Fact-checked by Grok 2 weeks ago

Jejunum

The jejunum is the middle portion of the small intestine, situated between the duodenum and the ileum, and measures approximately 2.5 meters in length in adults.^[1] It begins at the duodenojejunal flexure and extends through the peritoneal cavity, anchored by the mesentery to the posterior abdominal wall.^[1] Anatomically, the jejunum features prominent plicae circulares (valves of Kerckring), which are circular folds that enhance its absorptive surface area, along with numerous villi and microvilli on its mucosal lining.^[1] Compared to the ileum, it has a thicker wall, deeper red coloration due to greater vascularity, and less surrounding mesenteric fat, with fewer lymphoid nodules such as Peyer's patches.^[1] These structural adaptations distinguish it from adjacent intestinal segments and support its role in nutrient processing.^[1] Physiologically, the jejunum serves as the primary site for the absorption of carbohydrates, amino acids, and fatty acids, utilizing carrier-mediated transporters like SGLT1 for glucose and galactose, and GLUT5 for fructose.^[2] It also facilitates the uptake of folate (vitamin B9) and dietary iron, particularly in its proximal region, while contributing to mechanical digestion through segmentation and peristaltic movements that mix and propel chyme.^[2] Overall, it absorbs about 95% of carbohydrates and proteins, as well as 90% of ingested water, playing a crucial role in postprandial nutrient homeostasis.^[2]

Anatomy

Location and gross anatomy

The jejunum forms the proximal two-fifths of the small intestine, beginning at the duodenojejunal flexure—located at the level of the second lumbar vertebra—and extending distally to the indistinct jejunoileal junction, just proximal to the ileocecal valve.^[1]^[3] In adults, it typically measures about 2.5 meters in length (ranging from 1.5 to 3.5 meters), with a greater diameter of approximately 4 cm and thicker walls compared to the ileum, giving it a deeper red color due to enhanced vascularity.^[1]^[3]^[4] This segment is entirely intraperitoneal and suspended from the posterior abdominal wall by the mesentery, a broad double layer of peritoneum that conveys blood vessels, lymphatics, and nerves while permitting extensive mobility and coiling within the peritoneal cavity.^[1]^[5] The mesentery's root extends obliquely from the duodenojejunal flexure to the ileocecal junction, anchoring the jejunum primarily in the upper left quadrant of the abdomen and allowing it to occupy variable positions during peristalsis.^[5]^[4] Macroscopically, the jejunum exhibits prominent plicae circulares (valves of Kerckring), which are permanent transverse mucosal folds that spiral around the lumen and are most pronounced in the proximal portion, thereby increasing the internal surface area.^[3]^[1] It relates anteriorly and laterally to loops of the colon, which frame it superiorly and on either side, while its mesentery positions it in close proximity to major abdominal vessels such as the superior mesenteric artery and vein.^[4]^[6] These structural adaptations support the jejunum's key role in nutrient absorption.^[1]

Microscopic anatomy

The microscopic anatomy of the jejunum is characterized by a stratified wall structure optimized for nutrient absorption, consisting of four primary layers: mucosa, submucosa, muscularis externa, and serosa.^[1] The mucosa, the innermost layer, features prominent villi and crypts of Lieberkühn that dramatically increase the surface area for absorption.^[7] Villi are finger-like projections, tallest in the jejunum compared to other small intestinal segments, measuring approximately 0.5-1.6 mm in length, with a villus-to-crypt ratio of 3-5:1.^[8]^[7] Crypts of Lieberkühn, located in the lower 20% of the mucosa, house stem cells that differentiate into various epithelial cell types, facilitating continuous renewal of the epithelial lining.^[1]^[7] The epithelial lining of the jejunal mucosa comprises several specialized cell types. Enterocytes, the predominant absorptive cells, form a columnar layer covered by microvilli (1.5-2 µm long) that constitute the brush border, enhancing nutrient uptake.^[7] Goblet cells, interspersed among enterocytes, secrete mucus to protect the epithelium and lubricate the luminal surface, though their density is lower in the jejunum than in the ileum.^[1]^[7] Paneth cells, located at the base of the crypts, produce antimicrobial peptides to defend against pathogens.^[1] Enteroendocrine cells release hormones such as cholecystokinin in response to luminal contents.^[1] Beneath the epithelium lies the lamina propria, a loose connective tissue layer containing lacteals—blind-ended lymphatic capillaries within the villi cores—that facilitate fat absorption by transporting chylomicrons.^[1] This layer also harbors immune components, including scattered lymphocytes, plasma cells, and fewer Peyer's patches compared to the ileum, contributing to mucosal immunity.^[7] The submucosa, composed of denser connective tissue, includes blood vessels, lymphatics, and the Meissner plexus for local neural regulation, but lacks Brunner's glands, which are prominent in the duodenum.^[1]^[7] The muscularis externa consists of an inner circular smooth muscle layer and an outer longitudinal layer, separated by the Auerbach plexus, which coordinates peristaltic motility.^[7] The outermost serosa, a thin mesothelial layer with underlying connective tissue, covers the intraperitoneal jejunum, providing a slippery surface for abdominal mobility.^[1] In comparison to the ileum, the jejunum exhibits less lymphoid tissue and fewer goblet cells, reflecting its primary role in broad nutrient absorption rather than specialized immune surveillance or distal processing.^[7]

