pores that allow larger molecules to pass and are common in organs involved in absorption and filtration, such as the intestines and kidneys.

• Sinusoid capillaries have large openings and incomplete walls, allowing proteins and cells to move through. They are found in the liver, spleen, bone marrow, and some endocrine organs.

Veins

A vein is a blood vessel that carries blood toward the heart. In comparison to arteries, veins have thinner walls and larger, often irregularly shaped lumens. These structural features reflect the fact that veins transport blood under much lower pressure than arteries.

Because venous blood pressure is low, especially in vessels far from the heart, many larger veins (particularly those in the limbs) are equipped with valves. These valves ensure the one-way flow of blood toward the heart by preventing backflow toward the capillaries. 

Venous valves are especially important in counteracting the effects of gravity and maintaining efficient circulation, as blood flow in veins would otherwise become sluggish or reversed.