Structure of Vein
Three different types of the layer are present in veins:

Tunica intima -The layer which is present on the inner side of the vein is the tunica intima. Tunica intima is composed of monolayer endothelium cells along with some consecutive tissues.

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Valves are present in the veins which prevent backflow of blood and allow one-way movement.

Valves are mainly present in the veins of legs and arms.

Tunica externa- Tunica externa is present on the outer side of the vein. This layer is the thickest among the three-layer. Small vessels of blood known as vasa vasorum are present in the tunica extercolorer of the vein. This layer supplies blood to the vein’s walls.

Tunica media -It is the thin middle layer of the vein. In this layer, collagen is present in the highest amount. The main component of connective tissue is collagen.

The color of veins from the outside depends on the blood they carry, which appears dark red because the content of oxygen is very minimum in the blood.

Due to the absorption of low-frequency light by simultaneous fat, only high-energy blue waves can pass through the dark vessels of veins and be reflected by the viewer and blood vessels appear blue.

The color of blood vessels can be influenced by the level of oxygen in the blood, the size, the depth of the blood vessels, and the person’s skin characteristics. When the veins are cleared of blood and removed from the body, they appear greyish-white.

In the body, blood circulates through two channels: pulmonary circulation and systemic circulation. Based on the type of circulation, veins are following types: pulmonary and systemic veins.

Pulmonary veins: In pulmonary circulation, oxygen-free blood is transferred to the lungs from the heart. After the oxygenation of blood occurs in the lungs, blood is returned to the heart through the pulmonary circulation. Four pulmonary veins in total are present. They are different from the other veins because they carry oxygenated blood instead of deoxygenated blood. In all other veins, only oxygen-free blood passes.

Systemic veins: The systemic veins transport oxygen-free blood to the heart from the whole body, entering the pulmonary circuit. Systemic veins are the most abundant of the veins.

Classification of Systemic Vein:
Systemic veins are further classified into superficial veins and deep veins.

Deep veins –Deep veins are located along with the bones or in muscles. In these veins, one valve is present in tunic-like manner, which prevents the backflow of blood. To allow the movement of blood in forwarding direction, muscles which are nearly situated compress these veins.

Superficial veins- They are found in a layer of fat under the skin. The superficial vein has one valve present in the tunica intima. But in a superficial vein, nearby muscles are absent so compression by the muscle is absent. In these veins, the movement of blood is so slow in comparison to deep veins.

Connecting veins- From superficial veins, blood enters deep veins by connective veins which are short. The valve of this vein permits flow of blood from a superficial vein to a deep one, but the converse is not true.

Portal hypertension-Above the abdomen, the portal vein is found, through which blood enters the liver. Portal hypertension is related to or blood clots in the liver or cirrhosis or liver disease or other conditions such as compression by tuberculous tumors or lesions. When pressure on the portal vein increases, collateral circulation occurs, causing the veins in the esophagus to look like varicose veins.