Vascular and neural supply

The arterial supply to the jejunum originates from the superior mesenteric artery (SMA), which arises from the abdominal aorta at the level of the L1 vertebra.^[1] Multiple jejunal branches (typically 3–5) emerge from the left side of the SMA and course through the mesentery to reach the intestinal wall.^[1] These branches interconnect to form a series of arterial arcades, with fewer and simpler arcades in the proximal jejunum compared to the more complex networks distally; from these arcades, straight end-arteries termed vasa recta penetrate the intestinal wall to supply the mucosa and submucosa.^[1] This anastomotic arcade system provides collateral circulation, which helps maintain blood flow and prevent ischemia during events such as intestinal twisting.^[1] Venous drainage from the jejunum parallels the arterial supply, with jejunal veins collecting blood from the vasa recta and draining into the superior mesenteric vein (SMV).^[1] The SMV runs alongside the SMA within the mesentery and joins the splenic vein posterior to the neck of the pancreas to form the portal vein, which delivers nutrient-rich blood to the liver.^[1] Lymphatic drainage begins in the jejunal villi via specialized vessels called lacteals, which absorb lipids and fat-soluble vitamins, facilitating their transport as chylomicrons into the lymphatic system.^[1] These lacteals converge into larger collecting vessels within the mesentery, draining into mesenteric lymph nodes associated with the arterial arcades and SMA; efferent vessels from these nodes then proceed to the cisterna chyli and ultimately the thoracic duct for return to the systemic circulation.^[1] The jejunum receives dual extrinsic innervation from the autonomic nervous system, supplemented by the intrinsic enteric nervous system. Parasympathetic fibers travel via the anterior and posterior vagus nerves, stimulating secretory activity and peristaltic motility in the jejunal wall.^[1] Sympathetic innervation arises from the celiac and superior mesenteric plexuses, originating from the greater and lesser splanchnic nerves (T5–T11), and generally inhibits motility while constricting local blood vessels.^[1] Locally, the enteric nervous system coordinates gut function through the myenteric (Auerbach's) plexus between the muscular layers, which primarily regulates motility, and the submucosal (Meissner's) plexus, which controls secretion and local blood flow.^[1]

Physiology

Nutrient absorption

The jejunum serves as the primary site for the absorption of most nutrients from digested chyme, facilitating the uptake of carbohydrates, proteins, fats, and water-soluble vitamins into the bloodstream or lymphatics.^[9] This process occurs across the apical membrane of enterocytes via specialized transporters and channels, with subsequent basolateral transport ensuring delivery to circulation.^[10] Monosaccharides, such as glucose and galactose, are absorbed in the jejunum primarily through the sodium-glucose linked transporter 1 (SGLT1) on the apical membrane, which couples glucose uptake with sodium ions using the sodium gradient established by the Na+/K+-ATPase.^[11] Fructose enters via facilitative diffusion through GLUT5, while glucose and other monosaccharides exit the enterocyte basolaterally via GLUT2.^[9] Amino acids are absorbed via specific transporters, including the proton-coupled peptide transporter 1 (PEPT1) for di- and tripeptides, which hydrolyze intracellularly to release free amino acids, alongside dedicated amino acid transporters like B0AT1 for neutral amino acids.^[9] Water-soluble vitamins, including thiamine (vitamin B1), riboflavin (B2), niacin (B3), pantothenate (B5), biotin (B7), folate (B9), pyridoxine (B6), and ascorbic acid (vitamin C), are taken up by jejunal enterocytes through carrier-mediated mechanisms, such as the thiamine pyrophosphate transporter for B1 and sodium-dependent transporters for folate. Additionally, dietary iron is absorbed primarily in the duodenum and proximal jejunum. Ferric iron is reduced to ferrous form by duodenal cytochrome B (Dcytb) and taken up via the divalent metal transporter 1 (DMT1) on the apical membrane of enterocytes.^[9]^[12] Fat absorption in the jejunum begins with emulsification by bile salts secreted from the gallbladder, forming mixed micelles that solubilize monoglycerides, free fatty acids, cholesterol, and fat-soluble vitamins for diffusion across the unstirred water layer to the enterocyte brush border.^[13] Within enterocytes, these lipids are re-esterified into triglycerides and packaged into chylomicrons, which are exocytosed into lacteals—the jejunal lymphatic vessels—for transport via the lymph to the bloodstream.^[9] The absorptive capacity of the jejunum is greatly amplified by structural adaptations, including plicae circulares, villi, and microvilli on enterocytes, significantly expanding the effective surface area of the small intestine to approximately 30 m², with the jejunum contributing the majority due to its prominent plicae circulares.^[1]^[14] Final digestion of carbohydrates and proteins occurs at the jejunal brush border through membrane-bound enzymes: disaccharidases like lactase-phlorizin hydrolase (lactase) for lactose to glucose and galactose, sucrase-isomaltase for sucrose and maltose to glucose, and peptidases such as aminopeptidases and dipeptidases that cleave peptides into absorbable units.^[15] These enzymes are embedded in the glycocalyx of microvilli, ensuring proximity to substrates for efficient hydrolysis.^[16] Nutrient absorption in the jejunum is regulated by gastrointestinal hormones, including secretin, which stimulates bicarbonate secretion to neutralize chyme and optimize pH for enzymatic activity, and cholecystokinin (CCK), released from duodenal and jejunal I cells in response to fats and proteins, promoting bile salt release and pancreatic enzyme secretion to enhance lipid and protein digestion.^[9] These hormones also modulate enterocyte transporter expression and function, fine-tuning uptake rates based on luminal content.^[17]

Secretion and motility

The jejunum contributes to digestion through the secretion of succus entericus, a fluid produced by the intestinal mucosa that includes enzymes released from the brush border of enterocytes and protective mucus from goblet cells. Enterokinase (also known as enteropeptidase), a key brush-border enzyme secreted by enterocytes, activates pancreatic trypsinogen into active trypsin, initiating protein digestion in the small intestine.^[18] Nucleosidases, another group of enterocyte-derived enzymes, hydrolyze nucleotides into nucleosides and phosphate, facilitating nucleic acid breakdown.^[19] Goblet cells in the jejunal mucosa secrete mucus that forms a protective barrier against mechanical and chemical irritation, with higher density observed toward the ileum but still significant in the jejunum.^[19] The jejunal epithelium also secretes bicarbonate ions to help neutralize the acidic chyme entering from the stomach, maintaining an optimal pH for enzymatic activity and preventing mucosal damage. This secretion occurs via enterocyte transporters, including apical chloride-bicarbonate exchangers and basolateral sodium-bicarbonate cotransporters, stimulated by luminal acid sensing.^[20] In the proximal jejunum, substances like substance P can enhance electrogenic sodium and bicarbonate secretion, contributing to fluid balance. Jejunal motility involves two primary patterns: segmentation contractions for mixing chyme with digestive secretions and peristaltic waves for propulsion toward the ileum. Segmentation consists of rhythmic, localized contractions that enhance contact between contents and the absorptive surface, while peristalsis propels boluses distally through coordinated circular muscle activity.^[2] These patterns are primarily coordinated by the enteric nervous system (ENS), which integrates sensory input from mechanoreceptors with motor outputs via the myenteric plexus, using neurotransmitters such as acetylcholine for excitation and nitric oxide for inhibition.^[21] During fasting, the jejunum participates in the migrating motor complex (MMC), a cyclic motility pattern that clears residual debris and bacteria from the small intestine to prevent overgrowth. The MMC originates in the stomach or duodenum and propagates through the jejunum as phase III bursts of high-amplitude contractions every 90-120 minutes, driven by the hormone motilin released from duodenal enteroendocrine cells.^[22]^[23] Neural and hormonal controls modulate jejunal motility: vagal parasympathetic stimulation via the ENS increases segmentation and peristalsis to promote digestion, whereas sympathetic innervation inhibits these activities by reducing acetylcholine release and smooth muscle tone.^[21] Motilin further enhances interdigestive motility, linking hormonal signals to ENS-mediated propulsion.^[23]

