What are veins and what is the systemic venous system?

Veins are blood vessels that are responsible for carrying blood from peripheral parts of the body to the heart (centripetal force). With exception of the pulmonary veins, which carry oxygen-rich blood and nutrients, all veins generally carry deoxygenated blood to be transformed and waste products.

Depending on the diameter of the blood vessel, veins are divided by smallest to the largest in diameter: venous capillaries, venules, small veins, medium veins and large veins. The walls of the veins are made ​​up of three layers superimposed on each other (outer, middle and inner layer), are thinner than the walls of the arteries and are characterized by the presence of an elastic muscular component.  

The main structures that form the venous system are:

  • The pulmonary veins
  • The inferior vena cava
  • The superior vena cava


The pulmonary veins

The pulmonary veins are responsible for transporting oxygenated blood from the lungs to the heart (to the left atrium), which is then pumped outward to the rest of the body. Together with the pulmonary arteries, they make up the pulmonary circulation (also called "heart-lung circulation" or "small circulation") or part of the circulatory system. Pulmonary circulation is a part of the cardiovascular system and is responsible for carrying deoxygenated blood and waste products from all the organs and tissues of the body away from the heart, to the lungs, and returning oxygenated blood back to the heart, as well as enriched nutrients to all other organs and tissues.


The inferior vena cava

The inferior vena cava, also referred to as the posterior vena cava, is the largest vein in the human body. It is responsible for carrying deoxygenated blood from the lower body (lower limbs and all organs located below the diaphragm) to the heart. Located mainly in the abdominal cavity (only a small portion starts in the chest cavity), it lies posterior to the abdominal cavity and runs vertically along the right side of the spine. It enters the right atrium at the lower right, back side of the heart. 

The inferior vena cava is approximately 22 cm in length (18 of which run through the abdomen) and about 30mm in diameter.


The superior vena cava

The superior vena cava is a large vein located in the upper anterior mediastinum. It is responsible for carrying deoxygenated blood from the upper body (head, neck, upper limbs, and some organs of the chest) to the heart. The heart then sends the deoxygenated blood to the lungs so it can be transformed into oxygenated blood to be pumped throughout the entire body.

The superior vena cava is approximately 7 cm in length and about 22mm in diameter.


What functions do veins and the systemic venous system serve?

As part of the systemic circulation, the venous system is responsible for transporting deoxygenated blood from the muscle and tissues to the heart. It is made up of three units (superficial, deep and connection system). The body has superficial veins, which are located in the fatty layer under the skin; deep veins, which are located in the muscles and along the bones and; short veins (also called connecting veins), which link the superficial and deep veins.