Introduction[edit | edit source]
Beneath your skin, and deeper within your body, run networks of veins. These thin, tube-like structures are an essential part of the circulatory system, which distributes blood and nutrients throughout the body. At any point in time, nearly three-fourths of the circulating blood volume is contained in the venous system.
Three types of blood vessels make up the human circulatory system: arteries, veins, and capillaries. All three of these vessels transport blood, oxygen, nutrients, and hormones to organs and cells. While arteries carry oxygenated blood away from the heart to the tissues of the body, veins carry oxygen-depleted blood from the tissues back to the heart, and in fact have special valves that help them to achieve this directional flow. Capillaries are tiny blood vessels that connect arteries to veins and allow nutrients in the blood to diffuse to the body's tissues.
Structure and Function[edit | edit source]
Veins, like the arterial system, have 3 three layers making up their walls.
- 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.
Unlike the arteries, the venous pressure is low and their walls are thinner and are less elastic. This feature permits the veins to hold a very high percentage of the blood in circulation. The venous system can accommodate a large volume of blood at relatively low pressures, a feature termed high capacitance.
Features to aide venous blood flow forwards to the heart
- Veins have one-way valves inside that allow for blood flow to move toward the heart.
- Veins contain only smooth muscle cells. These muscle cells reside within the tunica media along with elastic fibers and connective tissue. Although veins only contain smooth muscles, the contraction of skeletal muscle plays an important role in the movement of blood from the periphery towards the heart in the venous system. 
- The forward blood flow from the lower extremities to the heart is also influenced by respiratory changes that affect pressure gradients in the abdomen and chest cavity. This pressure differential is highest during deep inspiration, but a small pressure differential is observable during the entire respiratory cycle.
Vein Size: A vein can range in size from 1 millimeter to 1-1.5 centimeters in diameter. The smallest veins in the body are called venules. They receive blood from the arteries via the arterioles and capillaries. The venules branch into larger veins which eventually carry the blood to the largest veins in the body, the vena cava. Blood is then transported from the superior vena cava and inferior vena cava to the right atrium of the heart.
Deep veins and superficial veins[edit | edit source]
Systemic veins are 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 the 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 deep veins, but not the other way.
Conditions of the venous system[edit | edit source]
Some of the most common ones include:
- Deep vein thrombosis (DVT). A blood clot forms in a deep vein, usually in the 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.
- Venous Ulcers
Blood in your veins is not blue[edit | edit source]
Sometimes blood can look blue through our skin. Maybe you’ve heard that blood is blue in our veins because when headed back to the lungs, it lacks oxygen. But this is wrong; human blood is never blue. The bluish color of veins is only an optical illusion. Blue light does not penetrate as far into tissue as red light. If the blood vessel is sufficiently deep, your eyes see more blue than red reflected light due to the blood’s partial absorption of red wavelengths .
Physiotherapy[edit | edit source]
See Cardiovascular Disease and links provided in Conditions of the venous system above.
As physiotherapists we can educate our clients regarding venous health:
- 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.
References[edit | edit source]
- Tucker WD, Arora Y, Mahajan K. Anatomy, Blood Vessels. [Updated 2021 Feb 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Available:https://www.ncbi.nlm.nih.gov/books/NBK470401/ (accessed 15.7.2021)
- Mental floss 11 Amazing facts about your veins Available: https://www.mentalfloss.com/article/524180/11-amazing-facts-about-veins (accessed 15.7.2021)
- Health line Vein structure Available: https://www.healthline.com/health/venous-system#vein-structure (accessed 15.7.2021)
- The Conversation Blood in your veins is not blue – here’s why it’s always red Available:https://theconversation.com/blood-in-your-veins-is-not-blue-heres-why-its-always-red-97064 (accessed 15.7.2021)