Clinical significance

Associated disorders

The jejunum is primarily affected by several malabsorption syndromes, which impair its critical role in nutrient uptake. Celiac disease, an autoimmune disorder triggered by gluten ingestion, leads to villous atrophy and crypt hyperplasia in the jejunal mucosa, resulting in symptoms such as chronic diarrhea, weight loss, abdominal pain, and nutritional deficiencies including iron, folate, and vitamin B12 malabsorption.^[24] Management generally involves a strict lifelong gluten-free diet to promote mucosal healing and symptom resolution, alongside nutritional supplementation as needed.^[25] Tropical sprue, an infectious malabsorption syndrome endemic to tropical regions, causes similar jejunal villous blunting and inflammation, often due to bacterial overgrowth or enterotoxins, manifesting as prolonged diarrhea, steatorrhea, and deficiencies in vitamins B12 and folate.^[26] Initial treatment typically includes antibiotics like tetracycline combined with folic acid supplementation to address the infectious etiology and restore absorption.^[27] Inflammatory conditions such as Crohn's disease frequently involve the jejunum, where transmural inflammation can progress to fibrostenotic strictures, narrowing the lumen and causing obstructive symptoms like postprandial pain, bloating, and vomiting.^[28] These strictures arise from chronic relapsing inflammation, affecting up to 70% of patients within 10 years of diagnosis.^[29] General management focuses on anti-inflammatory therapies, including corticosteroids and immunomodulators, to reduce inflammation and prevent stricture progression, with nutritional support to mitigate malnutrition.^[30] Jejunal obstructions can result from mechanical causes, leading to acute abdominal emergencies. Intussusception occurs when a segment of jejunum telescopes into an adjacent portion, often in adults due to a lead point like a polyp, presenting with intermittent colicky pain, nausea, and bloody stools.^[31] Volvulus involves twisting of the jejunal loop around its mesentery, compromising blood flow and causing sudden severe pain, distension, and vomiting.^[32] Adhesions, commonly forming post-abdominal surgery, represent the leading cause of small bowel obstruction including the jejunum, accounting for 65-75% of cases, with symptoms of crampy pain and obstipation.^[33] Initial management emphasizes fluid resuscitation, nasogastric decompression, and bowel rest to alleviate symptoms and prevent complications like ischemia.^[34] Tumors of the jejunum are rare but can significantly disrupt function. Jejunal adenocarcinoma, accounting for approximately 29% of small bowel adenocarcinomas (which comprise 30-40% of small bowel malignancies), originates from glandular epithelium and often presents late with obstruction, bleeding, or perforation due to its insidious growth.^[35]^[36] Leiomyosarcoma, a mesenchymal tumor arising from smooth muscle, accounts for roughly 1-2% of small bowel cancers and typically manifests as abdominal mass, pain, or gastrointestinal bleeding from ulceration.^[37] Both types are managed primarily through multidisciplinary approaches involving oncologic evaluation, with emphasis on early detection to improve outcomes.^[38] Resection of substantial jejunal length, often due to trauma, ischemia, or malignancy, can precipitate short bowel syndrome, characterized by inadequate absorptive surface leading to diarrhea, dehydration, and electrolyte imbalances.^[39] The jejunum's removal particularly impairs fluid and electrolyte absorption, exacerbating maladaptive responses in the remaining gut.^[40] Management centers on parenteral nutrition support, fluid replacement, and antidiarrheal agents to stabilize patients and promote intestinal adaptation.^[41]

Surgical and diagnostic procedures

Jejunostomy involves the surgical creation of an opening into the jejunum to insert a feeding tube for enteral nutrition, particularly in patients unable to tolerate oral or gastric feeding due to conditions like esophageal cancer or severe dysphagia.^[42] The procedure can be performed via open, laparoscopic, or percutaneous methods, with laparoscopic approaches showing reduced postoperative complications compared to open techniques, such as lower rates of wound infections and ileus.^[43] During insertion, the tube is typically placed 20-30 cm distal to the ligament of Treitz to ensure adequate nutrient absorption while minimizing reflux.^[44] Small bowel resection of the jejunum is indicated for localized tumors, such as neuroendocrine tumors or gastrointestinal stromal tumors, and for ischemic segments due to mesenteric vascular occlusion.^[45] The surgery entails removing the affected jejunal segment followed by primary anastomosis, with oncologic resections including regional lymphadenectomy to address potential metastasis.^[46] In cases of ischemia, urgent resection prevents perforation, and the procedure prioritizes restoring bowel continuity while assessing viability of remaining segments intraoperatively.^[47] Jejunal bypass procedures are utilized in modern bariatric surgery variants, such as single-anastomosis duodenal-jejunal bypass with sleeve gastrectomy, to promote weight loss and metabolic improvements in morbidly obese patients by altering nutrient flow and reducing caloric absorption.^[48] This technique involves transecting the jejunum distal to the ligament of Treitz and creating a gastrojejunal or duodenojejunal anastomosis, achieving significant excess weight loss of 60-80% at one year with remission of type 2 diabetes in up to 70% of cases.^[49] Diagnostic evaluation of the jejunum often employs capsule endoscopy, a noninvasive method where a swallowed camera traverses the small bowel to visualize mucosal abnormalities like ulcers or polyps, offering high sensitivity for obscure gastrointestinal bleeding sources.^[50] Computed tomography (CT) enterography and magnetic resonance (MR) enterography are preferred for detecting obstructions, providing detailed imaging of bowel wall thickening, luminal narrowing, and extraluminal involvement with diagnostic accuracies exceeding 90%.^[31] For histological confirmation, device-assisted enteroscopy allows targeted biopsy of jejunal lesions, enabling definitive diagnosis of conditions such as celiac disease or neoplasms through villous architecture assessment.^[51] Intraoperative considerations during jejunal surgery emphasize preserving the vascular arcades formed by the superior mesenteric artery branches to maintain perfusion and reduce ischemia risk at the anastomotic site, as detailed in the vascular supply section.^[52] Common complications following jejunal procedures include anastomotic leaks, occurring in 5-10% of cases due to technical factors or poor tissue perfusion, potentially leading to peritonitis if untreated.^[45] Enterocutaneous fistulas may arise from anastomotic breakdown, with output management crucial to prevent dehydration and electrolyte imbalances.^[45] Post-resection bacterial overgrowth can develop in the remaining small bowel, resulting from altered motility and anatomy, and is characterized by symptoms managed through antibiotics or prokinetics.^[45] Recent advances include the wireless motility capsule, a swallowable device that measures pH, pressure, and transit times to assess jejunal function noninvasively, providing regional small bowel transit data in 4-6 hours with over 80% correlation to scintigraphy.^[53] This tool aids in evaluating motility disorders without radiation exposure, facilitating ambulatory diagnosis.^[54]

