Blood Supply and Innervation of the Liver
The liver is the largest visceral tissue mass in the human body and consists of four lobes, which include the right, left, quadrate and caudate lobes. It can weigh up to two kilograms and accounts for 2% of an adults overall body weight. The right lobe is the largest and fills the majority of the right upper quadrant of the abdomen (RUQ), while the left lobe is much smaller and takes up only a medial portion of the left upper quadrant (LUQ) from the midline towards the left side. The quadrate lobe is situated on the inferior aspect of the liver between the gallbladder and the round ligament of the liver. The caudate lobe is also found facing caudally and is surrounded by the inferior vena cava, the ligamentum venosum and the porta hepatis.
It functions by receiving blood from the alimentary canal, the accessory organs of the digestive tract and the spleen and filtering it. The filtration process allows it to perform several important tasks such as glycogen storage, fatty acid synthesis, clotting factor production, toxin and drug metabolism, hormone modification, bile acid synthesis, bilirubin secretion, iron and fat soluble vitamin storage and phagocytosis of foreign materials that enter the portal circulation from the bowels.
The blood supply of the liver is delivered through the portal vein and the hepatic artery. The hepatic artery brings oxygenated blood to the hepatic tissues, while the portal vein collects the deoxygenated blood from the abdominal contents and filters it, eliminating toxins and processing the nutrients it collects during absorption from the alimentary canal. The portal venous system will be discussed below, so for now, the pathway of the hepatic artery, which contributes approximately 30% towards the hepatic blood supply will be mentioned. The celiac trunk branches off the abdominal aorta at the level of the twelfth thoracic vertebra and gives rise to the left gastric artery, the splenic artery and the common hepatic artery. The common hepatic artery veers retroperitoneally to the right and enters the hepatoduodenal ligament of the liver. It then divides into the gastroduodenal and proper hepatic arteries, which supply the stomach and duodenum as well as the liver respectively. The proper hepatic branches consist of the right and left hepatic arteries that supply the right and left lobes of the liver. The right gastric artery runs up the lesser curvature of the stomach and anastomoses with the left gastric artery. Finally, the cystic artery branches off the right hepatic artery to supply the gallbladder and the cystic duct.
Venous and Lymphatic Drainage
The two major venous plexuses that are responsible for draining the abdominal contents are that of the portal vein, which filters the blood directly into the liver and provides it with 70% of its blood supply and the inferior vena cava. The hepatic portal vein is formed by the merger of the splenic vein and the superior mesenteric vein. The veins that contribute to the splenic vein include the inferior mesenteric vein and its branches, the pancreatic veins, the left gastroepiploic vein and the short gastric veins. The superior mesenteric vein collects blood from the inferior pancreaticoduodenal vein, the right gastroepiploic vein, the right colic vein, the ileocolic vein, the jejunal veins and the ileal veins. The veins that drain as direct branches into the portal vein include the cystic vein, the superior pancreaticoduodenal vein and the left and right gastric veins. The venous drainage of the liver itself occurs via the three hepatic veins which consist of an accumulation of central veins and take the deoxygenated hepatic blood directly to the inferior vena cava just before it passes up through the diaphragm.
The lymph that is produced by the liver is collected mainly by the hepatic nodes which are situated around the porta hepatis. From there the fluid is carried to the celiac nodes which are part of the intestinal lymphatic trunks and merge directly into the thoracic duct, when the cisterna chyli is anatomically not present. It ascends into the thoracic region via the aortic hiatus of the diaphragm. If the cisterna chyli is present, this thin-walled sac can be found at the site in which all the main lymphatic ducts of the abdomen accumulate, which is at the level of the second lumbar vertebra. It allows the lymph to drain indirectly from the bowels to the thoracic duct, which then travels cranially through the posterior mediastinum and empties all the lymph that is collected from around the entire body into the junction between the left subclavian vein and the left jugular vein, which can be found just below the clavicle, towards the left shoulder.
The innervation of the liver is governed by the hepatic nerve plexus which runs along the hepatic artery and portal vein. It receives sympathetic fibers from the celiac plexus and parasympathetic fibers from the anterior and posterior vagal trunks. The exact role of the hepatic nervous supply is largely unknown although it does control vasoconstriction.
Referred pain is visceral pain perceived as somatic pain through the dermatomes of the skin which are innervated by the cutaneous nerves of the spinal vertebrae T5 to L3. It is essentially information that is carried by visceral afferent fibers via the thoracic and lumbar splanchnic nerves. The liver and the gallbladder are governed by the sixth to the ninth thoracic spinal nerves and present as referred pain in the epigastric region of the abdomen as well as to the right hypochondrium.