Nerves, vessels and lymphatics of the abdomen
The abdomen consists of the organs required for digestion which include the stomach, pancreas, liver, gallbladder and small and large intestines. The abdomen also contains the kidneys and spleen. These organs are interconnected by connecting tissues called the mesentery, which allows the organs to move relative to one another and expand. Additionally, the abdomen is covered by a protective layer of tissue called the transversalis fascia. The anterolateral abdominal muscles are situated in front of this fascia and on the outermost layer is the skin. The posterior aspect of the abdomen consists of back muscles and the spine.
Many important blood vessels are located in and travel through the abdomen. These include the abdominal aorta, inferior vena cava along with their branches or tributaries. The abdominal aorta gives rise to the celiac trunk, which divides into the left gastric artery, common hepatic artery and the splenic artery. Next, the left and right renal arteries branch off laterally from the abdominal aorta. Anteriorly, the superior and inferior mesenteric artery branch off the abdominal aorta while the gonadal arteries come off laterally. Inferiorly, near the pelvis, the common iliac artery divides into internal and external iliac arteries within the pelvic cavity.
The majority of thoracic and abdominal visceral organs are dually innervated by parasympathetic and sympathetic outflows. More specifically, the thoracic viscera and upper abdominal viscera are primarily innervated by the vagus (cranial nerve X) and spinal thoracolumbar outflows.
In regards to the peritoneum, the serous membrane which lines the walls of the abdominal cavity as well as abdominal and pelvic organs, receives both autonomic and somatic innervation. The autonomic nerves (sympathetic and parasympathetic systems) supply the visceral peritoneum, whereas the parietal peritoneum receives somatic innervation from spinal nerves.
The lymphatic system of the abdomen and pelvis represent a network of vessels and nodes responsible for collecting and transporting lymph. Lymph from abdominal organs reaches the lumbar and intestinal lymphatic trunks by passing through relay lymph nodes surrounding the great abdominal vessels. These trunks unite at the cisterna chyli, subsequently forming the thoracic duct. This major lymphatic vessel empties into the venous circulatory system at the junction between the left subclavian and internal jugular veins.
The lumbar plexus is a collection of spinal nerves located deep in the lumbopelvic region, close to the psoas major muscle. Formed from the anterior rami of spinal nerves L1–L4, this nerve network provides innervation to the muscles, joints and skin of the anterolateral aspects of the pelvis and thigh.
The lumbar plexus has six major terminal branches. According to their anatomical position, the branches can be grouped into anterior or posterior parts: the anterior part largely providing innervation to the pelvis and the posterior part supplying branches to the thigh. Fibers from spinal nerve L4 join with fibers from L5 to form the lumbosacral trunk. This large nerve links the lumbar plexus with the sacral plexus, thus these nerve complexes are often given the combined name of lumbosacral plexus. Together these two plexuses supply all innervation to the pelvis and lower limb.
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Arteries of the stomach, liver and gallbladder
The abdominal aorta and its branches supply the organs of the abdomen with oxygenated blood. One of its major branches is the celiac trunk which arises from the abdominal aorta at the level of the vertebra T12. It gives off three major branches known as the left gastric, common hepatic and splenic arteries which supply the abdominal foregut, liver, gallbladder, pancreas and spleen.
The stomach is supplied by branches of the celiac trunk, which include the right and left gastric, right and left gastroomental, short gastric and posterior gastric arteries. The liver is supplied by the common hepatic artery and, interestingly, also by the hepatic portal vein. The hepatic arteries brings oxygenated blood to the hepatic tissues while the hepatic portal vein carries deoxygenated but otherwise nutrient-rich blood from the gastrointestinal tract and the spleen to the liver. The main arterial supply to the gallbladder is the cystic artery which is a branch of the right hepatic artery.
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Arteries of the pancreas, duodenum and spleen
The supply of blood to the duodenum, as well as the head and uncinate process of the pancreas is mainly carried by the anterior and posterior superior pancreaticoduodenal arteries (branches of the gastroduodenal artery) and the anterior and posterior inferior pancreaticoduodenal arteries (branches of the superior mesenteric artery), which form an arterial arcade. The neck, body and tail of the pancreas are supplied by numerous branches that stem mainly from the splenic artery.
The arterial supply of the spleen comes from the splenic artery, which is a direct branch of the celiac trunk.
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Hepatic portal system
The hepatic portal system is responsible for the venous drainage of many structures within the abdomen. One of the most important structures of this is the hepatic portal vein.
The hepatic portal vein is formed by the union of the superior mesenteric and splenic veins and as a result receives nutrient-rich venous blood from the spleen, pancreas, gallbladder and upper parts of the gastrointestinal tract.
It travels within the hepatoduodenal ligament, alongside the proper hepatic artery and bile duct to reach the porta hepatis of the liver. The hepatic portal vein functions to transport venous blood from regions within the abdomen to the sinusoids of the liver, where it can then be processed and filtered. Within the liver, filtered venous blood is received by the right, intermediate and left hepatic veins which empty into the inferior vena cava of the systemic venous system.
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Lymphatics of the pancreas, duodenum and spleen
The lymphatic drainage of the pancreas, duodenum and spleen is closely related to their arterial supply. For example, as in the case of the arterial supply of the pancreas, the lymphatic drainage of the head of the pancreas differs from that of its body and tail. Similarly, the lymphatic drainage of the superior part of the duodenum varies from its inferior part.
Lymphatic capillaries within the pancreas, duodenum and spleen collect lymph from these tissues and transport it into adjacent lymph nodes. The lymph is then passed along a series of lymph nodes to reach the terminal lymph nodes, which in this case, are known as the preaortic lymph nodes. The preaortic lymph nodes consist of three groups of nodes: the celiac, superior mesenteric and inferior mesenteric lymph nodes, with the first two groups primarily responsible for draining the pancreas, duodenum and spleen.
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Lymphatics of the stomach, liver and gallbladder
The lymphatic drainage of the stomach is somewhat variable between individuals however several major lymph nodes are generally involved. These are the:
- Juxtacardial nodes, located in the region of the cardia of the stomach
- Right/left gastric nodes, short gastric nodes, right/left gastroomental (gastroepiploic) nodes, each corresponding to arteries of the same names)
- Pyloric nodes (made up of the supra-, sub- and retropyloric groups), related to the pylorus of the stomach.
The lymphatic drainage of the liver is elaborate, but can be split into superficial and deep pathways:
- The superficial pathway transports lymph via channels in the subserosal areolar tissue which envelopes the liver. The anterior and inferior surfaces largely drain to hepatic nodes, while lymph from other surfaces is mainly received by various node groups of the inferior mediastinum or the celiac/superior mesenteric nodes.
- The deep pathway consists of hepatic lymph vessels which follow branches of the hepatic arteries and portal vein and flow towards the hepatic nodes at the hilum of the liver. Other lymphatic vessels course along the hepatic veins which exit via the bare area of the liver; these are received by the right lumbar (a.k.a. caval) nodes or inferior diaphragmatic nodes.
Lymph drained from the gallbladder is mainly received either directly by hepatic nodes or first via a cystic lymph node.
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Lymphatics of the posterior abdominal and pelvic wall
There are several groups of lymph nodes present in the posterior abdominal and pelvic walls. Most of those groups are named according to their anatomical relationship to the aorta/inferior vena cava and their branches/tributaries.
The main lymph node groups of this region are the lumbar lymph nodes, composed of the right lumbar (caval), intermediate lumbar and left lumbar (aortic) nodes. Those structures receive lymph from abdominal organs and also from the common, internal and external iliac lymph nodes, which drain the pelvic organs.
The lymph is then drained by lymph trunks:
- Left lumbar lymph trunk: drains left lumbar lymph nodes
- Right lumbar lymph trunk: drains right lumbar and intermediate lumbar lymph nodes.
- Intestinal lymph trunk: drains celiac, superior mesenteric and phrenic lymph nodes.
- Cisterna chyli: receives the lymph from the trunks mentioned above (directly or indirectly) and ultimately drains all the lymph from the posterior abdomen and pelvis.
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