Blood Supply and Innervation of the Small Intestine
The neurovascular supply of any area of the body is the network in which the blood is pumped and the structures are innervated. The blood provides the necessary nutrition to that region in order for it to function and the nervous tissue circulates information between the brain and the periphery via action potentials. This article aims to give a brief overview of the arterial supply, the venous and lymphatic drainage and the innervation of the duodenum, the jejunum and the ileum.
The duodenum is supplied proximally by a branch of the common hepatic artery called the gastroduodenal artery that stems from the celiac trunk and gives the superior anterior and posterior pancreaticoduodenal arteries. It also receives blood distally from the anterior and posterior inferior pancreaticoduodenal arteries which are branches of the superior mesenteric artery. The terminal branches of the duodenal arteries form an important anastomoses between the celiac trunk and the superior mesenteric artery.
The main arterial supply of the jejunum and the ileum is from a single artery known as the superior mesenteric and between fifteen to eighteen of its branches which form anastomoses loops known as arterial arcades with terminal vasa recta or straight branches. It should be noted that a double row of arcades supplies the ileum.
Venous and Lymphatic Drainage
The duodenal veins drain directly from the duodenum into the pancreaticoduodenal veins. From here they merge back into the two largest vessels which are the superior mesenteric vein and the common hepatic vein. Generally speaking, the veins follow the arteries in this region and they all drain either directly or indirectly into the portal vein. The lymph nodes that collect lymph from around the duodenum encircle the arteries and drain into the pancreaticoduodenal nodes, the pyloric nodes, the superior mesenteric nodes and the celiac lymph nodes.
The superior mesenteric vein collects blood from the venous arcades of the small intestine and merges with the splenic vein posterior to the head of the pancreas to form the portal vein. The lymphatic tissue in the villi of the jejunum and the ileum is unique due to the fact that it contains special vessels known as lacteals which have the ability to absorb fat. The absorbed fat is known as chyle and is drained from the finger-like mucosa of the small intestine into the surrounding lymphatic plexuses that run within its walls. The lymphatic system of this region travels between the layers of the mesentery and drains firstly into the mesenteric lymph nodes, before continuing into the superior mesenteric or ileocolic nodes before finally reaching the cysterna chyli.
The duodenum receives both sympathetic and parasympathetic innervation. The vagus nerve (CN X), provides parasympathetic fibers via the celiac and superior mesenteric plexuses, while the sympathetic trunk also gives fibers to the intestinal plexuses that travel along the pancreaticoduodenal arteries.
Like the duodenum, the lower section of the small intestine is also innervated by both parasympathetic and sympathetic fibers. The sympathetic nerves arrive from the fifth to the ninth thoracic spinal cord segments, which enter the sympathetic trunk and synapse on the postganglionic cells in the celiac and superior mesenteric ganglia before becoming the greater and lesser splanchnic nerves. The parasympathetic system consists of preganglionic fibers that stem from the posterior vagal trunks and synapse on postganglionic cells in the myenteric and submucosal plexuses of the intestinal wall.