Duodenum

What is the Duodenum?

The duodenum is an important part of the digestive system (or the gastrointestinal tract) in humans and some other vertebrates. It is the initial portion of the small intestine, connecting the distal end (i.e. the latter part) of the stomach to the proximal end (i.e. the upper part) of the jejunum. It receives partially digested food (chyme) with high acidity from the stomach, and involves in neutralizing the acidity of the food and propelling it through the rest of the small intestine for further digestion and absorption.

Location

Picture 1: Location of the duodenum

In humans, it is located in the upper part of the abdominal cavity, slightly more towards the right side from the midline. It is a C-shaped (or horse-shoe shaped) hollow tube that extends from L1 to L3 vertebral levels, in a curve around the head of the pancreas. Its length varies in a range of 10 to 12 inches (25-30 cm). Its surface marking is roughly at the midpoint between the suprasternal notch and the pubic symphysis, i.e. about a hand’s breadth below the xiphisternum (the transpyloric plane) in supine position. It begins at the gastroduodenal junction and ends at the duodenojejunal junction.

Anatomy: Structure of the duodenum

For description, the duodenum is divided into four parts - first, second, third, and fourth. These parts may also be called as superior, descending, horizontal, and ascending parts respectively. The average length of each of these parts (in inches) are: 2, 3, 4, and 1 respectively (or 5 cm, 7.5 cm, 10 cm, and 2.5 cm).^[7]

Picture 2: Diagram of the duodenum, its location and different parts

The first part of the duodenum begins from the pyloric orifice at the end of the stomach (at L1 vertebral level), ascends upwards and backwards to the right until it reaches the inner border of the right kidney. The initial half of this part is known as the ‘duodenal cap’ which is surrounded by a part of the peritoneum. The rest of the duodenum lies outside the peritoneum and is hence said to be retroperitoneal. The first part lies in front of the gastroduodenal artery, bile duct, and portal vein. The gallbladder and the lower part of the right lobe of the liver are situated in front of the first part of the duodenum. The second part of the duodenum curves and descends downwards (convex towards the right side) along the hilum of the right kidney, adjacent to the L2 vertebra. ^[8]Thus, it lies in front of the right kidney and ureter. The right colonic flexure (hepatic flexure) and some coils of jejunum lie in front of this part of the duodenum. The head of the pancreas lies right next to the inner (or medial) part of this in-curved portion of the duodenum. The inner wall of this part (posteromedial wall), roughly at its mid-length, receives the common opening of the main pancreatic duct and the bile duct at the ampulla of Vater. This opening is seen on the inner surface as being surrounded by a small mucosal fold known a the major duodenal papilla. About 2 cm above this opening lies another opening for the accessory pancreatic duct, which is surrounded by minor duodenal papilla on the inside. The third part of the duodenum curves forwards and to the left, in a horizontal plane, across the L3 vertebral level. It begins just over the upper part of the right psoas muscle and reaches towards the upper part of the left psoas muscle. Thus, it is situated in front of the inferior vena cava and the aorta. The superior mesenteric artery passes down in front of the third part of the duodenum. The upper border of this part of the duodenum is in close contact with the lower border of the pancreatic head. The fourth part of the duodenum ascends upwards and to the left, on the left psoas muscle, adjacent to the L2 vertebra and the aorta. It ends by curving forwards and to the right, forming the duodenojejunal junction. The duodenojejunal junction can be identified by the presence of the suspensory ligament of Treitz (suspensory muscle of the duodenum) – a thin band of connective tissue (which may contain some muscle fibers as well) extending from the right crus of the diaphragm.

Picture 3: Anatomical relations of the duodenum

The wall of the duodenum is divided into four layers, namely (from the innermost to outermost) the mucosa, the submucosa, the muscularis propria, and the serosa.

• The mucosa is the innermost layer which consists of three sub-layers - the epithelium, the lamina propria, and the muscularis mucosa. The epithelium is made up of simple columnar cells with microvilli. The epithelium and the lamina propria together are arranged into mucosal folds, increasing the total inner surface area.^[6]
• The submucosa is the next layer which is mainly made up of connective tissue. Many blood vessels and nerves run across this layer. The Brunner’s glands made from the epithelial layer reach through the mucosa down to the level of this layer.
• The muscularis propria is made up of two smooth muscle layers; the inner circular layer and the outer longitudinal layer. These muscle layers contract and relax in an orderly and regulated manner, providing motility to the duodenum.
• The outermost serosa consists of a thin layer of areolar connective tissue lined by a simple squamous epithelium.

Picture 4: Layers of cells in the duodenal wall

Blood supply and lymph drainage

The duodenum receives its blood supply mainly via the superior and inferior pancreaticoduodenal arteries. But the first part receives blood from other vessels also, including the hepatic, gastroduodenal, supraduodenal, right gastric, and right gastroepiploic arteries. Venous drainage is via the branches of superior mesenteric and portal veins. Lymph drainage is to the celiac and superior mesenteric nodes. It is innervated by both parasympathetic and sympathetic neurons.

