Veins are a type of blood vessel that return deoxygenated blood from your organs back to your heart. These are different from your arteries, which deliver oxygenated blood from your heart to the rest of your body.

Deoxygenated blood that flows into your veins is collected within tiny blood vessels called capillaries. Capillaries are the smallest blood vessels in your body. Oxygen passes through the walls of your capillaries to your tissues. Carbon dioxide can also move into your capillaries from the tissue before entering your veins.

The venous system refers to the network of veins that work to deliver deoxygenated blood back to your heart.

The walls of your veins are made up of three different layers:

  • Tunica externa. This is the outer layer of the vein wall, and it’s also the thickest. It’s mostly made up of connective tissue. The tunica externa also contains tiny blood vessels called vasa vasorum that supply blood to the walls of your veins.
  • Tunica media. The tunica media is the middle layer. It’s thin and contains a large amount of collagen. Collagen is one of the main components of connective tissue.
  • Tunica intima. This is the innermost layer. It’s a single layer of endothelium cells and some connective tissue. This layer sometimes contains one-way valves, especially in the veins of your arms and legs. These valves prevent blood from flowing backward.

Veins are often categorized based on their location and any unique features or functions.

Pulmonary and systemic veins

Your body circulates blood on two different tracks called the systemic circuit and the pulmonary circuit. Veins are based on the circuit they’re found in:

  • Pulmonary veins. The pulmonary circuit carries deoxygenated blood from your heart to your lungs. Once your lungs oxygenate the blood, the pulmonary circuit brings it back to your heart. There are four pulmonary veins. They’re unique because they carry oxygenated blood. All other veins carry only deoxygenated blood.
  • Systemic veins. The systemic circuit carries deoxygenated blood from the rest of the body back to your heart, where it then enters the pulmonary circuit for oxygen. Most veins are systemic veins.

Deep veins and superficial veins

Systemic veins are further classified as being either:

  • Deep veins. These are found in muscles or along bones. The tunica intima of a deep vein usually has a one-way valve to prevent blood from flowing backward. Nearby muscles also compress the deep vein to keep blood moving forward.
  • Superficial veins. These are located in the fatty layer under your skin. The tunica intima of a superficial vein can also have a one-way valve. However, without a nearby muscle for compression, they tend to move blood more slowly than deep veins do.
  • Connecting veins. Blood from superficial veins is often directed into the deep veins through short veins called connecting veins. Valves in these veins allow blood to flow from the superficial veins to your deep veins, but not the other way.

Use this interactive 3-D diagram to explore the venous system.

Many conditions can affect your venous system. Some of the most common ones include:

  • Deep vein thrombosis (DVT). A blood clot forms in a deep vein, usually in your leg. This clot can potentially travel to your lungs, causing pulmonary embolism.
  • Superficial thrombophlebitis. An inflamed superficial vein, usually in your leg, develops a blood clot. While the clot can occasionally travel to a deep vein, causing DVT, thrombophlebitis is generally less serious than DVT.
  • Varicose veins. Superficial veins near the surface of the skin visibly swell. This happens when one-way valves break down or vein walls weaken, allowing blood to flow backward.
  • Chronic venous insufficiency. Blood collects in the superficial and deep veins of your legs due to improper functioning of one-way valves. While similar to varicose veins, chronic venous insufficiency usually causes more symptoms, including coarse skin texture and ulcers in some cases.

While the symptoms of a venous condition can vary widely, some common ones include:

  • inflammation or swelling
  • tenderness or pain
  • veins that feel warm to the touch
  • a burning or itching sensation

These symptoms are especially common in your legs. If you notice any of these and they don’t improve after a few days, make an appointment with your doctor.

They can perform a venography. In this procedure, your doctor injects contrast die into your veins to produce an X-ray image of a particular area.

Follow these tips to keep your vein walls and valves strong and properly functioning:

  • Get regular exercise to keep blood moving through your veins.
  • Try to maintain a healthy weight, which reduces your risk of high blood pressure. High blood pressure can weaken your veins overtime due to added pressure.
  • Avoid long periods of standing or sitting. Try to change positions regularly throughout the day.
  • When sitting down, avoid crossing your legs for long periods of time or regularly switch positions so one leg isn’t on top for a long period of time.
  • When flying, drink plenty of water and try to stand up and stretch as often as possible. Even while sitting, you can flex your ankles to encourage blood flow.