Comparative and developmental aspects

In other animals

In herbivores such as ruminants, the jejunum forms part of an extensively elongated small intestine, often measuring around 150 feet in total length in mature cattle, approximately 20 times the animal's body length, to facilitate the absorption of fermentation products from the rumen.^[55]^[56] This adaptation supports the processing of a cellulose-rich diet, where rumen microbial populations break down plant fibers into volatile fatty acids, amino acids, and vitamins, which the jejunum then absorbs via its villi and microvilli-lined epithelium.^[56] The increased length and surface area in ruminants like cows and sheep enhance the efficiency of extracting nutrients from fermented digesta, compensating for the energy-intensive microbial breakdown of complex carbohydrates.^[57] In contrast, carnivores such as dogs exhibit a comparatively shorter small intestine, ranging from 1 to 5 meters in length depending on body size, with the jejunum comprising the majority and optimized for rapid transit and absorption of high-protein and high-fat meals.^[18] This reduced length relative to body length—about 3 to 4 times the animal's body length—reflects an evolutionary emphasis on quick digestion of easily digestible animal tissues, where the jejunum primarily absorbs amino acids and peptides via sodium-linked carriers in enterocytes, and fats as fatty acids and monoglycerides incorporated into chylomicrons for lymphatic transport.^[18] In cats, a strict carnivore relative, the small intestine measures 1 to 1.5 meters, further underscoring the streamlined structure for protein and lipid uptake without the need for extensive fiber processing.^[18] The avian jejunum, as part of the small intestine's looped configuration in birds, lacks the plicae circulares found in mammals and instead relies on densely packed villi with microvilli to maximize surface area for nutrient absorption in a relatively short tract adapted to rapid digesta passage.^[59]^[60] In species like chickens and geese, the jejunum follows the duodenum and contributes to the breakdown and uptake of carbohydrates, proteins, and fats using pancreatic enzymes and bile, with histological studies showing progressive maturation of villus height in the jejunum during early development to support efficient extraction from varied diets including seeds and insects.^[61] This structure aligns with birds' high metabolic demands, where the small intestine overall is shorter than in equivalent-sized mammals, prioritizing speed over length for fermentation-independent digestion.^[59] Rodents display notable variations in jejunal architecture, characterized by high villus density and asymmetric mucosal folding, with antimesenteric villi significantly taller (e.g., 261 μm in mice versus 226 μm mesenterically) and contributing to a greater surface enlargement factor (up to 5.3 in mice), which enhances nutrient extraction efficiency in small-bodied omnivores.^[62] In rats and mice, this dense villus arrangement in the jejunum, combined with rapid epithelial turnover, optimizes the absorption of diverse nutrients from mixed diets, allowing for maximal uptake per unit length in compact gastrointestinal tracts.^[62] Such adaptations are particularly vital for rodents' high metabolic rates and opportunistic feeding, where the jejunum's enhanced surface area supports the digestion of both plant and animal matter without relying on extensive fermentation chambers.^[63] Across mammals, evolutionary trends show that small intestine length, including the jejunum, has increased in correlation with dietary complexity, particularly in herbivores, where longer tracts (phylogenetically adjusted) accommodate the breakdown of fibrous, cellulose-based foods compared to the shorter intestines of faunivores focused on protein-rich prey.^[57] Surface area expansions via villi and folds have similarly scaled with trophic level shifts toward herbivory, enabling greater absorption capacity for volatile fatty acids and other fermentation byproducts, as seen in the diversification of mammalian lineages adapting to varied ecological niches.^[64] These patterns highlight how intestinal elongation and mucosal enhancements evolved to match increasing dietary demands, from simple carnivory to complex plant-based foraging.^[65]

Embryological development

The jejunum develops from the midgut portion of the primitive gut tube, which forms during the third week of embryonic development following gastrulation, when endodermal cells are surrounded by splanchnic mesoderm to create a hollow structure.^[66] This midgut endoderm gives rise to the distal duodenum, jejunum, ileum, cecum, ascending colon, and proximal two-thirds of the transverse colon, with the jejunum specifically comprising the proximal segment of this loop.^[67] Early patterning along the anterior-posterior axis is influenced by signaling gradients, establishing regional identities before further morphogenesis.^[68] Between weeks 5 and 10, the rapidly growing midgut undergoes a complex 270-degree counterclockwise rotation around the superior mesenteric artery axis, positioning the jejunum immediately distal to the duodenum in its final central abdominal location.^[66] This process begins with herniation of the midgut loop into the umbilical cord around week 6, forming a U-shaped structure that accommodates the organ's disproportionate growth relative to the abdominal cavity.^[69] The loop returns to the coelomic cavity by approximately week 10 in a proximodistal manner, during which an additional 180-degree rotation occurs, fixing the jejunum's broad mesentery derived from dorsal mesentery fusion and establishing its vascular attachments.^[67] This return and fixation are critical for preventing abnormal mobility in the mature structure.^[66] Histological differentiation of the jejunal mucosa progresses through fetal stages, with villi emerging around weeks 9 to 12 from mesenchymal-endodermal interactions that cluster epithelial cells atop mesodermal cores to increase surface area.^[67] Crypts of Lieberkühn form shortly thereafter, between weeks 10 and 12, via endodermal invaginations into the mesenchyme, housing stem cells and secretory lineages essential for epithelial renewal.^[1] Maturation of nutrient transporters, such as those for glucose and amino acids, occurs progressively from mid-gestation onward, enabling fetal nutrient uptake and aligning with the adult microscopic features of absorptive enterocytes on villi.^[70] These processes are regulated by key signaling pathways, including Hedgehog (Sonic Hedgehog and Indian Hedgehog), which coordinates mesenchymal proliferation and villus clustering, and Wnt, which drives anterior-posterior patterning through transcription factors like Cdx2 to specify midgut identity.^[71] ^[68] Disruptions in these developmental events can lead to congenital anomalies, such as malrotation, where incomplete midgut rotation fails to properly position the jejunum and mesentery, predisposing to midgut volvulus and intestinal obstruction.^[72] Jejunal atresia, a complete blockage resulting in bowel discontinuity, arises from vascular accidents during herniation or rotation that compromise mesenteric blood supply to the midgut loop.^[67] These anomalies often manifest in the neonatal period and may associate with other syndromic features, underscoring the precision required in midgut morphogenesis.^[73]

