Vein
A vein is a type of blood vessel in the body. All veins carry blood to the heart. Most veins carry blood that is low in oxygen, except for the pulmonary vein and the umbilical veins which carry blood that is high in oxygen.
A vein has a large lumen (width) and less pressure than an artery. There are smaller amounts of smooth muscle and elastic fibres in the vessel wall. Most veins have one-way valves that keep blood from going backwards.
The vena cava, the biggest vein in human body, collects blood from the other veins and delivers it to the heart. Other important veins are the coronary veins (veins on the heart) and renal veins (veins on the kidney). Veins are mainly seen just below the skin, and are easily seen from the outside, where they look blue because of the lack of oxygen. The blood carried by veins is dark red, but when a vein is cut or pierced, the dark blood immediately reacts with the oxygen in the air and becomes bright red colored. The vein appears blue because of the scattering of light through the skin and the way the eye perceives color and light. Veins contain blood even after death. This is why a dead body appears blue.
Vein Media
Layers of vein wall shown in comparison to arterial wall
Positions of venae cavae and vessels of the pulmonary circulation
- 2134 Thoracic Upper Limb Veins.jpg
Deep and superficial veins of the arm and near thorax
- Venous valve 00013.gif
Video of a valve in the popliteal vein opening to allow blood to flow through and closing to prevent backflow
- Venous valve.svg
Venous valve stopping backflow
Diagram showing venous blood flow from capillary beds in some specific locations including the lungs, liver, and kidneys
- 2106 Large Medium Vein Venule.jpg
Diagram of different sized veins with differing component proportions
- William Harvey ( 1578-1657) Venenbild.jpg
Image of veins from William Harvey's Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus