In human, the duodenum is the short section of the small intestine connecting the stomach to the jejunum. It is a hollow jointed tube about 25-38cm (10-15 inches) long, while the entire small intestine measures about 20 feet (6.5 meters). The name duodenum is from the Latin duodenum digitorum, or "twelve fingers' breadth". It begins with the duodenal bulb and ends at the ligament of Treitz. Function The duodenum is largely responsible for the breakdown of food in the small intestine, using enzymes. The villi of the duodenum have a leafy-looking appearance, a histologically identifiable structure. It is lined with Brunner's glands, which secrete an alkaline mucus that supports the intestinal enzymes and aids in the absorption of nutrients. Brunner's glands are found in the duodenum only. In addition, the mucus secreted by the Brunner's glands helps protect the duodenum from the acidity, making it much less sensitive than the rest of the small intestine to the material. This allows it to help protect the rest of the small intestine by neutralizing the chyme to some extent before it passes into the jejunum. The duodenum wall is composed of a very thin layer of cells that form the muscularis mucosae. The duodenum is almost entirely retroperitoneal. It has three parts and each parts has its own significance. The duodenum also regulates the rate of emptying of the stomach via hormonal pathways. Secretin and cholecystokinin are released from cells in the duodenal epithelium in response to acidic and fatty stimuli present there when the pylorus opens and releases gastric chyme into the duodenum for further digestion. These cause the liver and gall bladder to release bile, and the pancreas to release bicarbonate and digestive enzymes such as trypsin, lipase and amylase into the duodenum as they are needed. Sections The duodenum is divided into four sections for the purposes of description. The first three sections curve in a "C"-loop concavity in which the head of the pancreas lies. Only the first 2 cm of the superior part is mobile (covered by peritoneum) – the distal 3 cm of the first part along with the rest of the duodenum is retroperitoneal (immobile). First part The first (superior) part begins as a continuation of the duodenal end of the pylorus. From here it passes laterally (right), superiorly and posteriorly, for approximately 5 cm, before making a sharp curve inferiorly into the superior duodenal flexure (the end of the superior part). It is the only intraperitoneal portion of the duodenum. Relations: Anterior Gallbladder Quadrate lobe of liver Posterior Bile duct Gastroduodenal artery Portal vein Inferior vena cava Head of pancreas Superior Neck of gallbladder Hepatoduodenal ligament (lesser omentum) Inferior Neck of pancreas Greater omentum Head of pancreas Second part The second (descending) part of the duodenum begins at the superior duodenal flexure. It passes inferiorly to the lower border of vertebral body L3, before making a sharp turn medially into the inferior duodenal flexure (the end of the descending part). The pancreatic duct, which introduces bile and pancreatic juice into the small intestine, is directly connected to the descending duodenum. Pancreatic juice contains enzymes that help break down food, while bile aids in the digestion and absorption of fats. This part of the small intestine is responsible for secreting hormones that trigger the pancreatic duct to release pancreatic juice and bile. The pancreatic duct and common bile duct enter the descending duodenum, commonly known together as the hepatopancreatic duct (or pancreatic duct in the United States), through the major duodenal papilla (known as Ampulla of Vater). This part of the duodenum also contains the minor duodenal papilla, the entrance for the accessory pancreatic duct (of Santorini). The junction between the embryological foregut and midgut lies just below the major duodenal papilla. Third part The third (inferior/horizontal) part of the duodenum begins at the inferior duodenal flexure and passes transversely to the left, crossing the right ureter, right testicular/ovarian vessels, inferior vena cava, abdominal aorta and the vertebral column. Fourth part The fourth (ascending) part passes superiorly, either anterior to, or to the left of, the aorta, until it reaches the inferior border of the body of the pancreas. Then, it curves anteriorly and terminates at the duodenojejunal flexure where it joins the jejunum. The duodenojejunal flexure is surrounded by a peritoneal fold containing muscle fibres: the ligament of Treitz. Blood supply The duodenum receives arterial blood from two different sources. The transition between these sources is important as it demarcates the foregut from the midgut. Proximal to the 2nd part of the duodenum (approximately at the major duodenal papilla – where the bile duct enters) the arterial supply is from the gastroduodenal artery and its branch the superior pancreaticoduodenal artery. Distal to this point (the midgut) the arterial supply is from the superior mesenteric artery (SMA), and its branch the inferior pancreaticoduodenal artery supplies the 3rd and 4th sections. The superior and inferior pancreaticoduodenal arteries (from the gastroduodenal artery and SMA respectively) form an anastomotic loop between the celiac trunk and the SMA; so there is potential for collateral circulation here. The venous drainage of the duodenum follows the arteries. Ultimately these veins drain into the portal system, either directly or indirectly through the splenic or superior mesenteric vein. Lymphatic drainage The lymphatic vessels follow the arteries in a retrograde fashion. The anterior lymphatic vessels drain into the pancreatoduodenal lymph nodes located along the superior and inferior pancreatoduodenal arteries and then into the pyloric lymph nodes (along the gastroduodenal artery). The posterior lymphatic vessels pass posterior to the head of the pancreas and drain into the superior mesenteric lymph nodes. Efferent lymphatic vessels from the duodenal lymph nodes ultimately pass into the celiac lymph nodes.