History

Etymology

The term "jejunum" originates from the Latin adjective jejunus (or ieiunus), meaning "empty," "barren," or "fasting." This nomenclature arose from observations during anatomical dissections, where the jejunum was frequently found devoid of contents after death, attributed to its efficient nutrient absorption or post-mortem peristaltic activity that empties it rapidly.^[74]^[75] The specific naming of the jejunum as such emerged in medieval European anatomy, with the description as the "empty intestine" due to its proximity to the liver, believed to draw chyle from it, appearing in Berengario da Carpi's anatomical texts in the early 16th century. Although ancient Greek anatomists like Herophilus (c. 335–280 BCE) conducted systematic human dissections and described intestinal structures, the Latin term "jejunum" was formalized later during the Renaissance, notably in texts by anatomists such as Andreas Vesalius in De humani corporis fabrica (1543), which standardized regional intestinal nomenclature based on classical observations.^[76]^[77] In medieval Arabic medical literature, the equivalent structure was termed sa'm or saim by scholars like Avicenna (Ibn Sina, 980–1037 CE) in his Canon of Medicine, translating to "fasting" or "empty intestine," reflecting a parallel recognition of its typical vacuity and aligning with Greco-Roman traditions preserved and expanded in Islamic scholarship. This terminology underscores a cultural and observational link to fasting, as the Latin jejunus also evokes religious abstinence from food, influencing how anatomists across traditions conceptualized the organ's physiological role.^[78]^[79]

Historical descriptions

The earliest systematic descriptions of the small intestine's components emerged in ancient Alexandria around 300 BCE, where physicians Herophilus and Erasistratus conducted human dissections, enabling the first detailed distinctions among its parts; Herophilus specifically named the duodenum as the initial segment.^[77] These pioneers differentiated the duodenum from the longer, coiled remainder of the small intestine, laying foundational anatomical observations that contrasted with earlier speculative accounts.^[80] In the 13th century, Islamic scholar Ibn al-Nafis advanced anatomical knowledge in his comprehensive medical encyclopedia Sharh Tashrih al-Qanun, providing corrected diagrams and descriptions of the digestive tract's structures, including the attachments supporting intestinal mobility.^[81] His work emphasized empirical corrections to prior Greek models, detailing the layered composition of abdominal organs and their interconnections, which influenced later European anatomists.^[82] The Renaissance marked a pivotal advancement with Andreas Vesalius's 1543 publication of De humani corporis fabrica, which featured precise woodcut illustrations of the jejunum's coiled configuration and its distinction as the middle segment of the small intestine between the duodenum and ileum.^[83] Vesalius's direct observations from dissections refuted Galenic errors, accurately depicting the jejunum's mesenteric suspensions and valvular folds to enhance understanding of its form.^[84] In the 19th century, French physiologist Claude Bernard's experimental investigations into digestion underscored the jejunum's critical absorptive function, demonstrating through vivisections and nutrient tracing how pancreatic secretions facilitate carbohydrate and fat uptake in the small intestine's mid-region.^[85] Bernard's 1849-1856 studies, including sugar absorption assays post-intestinal transit, established the jejunum as a primary site for converting chyme into absorbable forms, integrating anatomy with physiological mechanisms.^[86] Twentieth-century technological breakthroughs, particularly electron microscopy in the 1950s, unveiled the jejunum's microvilli—fine, finger-like projections on enterocyte surfaces that amplify absorptive area by up to 600-fold—as first visualized by Granger and Baker in rat intestinal epithelium.^[87] This revelation transformed comprehension of nutrient transport at the cellular level. Recent genomic research has illuminated the molecular basis of jejunal function, identifying differentially expressed transporter genes like those encoding SLC family proteins for nutrient and drug uptake, as profiled in human jejunal tissues via RNA sequencing.^[88] Studies from the 2010s onward, including transcriptomic analyses, have mapped over 30 key genes along the jejunum's axis, revealing regulatory variations that underpin selective absorption.^[89]

