In the circulatory system, veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart. The difference between veins and arteries is their direction of blood flow (out of the heart by arteries, returning to the heart for veins), not their oxygen content. Veins differ from arteries in structure and function. For example, arteries are more muscular than veins, veins are often closer to the skin, and veins contain valves to help keep blood flowing toward the heart, while arteries do not have valves and carry blood away from the heart.
In general, veins function to return deoxygenated blood to the heart, and are essentially tubes that collapse when their lumens are not filled with blood. The thick outermost layer of a vein is made of connective tissue, called the tunica adventitia or tunica externa. Below the tissues are thin bands of smooth muscle called the tunica media. The interior is lined with endothelial cells called the tunica intima. The precise location of veins is much more variable than that of arteries, since veins often display a lot of anatomical variation from person to person.
Vein color is determined in large part by the color of venous blood, which is usually dark red (and not blue as is commonly believed) as a result of its low oxygen content. Veins appear blue because the subcutaneous fat absorbs low-frequency light, permitting only the highly energetic blue wavelengths to penetrate through to the dark vein and reflect back to the viewer.
Veins serve to return blood from organs to the heart. Veins are also called "capacitance vessels" because most of the blood volume of the body (60%) is contained within veins. In systemic circulation, oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries. The blood then enters venules, then veins filled with cellular waste and carbon dioxide. The deoxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped through the pulmonary arteries to the lungs. In pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.
The return of blood to the heart is assisted by the action of the skeletal-muscle pump, and by the thoracic pump action of breathing during respiration. As muscles move, they squeeze the veins that run through them. Veins contain a series of one-way valves, and as the vein is squeezed, it pushes blood through the valves, which then close to prevent backflow . Standing or sitting for a prolonged period of time can cause low venous return from venous pooling. In venous pooling, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain, which can cause unconsciousness.
Although most veins take blood back to the heart, there is an exception. Portal veins carry blood between capillary beds. For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins, and blood is taken back into the heart. Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver.
Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small:
- Superficial veins - Superficial veins are close to the surface of the body, and have no corresponding arteries.
- Deep veins - Deep veins are deeper in the body and have corresponding arteries.
- Communicating veins - Communicating veins (or perforator veins) are veins that directly connect superficial veins to deep veins.
- Pulmonary veins - The pulmonary veins are a set of veins that deliver oxygenated blood from the lungs to the heart.
- Systemic veins - Systemic veins drain the tissues of the body and deliver deoxygenated blood to the heart.