The abdomen is drained by two distinct venous systems - the portal venous system and the systemic venous system. The primary purpose of the portal system is to transport venous blood, rich with nutrients extracted from ingested food, to the liver for processing. The systemic venous system, on the other hand, is responsible for returning deoxygenated blood to the right atrium of the heart. In this article, we will discuss the anatomy of these two venous systems, namely the major vessels involved, their anatomical course and their tributaries.
The systemic venous system is the primary venous system that carries deoxygenated blood to the right atrium of the heart. The major vessel in this system is the inferior vena cava, which is located on the posterior abdominal wall, anterior to the vertebral column and to the right of the abdominal aorta. This vein is formed by the union of the common iliac veins at the L5 vertebral level, and is responsible for the venous drainage of all structures below the diaphragm. It ascends in a superior direction, leaving the abdomen by piercing the central tendon of the diaphragm at the T8 level, and from here enters the thoracic cavity, where it finally drains into the right atrium of the heart.
The inferior vena cava shares an anatomical relationship with many abdominal structures, including the right common iliac artery, the root of the mesentery, the head of the pancreas, the bile duct, the portal vein and the liver.
The inferior vena cava is responsible for the venous drainage of all structures below the diaphragm. It receives tributaries from the following veins:
It is worth noting that no tributaries are received from the spleen, pancreas, gallbladder or the abdominal part of the gastrointestinal tract. This is because these structures are first drained into the portal venous system, before the blood they contain ultimately enters the inferior vena cava via the hepatic veins after being processed by the liver.
The portal venous system is primarily tasked with carrying venous blood, collected from the absorption of nutrients, to the liver for processing. The major vessel of this system is the portal vein. It is formed by the union of the splenic vein and the superior mesenteric vein, posterior to the neck of the pancreas, at the level of L2. This vein ascends towards the liver, while passing posteriorly to the superior part of the duodenum and the bile duct. Immediately before entering the liver, the portal vein divides into right and left branches, which then enter the parenchyma of the liver.
The portal vein receives tributaries from the following veins:
Unlike the splenic artery, the splenic vein is straight and maintains contact with the body of the pancreas as it traverses the posterior abdominal wall.
The inferior vena cava (IVC) is the major vessel in this system, responsible for the convergence of venous drainage from all structures below the diaphragm. It is located on the posterior abdominal wall, anteriorly to the vertebral column and to the right of the abdominal aorta. The vessel is formed by the union of the common iliac veins at the L5 vertebral level and ascends superiorly before piercing the central tendon of the diaphragm at the T8 level to enter the thorax and drain into the right atrium of the heart.
The inferior vena cava shares an anatomical relationship with numerous abdominal structures - including the right common iliac artery, the root of the mesentery, the head of the pancreas, the bile duct, the portal vein and the liver - and it receives tributaries from common iliac veins, lumbar veins, renal veins, right testicular or ovarian vein, right suprarenal vein, and inferior phrenic veins.
The portal system carries venous blood to the liver for processing and the major vessel of the portal system is the portal vein, formed by the union of the splenic vein and the superior mesenteric vein, posterior to the neck of the pancreas, at the level of L2.
The splenic vein is formed from the convergence of a variety of smaller vessels as they leave the hilum of the spleen. It has several tributaries, including short gastric veins, left gastro-omental vein, pancreatic veins, and inferior mesenteric vein.
The superior mesenteric vein drains blood from the small intestine, cecum, ascending colon, and transverse colon. It begins in the right iliac fossa as a convergence of the veins draining the terminal ileum, cecum, and appendix, and it has a number of tributaries:
Many of these tributaries are formed as an accompanying vein for each branch of the superior mesenteric artery.
A porto-systemic anastomosis is a connection between the veins of the portal venous system and the veins of the systemic venous system. The major sites of these anastomoses include:
When blood flow through the portal system is obstructed (e.g. due to cirrhosis, portal vein thrombosis, or external pressure from a tumour), the pressure within the portal system increases. A portal venous pressure in excess of 20mmHg is defined as portal hypertension. Under these circumstances, the pressure in the porto-systemic anastomoses is lower than in the portal system and, in an effort to equalize pressure, blood is re-directed through the anastomoses. Over a long enough period, if a large volume of blood passes through these anastomoses, the veins around the anastomosis can become abnormally dilated— known as varices. Rupture of oesophageal or rectal varices can result in fatal blood loss.
Porto-systemic anastomoses are connections between the portal venous system and the systemic venous system, allowing blood to travel around the liver instead of through it. These anastomoses have a variety of physiological functions, but the most important is to serve as an alternative route of blood flow should the portal system become obstructed. The major sites of these anastomoses are as follows:
Obstruction of the portal system, as can occur with cirrhosis, portal vein thrombosis, or external pressure from a tumour, results in increased pressure within the portal system. A portal venous pressure in excess of 20mmHg is defined as portal hypertension. When this occurs, pressure is relieved via the porto-systemic anastomoses as they are under a lower pressure than the rest of the portal system.
If a large volume of blood passes through these anastomoses over a long period of time, the veins around the anastomosis can become abnormally dilated, a condition known as varices. Rupture of oesophageal or rectal varices can result in significant and potentially fatal blood loss.
The porto-systemic anastomoses represent a crucial element of the venous system. While they have a number of physiological functions, their primary purpose is to act as a safety valve, ensuring blood is redirected should the portal system become obstructed or experience an increase in pressure. When used properly, they can help reduce the risk of severe and life-threatening complications such as variceal rupture.