References

[1]
Anatomy, Abdomen and Pelvis, Small Intestine - StatPearls - NCBI
Feb 18, 2025 · The jejunum primarily absorbs carbohydrates, amino acids, and fatty acids through the villi. Jejunal and ileal plicae circulares increase ...
[2]
Physiology, Small Bowel - StatPearls - NCBI Bookshelf
Jan 31, 2024 · The primary function of the small intestine is to break down food, absorb nutrients, extract water, and move food along the GI tract. The ...
[3]
Jejunum: Anatomy, histology, function, composition - Kenhub
The jejunum is the middle of the three parts of the small intestine between the duodenum and ileum. Its arterial supply is provided by the jejunal arteries.
[4]
Small Intestine Anatomy - Medscape Reference
Mar 19, 2025 · It lies between the stomach and the large intestine (large bowel) and includes the duodenum, jejunum, and ileum.
[5]
The Small Intestine - Duodenum - Jejunum - Ileum - TeachMeAnatomy
Characteristic Features of the Jejunum and Ileum ; Located in upper left quadrant, Located in lower right quadrant ; Thick intestinal wall, Thin intestinal wall.
[6]
Small Intestine: Function, Parts, Length & Location - Cleveland Clinic
The jejunum. This section of your small intestine is 8 feet long. It lays in many coils inside the lower abdominal cavity. The jejunum is dark red because ...Gastric neuroendocrine tumors · Meckel’s Diverticulum · DuodenumMissing: gross | Show results with:gross
[7]
Histology-small intestine - Pathology Outlines
Jan 20, 2022 · Small intestine extends from gastric pylorus to ileocecal valve; layers include mucosa, submucosa, muscularis propria (externa), ...
[8]
Digestion's Cutest Heroes: A Dive Into the Intestinal Villi - Visible Body
Sep 12, 2019 · Each one is approximately 0.5-1.6 mm long and has numerous projecting microvilli that help you absorb nutrients. Image from Human Anatomy Atlas ...
[9]
Physiology, Nutrient Absorption - StatPearls - NCBI Bookshelf
Oct 28, 2023 · The lacteals—the jejunal lymphatic vessels—aid in the absorption of lipids, which have become glycerol and free fatty acids in this segment.Cellular Level · Function · Mechanism · Pathophysiology
[10]
Physiology of Intestinal Absorption and Secretion - PMC - NIH
Virtually all nutrients from the diet are absorbed into blood across the highly polarized epithelial cell layer forming the small and large intestinal mucosa.
[11]
Mechanisms of Glucose Absorption in the Small Intestine in Health ...
Jul 20, 2021 · There is evidence that GIP may stimulate intestinal glucose absorption in the jejunum by increasing, at least in part, its active transport ...
[12]
Intestinal lipid absorption - PMC - PubMed Central - NIH
Bile salt micelles facilitate the transfer of cholesterol across the unstirred water layer. The mechanism by which the cholesterol in micelles is taken up by ...
[13]
Physiology, Digestion - StatPearls - NCBI Bookshelf
Sep 12, 2022 · The small intestines have a brush border made up of numerous microvilli lining their apical surface. This border is rich in enzymes. The ...
[14]
Small Intestinal Brush Border Enzymes
Jan 23, 2025 · Maltase-glucoamylase and sucrase-isomaltase are closely related enzymes embedded in the brush border membrane that execute the terminal stages of digestion.
[15]
Physiology, Gastrointestinal Hormonal Control - StatPearls - NCBI
CCK is secreted from I cells in the duodenum and jejunum in response to acids and monoglycerides (but not triglycerides), as well as the presence of protein ...
[16]
Small Intestine - PMC - PubMed Central
The brush-border peptidase enterokinase (enteropeptidase) is important in the initial activation of pancreatic trypsin from trypsinogen by cleaving a terminal ...Missing: nucleosidases | Show results with:nucleosidases
[17]
https://www.ncbi.nlm.nih.gov/books/NBK537284/
[18]
Bicarbonate secretion and acid/base sensing by the intestine - PMC
Feb 19, 2024 · The transport of bicarbonate across the enterocyte cell membrane regulates the intracellular as well as the luminal pH and is an essential part of directional ...
[19]
The Enteric Nervous System and Its Emerging Role as a ...
Sep 8, 2020 · This review will provide a general overview of the ENS to date and connect specific GI diseases including short bowel syndrome with neuronal pathophysiology ...
[20]
The Migrating Motor Complex
The migrating motor complex is a distinct pattern of electromechanical activity observed in gastrointestinal smooth muscle during the periods between meals.
[21]
Physiology, Motilin - StatPearls - NCBI Bookshelf
Sep 26, 2022 · Motilin stimulates gastric and small intestine motility, causing undigested food in these regions to move into the large intestine. This ...Missing: segmentation | Show results with:segmentation
[22]
Celiac Disease - StatPearls - NCBI Bookshelf - NIH
Feb 4, 2025 · Gastrointestinal symptoms arise from malabsorption due to villous atrophy in the small intestine, reducing the absorptive surface and digestive ...
[23]
Celiac Disease | MUSC Health | Charleston SC
Patients with celiac disease who ingest any wheat products develop abnormalities in the lining of the small intestine, particularly the upper part (jejunum).
[24]
Tropical Sprue - StatPearls - NCBI Bookshelf
Sep 14, 2025 · Tropical sprue is a malabsorption syndrome characterized by acute or chronic diarrhea, occurring in individuals residing in or with ...Introduction · Etiology · Evaluation · Differential Diagnosis
[25]
Tropical malabsorption - PMC - PubMed Central - NIH
Definition. Tropical sprue is an acquired disease of unknown aetiology, characterised by malabsorption, multiple nutritional deficiencies and mucosal ...Small‐bowel Mucosal... · Protozoal Infections · Tropical Sprue
[26]
Intestinal strictures in Crohn's disease: a 2021 update - PMC - NIH
Jun 21, 2022 · Approximately 70% of CD patients will develop fibrotic strictures after 10 years of CD diagnosis.
[27]
Prevention and treatment of stricturing Crohn's disease - NIH
Stricturing disease is a known complication of CD and is likely a result of chronic relapsing and remitting inflammation[1]. Up to approximately 20% of patients ...
[28]
Crohn's disease - Symptoms and causes - Mayo Clinic
Oct 29, 2024 · Over time, parts of the bowel can scar and narrow, which may block the flow of digestive contents, often known as a stricture. Surgery to widen ...Gastroenterology & Gi... · Symptoms · Risk Factors<|control11|><|separator|>
[29]
Small Bowel Obstruction - StatPearls - NCBI Bookshelf
Jan 19, 2025 · This condition is most frequently caused by postoperative adhesions, followed by hernias, tumors, or less common conditions like volvulus, ...
[30]
Intestinal Obstruction - Gastrointestinal Disorders - Merck Manuals
