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What is the difference between azygos and hemiazygos veins?

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 The human body is indeed a marvel. There are many organs and systems in a human body allowing humans to live, strive and survive. This is not only important for one human life but also affects the next generation. How? Think of it as when a person is alive and has a good reproductive system, they are able to continue the next generation through pregnancy and giving birth but when it is said that person has issues with their genetics such as genetic diseases, their offspring is likely to be affected by it. In this DoctorOnCall’s article, we will be learning about the differences between azygos and hemiazygos vein.

          Before we go through the differences, do you know these two structures belong to the circulatory system? The blood vessels in the blood circulation system itself has a length of around 1000 000 kilometres which is equivalent to twice the distance of around the world. It is so long, isn’t it? Have you ever pondered how amazing the blood flows smoothly throughout the body and enables the 37.2 trillion cells in the body to function properly? There is no surprise that a cardiologist specialising in cardiovascular systems takes around 14 years to complete their studies and a cardiovascular surgeon specialising on cardiovascular surgeries may take up to 15 years or more in total of studying and training to be able to operate on blood vessels. This shows how complex blood vessels can be and the vast effect it has on a person’s life.

          Roughly, the differences between azygos and hemiazygos veins lies in the definition itself. The azygos vein is the large vein that carries the blood from the back wall of the chest and abdomen of the heart. Hemiazygos vein is the vein that tributes to the azygos vein. This means that the azygos vein is the larger vein compared to the hemiazygos vein. Plus, the azygos term itself means “unpaired” and hemiazygos term itself means “half of the unpaired” and reflects the lower part of the azygos veins. Below are the details of the azygos and hemiazygos veins to help you understand the differences:

Azygos veins

It is located on the either side of the vertebral column and drains the blood from the back of the chest and abdomen to the heart within the mediastinum.

These veins arise from the lumbar azygos or from the back side of inferior vena cava around the level of renal veins.

Azygos veins pass through the diaphragm and reach the mediastinum (an area between the lungs) before draining into the superior vena cava and before entering the pericardium (a sac capsulating the heart).

This vein receives the lower eight right sided posterior intercostal veins and bronchial veins from the right lung. It also receives from the accessory hemiazygos vein at the level T8 and receives from the hemiazygos vein at the level T9 (T8 and T9 refers to the levels of the vertebra).

This vein has many anastomoses (cross connection) with inferior vena cava and vertebral venous plexuses. This connection is an important part in case of the inferior vena cava obstruction which is going to become an alternate drainage route.

Hemiazygos veins

Hemiazygos vein and the accessory hemiazygos vein tributes to the azygos vein.

Hemiazygos vein and the accessory hemiazygos vein have the same relative function and location to the azygos vein.

The hemiazygos vein usually arises from the left ascending lumbar vein and passes through the left of the diaphragm. It drains the 9th to 11th left posterior intercostal vein and left subcostal veins. At times, it also drains some of the esophageal veins. It ascends to the front of the vertebral column before crossing the column posterior of the aorta, esophagus and thoracic duct at level of T8. Hemiazygos vein joins the azygos vein at the level T9.

The accessory hemiazygos vein is formed by veins from the 4th to 8th posterior intercostal spaces. It drains from the superior left hemithorax and left bronchial veins with some from the esophagus veins. The accessory hemiazygos vein joins the azygos vein at the level T8.

          It can be concluded that there are similarities of the azygos and hemiazygos vein in terms of function but there are more differences that can be found. These differences are important for medical professionals to understand especially in surgery as cases such as injury to the azygos vein can be prevented when involving thoracic surgery or treating injuries from accidents like fall or sport injury. There can be variation of the azygos veins which means what is mentioned above regarding the azygos venous system which includes azygos vein, hemiazygos vein and accessory hemiazygos vein, can vary in many people. For instance, usually a person is presented with one azygos vein on the right side of the body but occasionally it can be two azygos veins as this may be formed during fetal development. Another variation is the azygos vein usually originates at the level of the lumbar vertebrae but it is possible to be from further up the lumbar. Rarely, the azygos vein may drain directly into the right atrium rather than to the many veins first. People azygos venous system that does not form well since they are born (congenital changes) are likely to have high risk for congenital heart diseases.

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