Anatomy
The small intestine extends from the pylorus of the stomach to the ileocecal valve and is divided into three parts: the duodenum, jejunum, and ileum. Its total length ranges from 3 to 6 meters, with considerable individual variation. The duodenum starts below the pylorus and ends at the duodenojejunal flexure. It measures about 20–25 cm and is the widest and most fixed portion of the small intestine. The junction between the duodenum and jejunum is anchored by the ligament of Treitz. The jejunum and ileum are coiled within the abdominal cavity below the mesocolon and are attached to the posterior abdominal wall by the mesentery. There is no clear anatomical boundary between the jejunum and ileum, but generally, the proximal two-fifths of the mesenteric small intestine is considered the jejunum, while the distal three-fifths is classified as the ileum.
The jejunum has a larger lumen, thicker walls, and numerous prominent and dense circular folds (plicae circulares), which can be felt through the intestinal wall. Toward the distal ileum, these folds become progressively lower and sparser, disappearing entirely at the terminal ileum.
The small intestine is richly supplied with blood vessels, lymphatic vessels, and nerves, all of which run through the mesentery to serve or innervate the intestine. With the exception of the proximal duodenum, which is supplied by branches of the celiac trunk, the rest of the small intestine receives its blood supply from the superior mesenteric artery. This artery originates from the abdominal aorta and gives rise to branches such as the inferior pancreaticoduodenal artery, middle colic artery, right colic artery, ileocolic artery, and 12–16 jejunal and ileal arteries. These branches form arterial arcades through mutual anastomosis, with straight arteries (vasa recta) extending to the intestinal walls. The number of arcades increases distally, from 1–2 in the proximal jejunum to 3–4 near the distal ileum, where the straight arteries become shorter and the density of mesenteric vessels decreases. Venous drainage is via veins that parallel their arterial counterparts, ultimately converging into the superior mesenteric vein. This vein runs alongside the superior mesenteric artery and joins the splenic vein behind the pancreatic neck to form the portal vein.
Lymphatic drainage of the small intestine occurs through lacteals located at the center of the intestinal villi and along lymphatic vessels accompanying the venous system. Lymph flows sequentially through peri-intestinal lymph nodes, lymph nodes near the mesenteric vascular arcades, and lymph nodes surrounding the superior mesenteric artery and vein, eventually reaching the cisterna chyli. The mucosa of the jejunum contains scattered solitary lymphoid follicles, while the ileum has aggregated lymphoid nodules (Peyer’s patches). This lymphatic drainage pathway is essential for transporting absorbed dietary lipids into the circulatory system and plays an important role in immune defense.
The small intestine is innervated by parasympathetic and sympathetic fibers that form the celiac plexus and superior mesenteric plexus, sending nerve branches to the intestinal wall. Sympathetic activation reduces peristalsis, causes vasoconstriction, and decreases intestinal gland secretion. In contrast, vagal (parasympathetic) stimulation enhances peristalsis and secretion activity. Pain sensations in the small intestine are transmitted via visceral afferent fibers of the sympathetic nervous system, typically radiating to the periumbilical regions innervated by the 9th to 11th thoracic nerves, and only rarely to the lumbar and back regions.
Physiology
The small intestine is the primary site of digestion and absorption of nutrients. Its mucosal glands secrete alkaline intestinal fluid containing various enzymes, among which enterokinase is essential for breaking down polypeptides into amino acids. Food substances in the chyme are digested into glucose, amino acids, and fatty acids before being absorbed by the intestinal mucosa. Additionally, the small intestine absorbs water, electrolytes, and various vitamins. The human body secretes 6–8 liters of digestive fluid daily, with about 80% being reabsorbed in the small intestine. Consequently, small intestinal diseases, such as intestinal obstruction or fistulas, can lead to severe water and electrolyte imbalances and nutritional deficiencies.
The small intestine functions as a critical endocrine organ, releasing numerous gastrointestinal hormones such as secretin, somatostatin, gastrin, and glucagon. These hormones regulate the activity of digestive glands and the small intestinal epithelium by influencing nutrition, secretion, and motility through endocrine, paracrine, autocrine, and neurotransmitter pathways.
The small intestine also serves as an effective barrier, with components such as immunoglobulin A (IgA), mucin, defensins, the brush border of intestinal epithelial cells, and gut-associated lymphoid tissue (GALT) contributing to its defense mechanism. Under normal conditions, this barrier prevents the translocation of pathogens and toxins from the intestinal lumen into the systemic circulation. However, conditions that cause intestinal ischemia or inflammation may compromise the barrier, resulting in the translocation of pathogens and toxins into the lymphatic and circulatory systems.