Jun 19, 2018 · Causes of Intestinal Obstruction ; Neonates. Atresia, volvulus, bands, annular pancreas ; Jejunum and ileum ; Adults. Hernias, adhesions (common), ...
[31]
Small-Bowel Obstruction - Medscape Reference
Dec 2, 2024 · The most common cause of SBO in developed countries is intra-abdominal adhesions, accounting for approximately 65% to 75% of cases, followed by ...
[32]
Intestinal obstruction - Symptoms & causes - Mayo Clinic
Intestinal obstruction is a blockage that keeps food or liquid from passing through your small intestine or large intestine (colon).
[33]
Jejunal Adenocarcinoma: A Rare Cause of Small Bowel Obstruction
Jejunal adenocarcinoma (JA) is both a rare type of gastrointestinal malignancy and an uncommon cause of small bowel obstruction (SBO).
[34]
Unveiling a Jejunal Leiomyosarcoma Presenting as a ... - NIH
Aug 15, 2024 · The five types of malignant small bowel tumors include leiomyosarcomas (LMSs) (1.2%), carcinoids (44.3%), adenocarcinomas (32.6%), lymphomas ( ...
[35]
Small bowel cancer - Symptoms and causes - Mayo Clinic
Aug 19, 2025 · Adenocarcinoma. Adenocarcinoma is the most common type of small bowel cancer. Adenocarcinoma starts in the gland cells that make mucus.Overview · Causes · Types Of Small Bowel Cancer
[36]
Management of patients with a short bowel - PMC - NIH
Jejunum-colon patients often appear well after their resection except for diarrhoea/steatorrhoea, but in the following months may lose weight and become ...
[37]
Short Bowel Syndrome: Symptoms, Causes & Treatment
If surgery removes your duodenum and a portion of your jejunum, your ileum can take on its role in absorbing nutrients.
[38]
Management of short bowel syndrome in adult patients - Mayo Clinic
Aug 19, 2022 · Adults typically develop SBS after massive surgical resection or significant damage to the small intestines.
[39]
Feeding Jejunostomy Tube - StatPearls - NCBI Bookshelf - NIH
Although it is a relatively simple procedure, it can be associated with complications like bowel obstruction and metabolic abnormalities. Minimally invasive ...Continuing Education Activity · Introduction · Indications · Technique or Treatment
[40]
Laparoscopic vs. open feeding jejunostomy: a systemic review and ...
Dec 13, 2022 · Our results indicate that laparoscopic feeding jejunostomy might reduce the postoperative overall complication rate compared with open ...
[41]
Jejunostomy Tube Insertion for Enteral Nutrition - NIH
May 24, 2020 · Laparoscopic and radiologic jejunostomy tube insertion both showed high success and low complication rates. Previous major abdominal surgery and ...
[42]
Small Bowel Resection - StatPearls - NCBI Bookshelf
Small bowel resection involving the ileum or jejunum is straightforward as long as the bowel is adequately mobile. Duodenal resections are exponentially more ...Anatomy and Physiology · Indications · Technique or Treatment · Complications
[43]
The Surgical Management of Small Bowel Neuroendocrine Tumors
Surgical resection of SBNETs should include a complete oncologic resection of the primary tumor(s), regional lymph nodes, and mesenteric fibrosis, if feasible.
[44]
Successful treatment of nonocclusive mesenteric ischemia in a ...
Aug 10, 2023 · We report a very rare case of jejunal necrosis caused by NOMI in the pedicled mesentery of the reconstructed jejunum after remnant gastric tube resection.
[45]
Single-Anastomosis Sleeve Jejunal Bypass as a Treatment for ...
Dec 29, 2023 · SASJ bypass is a safe and effective procedure for weight loss in patients with morbid obesity, with an acceptable rate of complications.
[46]
Comparison of Laparoscopic Single-Anastomosis Duodenal-Jejunal ...
The aim of this study was to compare postoperative course of laparoscopic single-anastomosis duodenal-jejunal bypass with sleeve gastrectomy (SADJB-SG) and ...
[47]
Capsule endoscopy camera - Mayo Clinic
Capsule endoscopy can find the irritated and inflamed areas in the small intestine in diseases such as Crohn's disease or ulcerative colitis.
[48]
Impact of enteroscopy on diagnosis and management of small ...
DAE has an important role, since it enables a biopsy sample and a definitive histological diagnosis. DAE can also precisely locate and mark single or ...
[49]
The technique of fourth jejunal artery-based jejunal conduit for ... - NIH
Sep 8, 2020 · ... arteries are divided at the base to preserve the vascular arcade and create a long pedicled jejunal conduit. Jejunojejunostomy was performed ...
[50]
A Technical Review and Clinical Assessment of the Wireless Motility ...
The WMC is a single-use, orally ingested, nondigestible capsule that is capable of measuring gastric emptying time (GET), small bowel transit time (SBTT), ...
[51]
Gastrointestinal motility revisited: The wireless motility capsule - NIH
The wireless motility capsule (WMC) provides a novel method of measuring whole and regional GI transit times, pH, and temperature in a single, ambulatory, ...
[52]
Understanding the Ruminant Animal Digestive System
Anatomy of the ruminant digestive system includes the mouth, tongue, salivary glands (producing saliva for buffering rumen pH), esophagus, four-compartment ...
[53]
The ruminant digestive system - University of Minnesota Extension
The rumen absorbs most of the VFAs from fermentation. A good blood supply to the rumen walls improves absorption of VFAs and other digestion products. Tiny ...
[54]
Mammalian intestinal allometry, phylogeny, trophic level and climate
The fact that there is a diet effect on the total intestinal tract length across all mammals, or within Chiroptera, contrasts with the absence of such an effect ...Missing: jejunum | Show results with:jejunum
[55]
Digestive Anatomy and Physiology of Birds
The proximal small intestine receives bile from the liver and digestive enzymes from the pancreas, and the absorptive epithelial cells are decorated with ...
[56]
Multiple GI Tracts? Jeez! – Lesson plans for GI physiology topics
Structurally, avian small intestines are like monogastric and ruminant digestive systems in that there are three portions: the duodenum, jejunum, and ileum.<|separator|>
[57]
Morphometric Analysis of Developmental Alterations in the Small ...
Oct 21, 2023 · This study concerned histological changes in the small intestine (duodenum and jejunum) and liver in the first weeks of life of geese.
[58]
Asymmetric mucosal structure, mesenteric versus antimesenteric, in ...
Dec 21, 2022 · Villus length as well as surface enlargement factor was significantly higher antimesenteric in both the mouse, rat, and human small intestines.
[59]
[PDF] Intestinal Adaptations of Male Rats to Endurance Exercise and Diet
