The vein is one of the most important parts of the circulatory system and the human body as a whole. Although they may be small in size, they branch out all over the body and connect to the heart. The most important function of veins is to return deoxygenated blood back to the heart, from where it can be cycled through the heart and lungs and once again obtain oxygen that can be sent out to the body and its vital organs. However, it is important to keep in mind that not all veins carry deoxygenated blood. Some actually carry oxygenated blood and have different functions.

Veins receive blood from the capillaries after the exchange of oxygen and carbon dioxide takes place in the lungs. It is important that the deoxygenated blood keeps moving in the right direction or problems can arise. Valves that are located inside the veins accomplish this flow. These valves force the blood to only move in one direction.

Veins are composed of three layers and these include tissue, muscle, and epithelial cells. The outer layer of the vein is composed of tissue, the middle is muscle, and the inner layer is the epithelial cells.

There are different types of veins and one of the most important types is the pulmonary vein. These veins carry blood containing oxygen from the lungs to the left atrium of the heart. These are the only veins in the human body with carries oxygenated blood. Other types of veins include portal veins, superficial veins, deep veins, jugular veins, and systemic veins.

Veins can also development problems or diseases and the most common of these problems is varicose veins. These veins are enlarged or twisted and very often blue in color due to deoxygenated blood pools. Blood clots can also form in veins, also known as deep vein thrombosis, which can lead to a pulmonary embolism.