Why are Thoracic and Abdominal Veins Important in the Human Body?

Thorax is the region that is referred to as the trunk’s superior part and is situated between the region of the abdomen and neck. On the other hand, the abdomen is the region that is situated between the pelvis and thorax. The thoracic veins and veins of the abdomen are responsible for the drainage of deoxygenated blood from the region of the thorax and abdomen in order to return them to the heart. Thus, it plays a significant role in the human body.

Anatomy of the Thorax

Thorax represents the chest region which forms the superior portion of the trunk. It is made up of various components namely the neurovasculature, thoracic wall, lymphatics, different cavities, breasts, and internal organs. The thoracic wall is composed of the skeletal framework, neurovasculature, muscles, and fascia. All these are connected to form a protective and strong cage but they exhibit flexible nature as well.  The thoracic wall normally encloses a space that is otherwise called a cavity that is generally filled with several anatomical structures. The region also contains various significant organs including lungs, trachea, esophagus, heart, and thymus apart from these organs, thorax contains thoracic arteries, lymphatics, and nerves.

Anatomy of the Abdomen

The superior aperture of the abdomen is pointed towards the thoracic region that is enclosed by the diaphragm. The abdomen is open to the pelvis region inferiorly and communication between them takes place through the pelvic aperture that is present superiorly. These 2 apertures, combined with the abdominal walls, define the boundary of the abdominal region. The organs present in the abdominal cavity include certain parts of structures such as the digestive system, urinary system, endocrine system and immune system.

Veins

Veins are the blood vessels that are elastic in nature and helps in transporting deoxygenated blood from different parts of the body to the heart. It forms an important component of the cardiovascular system and plays a significant role in circulating blood to replenish the oxygen and removes metabolic wastes from different tissues and cells which are metabolically active. This system usually relies on muscle contraction for returning blood supply to the region of the heart. The problems in circulation through the veins are caused either by clots or due to the defect in the veins.

Veins of the Thorax

The thoracic veins are categorized into 4 veins: the superior vena cava, internal thoracic vein, supreme intercostal vein and inferior vena cava. Of these 4 veins, superior vena cava and inferior vena cava are the major thoracic vein.

• Superior vena cava

The brachiocephalic veins that are present in the left and right sides of the thorax form the superior vena cava (on the right side of the upper chest). The superior vena cava helps in returning the deoxygenated blood from the body’s upper half portion such as the head, upper limbs, and neck via the jugular and thyroid veins. Superior vena cava is connected to the inferior vena cava just before entering into the heart with the help of the azygos vein that runs up the vertebral column of the thorax on its right side. Azygous vein helps in transporting blood from the thoracic cavity.

• Internal thoracic vein

A vessel that generally drains the organs such as breasts and the wall of the chest is termed an internal thoracic vein. This particular vein bilaterally emerges from the superior epigastric vein and it generally accompanies the thoracic artery in the internal portion along its course. And finally, it gets terminated in the vein named brachiocephalic vein.

• Supreme intercostal vein

A paired vein that is responsible for draining the 1^st intercostal space that is situated on its respective side is named as supreme intercostal vein. This normally drains into the brachiocephalic vein.

• Inferior vena cava

The inferior vena cava returns blood from the region of the lower limbs and abdomen to the heart’s right atrium. The liver’s hepatic veins and the renal veins from the kidney directly drain into the inferior vena cava. In addition to this, the inferior and superior phrenic veins that drain the diaphragm region, normally open into the inferior vena cava and internal mammary vein respectively.

Veins of the Abdomen

The venous system that drains the structures or organs present in the abdomen is the systemic and the portal venous system. The systemic system is involved in returning blood to the heart's right atrium whereas the portal system is involved in transporting the venous blood from the gastrointestinal (GI) tract to the liver for the purpose of processing the nutrients present in the blood from the GI tract.

Systemic Venous System

The major vessel of this system is the inferior vena cava. The systemic venous system acts as the common convergence of venous drainage from all the structures below the region of the diaphragm. It is situated on the abdominal wall’s posterior side and anteriorly to the vertebral column and present on the right side of the aorta of the abdomen. The common iliac veins present on the right and left sides join together in the region of the abdomen at the L5 vertebra’s level. The common iliac veins ascend and leaves the region of the abdomen by piercing the diaphragm’s central tendon at the level of T8. During its long course, the inferior vena cava tends to share an anatomical relationship with several abdominal structures namely the root of the mesentery, right common iliac artery, the bile duct, the portal vein, the head of the pancreas, and the liver. This receives tributaries from the veins of the common iliac, right testicular or ovarian veins, renal veins, inferior phrenic veins, hepatic veins, lumbar veins, and right suprarenal vein. Each vein drains a different portion of the body.   

Portal Venous System

Tributaries from the region of the pancreas, spleen, gastrointestinal tract’s abdominal part, and gallbladder are absent as these structures are first drained into the portal venous system. The portal vein or the hepatic portal vein is a very important vessel of the hepatic portal system. It is the convergence point for the venous drainage of the gastrointestinal tract present in the abdomen region, gallbladder, spleen, and pancreas. The formation of the hepatic portal vein occurs by the union of the superior mesenteric vein and the splenic vein. This takes place posterior to the neck portion of the pancreas, at the L2 level. It receives extra tributaries from the right and left gastric veins, cystic veins, and para-umbilical veins.

Different Veins of the Abdomen

The veins of the abdomen are listed and described as follows:

• A vein of the external iliac region is the femoral veins (present in the lower leg) which pass in the upward direction along the region of the pelvis and terminates to form the common vein of the ileum. The tributaries of these veins are deep iliac circumflex, pubic veins, and inferior epigastric.
• The veins of the internal iliac region are present and arises near the greater sciatic foramen’s upper part. The internal iliac vein passes in the upward direction, behind and moderately medial to the region of the internal iliac artery and at the pelvis’s brim. It further combines with the external vein of the ilium and forms a common iliac vein.
• The common iliac veins situated on the left and right sides combine together in the abdomen region at the L5 vertebra’s level results in the formation of inferior vena cava. This is vital for draining deoxygenated blood from the lower limbs and pelvis region to the right atrium.
• The superior epigastric vein drains into the thoracic veins that are present internally (or the internal thoracic vein). It articulates with the epigastric vein that is present in the inferior region at the umbilicus’ level and is characterized to drain the anterior part of the abdomen wall and a certain part of the diaphragm.
• The inferior epigastric is the vein that usually drains into the external iliac vein and articulates with the superior epigastric vein.
• The formation of the deep circumflex iliac vein is achieved by the combination of the deep iliac circumflex artery’s venae commutants and joins the external vein of the ilium about 2 cm above the inguinal ligament region.

Common Mistakes

The names of the vein and their location should not be confused. Hence, use anatomic diagrams to remember them easily.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses that are specifically dealt with the concepts of physiology and anatomy. Understanding the functions and anatomy of the veins helps in a better understanding of the cardiovascular and circulatory systems.

Practice Problems

From the understanding of the venous drainage of the abdomen, which vein does not directly drain into the inferior vena cava?

Answer: The splenic vein tends to get drained into the portal venous system due to which it does not drain into the structure of the vena cava that is present inferiorly.