The longer villi in the jejunum relative to the distal region is not surprising since the middle section of intestine, the jejunum, is largely responsible ...Missing: rodents | Show results with:rodents
[60]
Ontogenetic development of intestinal length and relationships to ...
Feb 25, 2013 · This study documents the phylogenetic development of intestinal length variability, and resultant correlation with dietary habits, within a ...Missing: jejunum | Show results with:jejunum
[61]
[PDF] Ecological Physiology of Diet and Digestive Systems
Jan 14, 2011 · The time course over which those prod- ucts are released and absorbed varies widely among food and substrate types. Accordingly, the foods at ...
[62]
Embryology, Midgut - StatPearls - NCBI Bookshelf - NIH
Following gastrulation, primitive gut tube development begins as a hollow chamber of endoderm is then surrounded by mesodermal cells. The endodermal cells ...Introduction · Development · Cellular · Mechanism
[63]
Embryology, Bowel - StatPearls - NCBI Bookshelf
This article will focus on the development of the midgut and hindgut and explore developmental abnormalities that may occur during this process.
[64]
Wnt Signaling Specifies and Patterns Intestinal Endoderm - PMC - NIH
Wnt signaling is active in posterior endoderm as early as E7.5. Genetic and chemical activation show that the Wnt pathway acts directly on endoderm.
[65]
The growth pattern of the human intestine and its mesentery - PMC
Aug 22, 2015 · The standard description of normal intestinal “rotation” encompasses a couple of sequential developmental steps. Initially, the midgut loop ...
[66]
Ontogeny, growth and development of the small intestine - NIH
Prenatal intestinal transporters are critical for the development of the fetus, as an estimated 10%-15% of fetal protein requirements in rhesus monkeys are met ...
[67]
Hedgehog Signaling in Intestinal Development and Homeostasis
The hedgehog (Hh) signaling pathway plays several diverse regulatory and patterning roles during organogenesis of the intestine and in the regulation of adult ...Missing: jejunum | Show results with:jejunum
[68]
Midgut Malrotation - StatPearls - NCBI Bookshelf
... developmental rotational anomaly of the embryonic bowel. Malrotation ... Mid-gut malrotation is associated with congenital anomalies, including congenital ...
[69]
Review of genetic factors in intestinal malrotation - PMC
Some chromosomal disorders have been associated with malrotation; in these cases, there are usually multiple other abnormalities: other malformations which may ...
[70]
Jejunum - Etymology, Origin & Meaning
Jejunum, from Late 14c. Modern Latin ieiunum, neuter of Latin ieiunus "empty," named for its typical emptiness during dissections, reflecting its origin and ...Missing: anatomy | Show results with:anatomy
[71]
jejunum - WordReference.com Dictionary of English
Etymology: 16th Century: from Latin, from jējūnus empty; from the belief that the jejunum is empty after death. 'jejunum' also found in these entries (note ...
[72]
History of the Stomach and Intestines
It is called the jejunum, hire, hilla, sterile, or empty. It is empty because it is near the liver, which empties the jejunum by drawing the chyle from it.
[73]
Greek anatomist herophilus: the father of anatomy - PMC - NIH
One of the most stirring controversies in the history of Anatomy is that Herophilus, an ancient Greek anatomist and his younger contemporary, Erasistratus ...Missing: Avicenna | Show results with:Avicenna
[74]
Avicenna's View on the Etiologies of Intestinal Obstruction - PMC - NIH
Upper intestines are thin with inner mucoid surface, named as “asna ashari” (duodenum), “saem” (jejunum), and “deghagh” (ileum). Lower intestines need ...
[75]
AVICENNA x. Medicine and Biology - Encyclopaedia Iranica
Avicenna wrote the medieval textbook of Galenic medicine the Qānūn (the Canon), as well as the central medieval statement of Aristotelian biology.
[76]
Herophilus | Research Starters - EBSCO
The sparse ancient evidence suggests that Herophilus (huh-RAHF-uh-luhs) left his native city of Chalcedon for an apprenticeship with the distinguished physician ...Missing: jejunum | Show results with:jejunum
[77]
SCIplanet - Ibn al-Nafis and the Exploration of the Human Body
Apr 14, 2016 · In this book, Ibn al-Nafis corrected the concept of the parts of the human body with diagrams to illustrate its function and working. He also ...Missing: mesenteric attachments
[78]
An untold story: The important contributions of Muslim scholars for ...
Nov 22, 2016 · Six intestines (S16) are described: duodenum (twelve or Al-ethna Ashar), jejunum ... The first contained the anatomy of organs (Tashrih Al ...
[79]
Newly Digitized 1543 Edition - Vesalius
One of them is a Basel copy of the 1543 edition of De humani corporis fabrica libri septem by Andreas Vesalius. The copy at the University of Basel is not ...Missing: jejunum | Show results with:jejunum
[80]
De humani corporis fabrica (Of the Structure of the Human Body)
When Andreas Vesalius (1514–1564) first published his radical De humani corporis fabrica (On the Structure of the Human Body), the ancient texts of ...Missing: jejunum | Show results with:jejunum
[81]
Claude Bernard, the Founder of Modern Medicine - PMC - NIH
May 20, 2022 · By showing that the liver secretes glucose into the blood, Claude Bernard showed that organs can also produce and secrete molecules into the ...
[82]
First Steps in Claude Bernard's Discovery of the Glycogenic ... - jstor
65) that Claude. Bernard "in testing the blood for its sugar content at various points after its departure from the intestine, where sugar is absorbed .
[83]
Electron microscope investigation of the striated border of intestinal ...
Electron microscope investigation of the striated border of intestinal epithelium. Anat Rec. 1950 Aug;107(4):423-41. doi: 10.1002/ar.1091070409.Missing: small | Show results with:small
[84]
(PDF) Herophilos, the great anatomist of antiquity - ResearchGate
Aug 4, 2025 · Herophilos (ca. 330 to ca. 260 BC) was a Greek physician and precursor in learning and teaching of human anatomy through systematic anatomical dissections.
[85]
Expression of Thirty-six Drug Transporter Genes in Human Intestine ...
This study was designed to quantitatively assess the mRNA expression of 36 important drug transporters in human jejunum, colon, liver, and kidney.Missing: genomics advances
[86]
Changes in the transcriptional profile of transporters in the intestine ...
The purpose of this work was to characterize the expression of drug and nutrient carriers along the anterior-posterior and crypt-villus axes of the intestinal ...