Picture 5: Blood supply of the duodenum

Function of the duodenum

The duodenum receives chyme (partially digested food mixed with acid, mucus and pepsin) from the stomach, and acts as an intermediate for the further digestion and motility of this food.

• The endocrine cells found in the duodenal epithelium secrete the hormones secretin and cholecystokinin (CCK) in response to the acids and fats present in chyme.^[5] These hormones act locally on the pyloric sphincter of the stomach, and thereby regulate the movement of food out of the pylorus (i.e. regulate gastric emptying).
• Secretin stimulates the release of bicarbonate and mucus from the duodenal epithelium, and hence helps bring the high acidity (or very low pH) of chyme back to normal, making it easier to be digested further.
• These hormones also stimulate the secretion of bile from the liver and gallbladder, and the secretion of digestive enzymes such as trypsin, lipase and amylase from the pancreas. These secretions are received by the duodenum via the main and accessory pancreatic ducts, and the food is mixed with these secretions and prepared for further chemical digestion.
• Bile and the digestive enzymes initiate and continue the chemical digestion of food inside the duodenum by breaking down all nutrient macro-molecules into their simpler monomeric forms.
• The duodenum is the main part where calcium and iron absorption takes place in the small intestine.
• The muscle layers of the duodenum contract and relax in a regulated manner by way of peristalsis and segmentation movements to transfer the food in an orderly way to the rest of the small intestine where further digestion and absorption takes place.

How is the ileum structurally different from the duodenum?

	Duodenum	Ileum
Location	Upper abdomen	Lower abdomen and Pelvis
Shape	C shaped tube	Irregular loops
Wall thickness	Thick, muscular wall	Relatively thin, less muscular
Attachment	Not attached to the mesentery	Attached to the mesentery
Lining	Not lined by peritoneum (except first part)	Lined by peritoneum
Blood supply	Superior and inferior pancreaticoduodenal arteries; no anastomosing arcades or vasa recta	Superior mesenteric artery; via anastomosing arcades and vasa recta

Clinical significance

Duodenal ulcers

Peptic ulcer disease is an important condition that comes under gastrointestinal diseases. A peptic ulcer may be a gastric ulcer or a duodenal ulcer. These ulcers are breaks in the mucosa that reach down to the level of the muscularis mucosa. In the duodenum, ulcers occur most commonly in its first part. Duodenal ulcers are more common than gastric ulcers, and are mostly caused by the action of the bacterium Helicobacter pylori, or due to long term NSAID use. These ulcers may heal with scar formation, or may cause further complications such as bleeding, perforation, or gastric outlet obstruction (due to edema surrounding the inflamed ulcer).

Duodenitis

Duodenitis means inflammation of the duodenum, which can be either acute (short-term) or chronic (long-term). It can be caused by various reasons, and the most common ones are - infection with Helicobacter pylori, and long term NSAID use. ^[4] Other less common causes include autoimmune conditions, Crohn’s disease, bile reflux, some viral infections, ingestion of corrosive substances, excessive cigarette smoking etc.

Duodenal cancer

Duodenal cancers are less common when compared to other gastrointestinal cancers such as gastric cancer and colorectal cancer.^[3]However, it is the most common site for cancers occurring in the small intestine. The condition named familial adenomatous polyposis (FAP) is an important risk factor for duodenal cancer, and hence screening is advised for patients with FAP.

Duodenal obstruction

Duodenal obstruction is usually caused by cancers, the most common cause being pancreatic cancer. Metastatic deposits from other cancers such as gastric or colorectal cancer may also cause duodenal obstruction. Primary duodenal cancer is a less common cause when compared with the aforementioned causes.

What causes pain in the duodenum?

Pain in the duodenum can be caused by any of the conditions mentioned above – the most common being duodenal ulcers and duodenitis. Duodenal pain may be felt as a gnawing type of pain in the upper middle region of the abdomen (i.e. the epigastric region) or on either sides if it (i.e. the right and left hypochondrial regions). Less commonly, it may also be felt in the lower back region – particularly if there is an ulcer on the posterior surface of the duodenum, eroding backwards.^[2] Pain alone is not an accurate clue to diagnose a condition in this region because there are a lot of other abdominal organs closely related to one another in the upper part of the abdomen. Pain that is related to meals (often relieved after meals) and associated with other symptoms such as abdominal bloating, belching, nausea, loss of appetite, and vomiting indicate the possibility of a duodenal ulcer or severe duodenitis.^[1] More severe symptoms or alarm symptoms such as difficulty in swallowing, severe loss of appetite, loss of weight, vomiting of blood (hematemesis), or passage of tarry black stools (melena) may indicate a serious underlying condition, and hence need proper medical advice and intervention.