Veins are elastic tubes, or blood vessels, that carry blood from your organs and tissues of the body back to your heart.

Each vein is made up of three layers:

• A layer of membranous tissue on the inside
• A layer of thin bands of smooth muscle in the middle
• A layer of connective tissue on the outside

Veins are different from arteries. But together veins and arteries form an important network to transport blood throughout the body.

Cycle of Oxygen Delivery

Most veins carry blood that is low in oxygen back to the heart. Contrary to popular belief, blood that is low in oxygen is not blue in color, but rather it is dark red. Veins only appear blue when seen through the skin.

After the oxygen and nutrients have been delivered to the body’s tissues by the arteries and capillaries, a second network of venules and veins carries blood and waste products back to the heart.

Smaller tubes, venules, pick up the oxygen-depleted blood and transfer it to the veins, which carry it back to the heart. Once the veins return blood to the heart, it is pumped to the lungs. Here, the carbon dioxide from the venous blood is expelled as we breathe out. The oxygen we breathe in is picked up by the blood flowing through the lungs. This blood that is replenished with oxygen returns to the heart where it is pumped back into the rest of the body and the process starts again.

The veins throughout the body support many systems and are critical to your cardiovascular health and many other functions of blood, including waste removal.

Your veins are strong and somewhat flexible, but they can be affected by the following medical conditions:

• Varicose veins and spider veins (caused by venous insufficiency, or faulty valves in the veins) are among the most common vein conditions. They usually affect the veins in your legs. Because the valves in the veins are not working correctly, gravity keeps the blood from flowing back up to the heart. The blood pools and enlarges the vein, causing swollen, twisted and sometimes painful veins. Chronic venous insufficiency can lead to chronic swelling, itchiness, discoloration and an increased risk of ulcers and cellulitis (an infection of the tissues) in the legs.
• Deep vein thrombosis (DVT) is a clot within a vein, which can be a serious condition. A clot is more likely to form after surgery, during a hospitalization, as a side effect from oral contraceptives or after periods of immobility. A DVT can travel through your veins and reach one of your lungs (called a pulmonary embolism), which could be life-threatening.

Making healthy lifestyle decisions and seeking out timely medical care can help prevent, manage and even correct diseases of the veins.