Small intestine

• 1. Introduction

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  The small intestine is roughly 6.5 metres long, yet it folds tightly enough to fit inside the abdomen. That single fact raises an immediate question: what is happening inside all those coils? Almost every nutrient your body will ever use passes through this one organ. The stomach breaks food down, the large intestine handles what remains, but the small intestine is where the actual transfer of energy and building material into the bloodstream occurs. Three distinct regions, each with a different specialty. A surface area that, counting every fold and projection, averages 30 square metres. A dual role as both a digestive organ and a front-line outpost of the immune system. And a developmental story that begins before birth with a loop of tissue so fast-growing it temporarily bursts through the navel. How does one organ manage all of this, and what happens when it goes wrong?

• 2. Three Rooms in One Tube

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  The duodenum is the shortest of the three sections, shaped like a "C" and curling around the head of the pancreas. Food arrives here as gastric chyme, the acidic slurry the stomach produces. The duodenum's first job is to neutralize that acid. Brunner's glands in the duodenum wall release a mucus-rich, alkaline secretion containing bicarbonate, and more bicarbonate arrives from the pancreas. The hormone cholecystokinin, produced in response to the presence of nutrients, triggers the release of bile from the gallbladder and digestive enzymes from the pancreas, all entering through the pancreatic duct.

  The jejunum picks up where the duodenum leaves off. About 2.5 metres long, it is the section most densely equipped for absorption. Circular folds run along its inner wall, and from those folds project villi, the microscopic finger-like projections whose Latin name means "shaggy hair." Each epithelial cell on the surface of a villus carries its own smaller projections called microvilli. The combined effect of folds, villi, and microvilli pushes the absorptive surface area to an average of 30 square metres, despite the organ fitting within the abdomen.

  The ileum is the final section, about 3 metres long, and it handles what the jejunum did not finish. Its specific responsibilities are vitamin B12 and bile salts. Vitamin B12 can only be taken up by the ileum after binding to a protein called intrinsic factor. At the ileum's far end, the ileocaecal junction connects to the cecum of the large intestine, marking the handoff point for whatever material was not absorbed.

• 3. A Chemical Assembly Line

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  Proteins begin breaking down in the stomach, but the serious chemical work happens in the small intestine. The pancreas secretes proteolytic enzymes, including trypsin and chymotrypsin, which cleave proteins into smaller peptides. Carboxypeptidase then splits off amino acids one at a time, while aminopeptidase and dipeptidase free the final end products for absorption.

  Fats present a different problem. Pancreatic lipase can break down triglycerides into free fatty acids and monoglycerides, but lipase is water-soluble and triglycerides are hydrophobic. Left alone, fat molecules clump together and present too small a surface for the enzyme to act on. Bile salts from the liver solve this by emulsifying the triglycerides into tiny droplets suspended in the watery intestinal environment, giving lipase enough surface area to work efficiently.

  Carbohydrates follow a more varied path. Pancreatic amylase breaks down starches into oligosaccharides, and brush border enzymes, including dextrinase, glucoamylase, maltase, sucrase, and lactase, break those down further into simple sugars. Lactase is absent in some adult humans, which means lactose and most polysaccharides pass through undigested. Cellulose is not digested at all, because humans lack the enzyme to split the beta-glucose bonds that hold it together; that task is left to bacteria in the large intestine.

• 4. How Nutrients Cross the Wall

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  Each villus contains a network of capillaries and fine lymphatic vessels called lacteals near its surface. Amino acids and carbohydrates pass from the intestinal lumen into the capillaries; lipids travel into the lacteals. From there, the bloodstream carries nutrients to the organs that need them.

  Different nutrients are absorbed in different parts of the small intestine, and the specificity is precise. Iron is absorbed in the duodenum. Folate, or vitamin B9, is taken up in the duodenum and jejunum. Most other nutrients are absorbed in the jejunum. Vitamin B12 and bile salts are reserved for the terminal ileum. Water moves by osmosis throughout the entire length of the organ, and lipids cross membranes by passive diffusion along the same route.

  The secretin hormone, produced in the small intestine itself, acts as a feedback signal back to the pancreas, prompting the release of bicarbonate into the duodenum to keep the pH stable. The pyloric sphincter, a muscle at the junction of the stomach and duodenum, controls how much gastric chyme enters at any one time, preventing the digestive system from being overwhelmed.

• 5. Immunity Behind the Lining

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  Peyer's patches, located within the ileum, are clusters of lymphoid tissue that act as sampling stations for the immune system. Potentially harmful bacteria and other microorganisms passing through the digestive tract are captured and presented to immune cells there, allowing the body to mount a response before an infection can take hold.

  Gut flora, the community of microorganisms living in the intestine, appear to contribute positively to the host's immune system. The small intestine is not passive in this relationship; its structure actively supports the microbial environment. Material that escapes digestion and absorption moves into the large intestine, where bacteria handle further breakdown, and the byproducts of that process feed back into the body's overall physiology.

  From a clinical standpoint, the small intestine is vulnerable to an extensive range of conditions. Some, such as irritable bowel syndrome, affect up to 10 percent of people at some point in their lives. Others, like certain tapeworm infections including Diphyllobothrium latum and Taenia solium, are far rarer. Crohn's disease, coeliac disease, and mesenteric ischemia each disrupt the organ in different ways, and cancers including adenocarcinoma, carcinoid, and gastrointestinal stromal tumors can arise from its tissues.

• 6. Born from the Navel

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  By the fifth week of embryological development, the ileum begins growing so rapidly that it cannot fit inside the abdomen. It forms a U-shaped fold called the primary intestinal loop and protrudes outward through the umbilicus. By week ten, the loop retracts back inside.

  Between weeks six and ten, the small intestine rotates anticlockwise as seen from the front of the embryo. After it returns to the abdomen, it rotates a further 180 degrees. This sequence of outward growth, rotation, and return is what produces the characteristic twisted arrangement of the large intestine. The entire geometry of the adult gut is a consequence of this early developmental choreography.

  Size at birth and in adults differs measurably. In newborns after 35 weeks of gestational age, the diameter is approximately 1.5 cm. In adults, the organ is considered abnormally dilated on a CT scan when its diameter exceeds 2.5 cm. The total length varies widely across individuals, ranging from as short as 3 metres to as long as 10.5 metres, with taller people generally having a longer small intestine.

• 7. Across the Animal Kingdom

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  In teleost fish, the small intestine is relatively short, typically about one and a half times the length of the fish's body, and it commonly carries pyloric caeca, small pouch-like structures that increase its digestive surface area. There is no ileocaecal valve in teleosts; the transition from small intestine to rectum is marked only by the end of the digestive epithelium.

  In sharks, sturgeons, and lungfish, there is no small intestine as such. Instead, a spiral intestine connects stomach to rectum. The intestinal tube is relatively straight, but a long internal fold runs along its length in a spiral, sometimes for dozens of turns. This dramatically increases both surface area and functional length. Lampreys have a very small spiral valve, consistent with their diet requiring little digestion. Hagfish have no spiral valve at all; digestion occurs along almost the entire length of an undivided intestine.

  Across tetrapods, the ileocaecal valve is always present. The small intestine is longer relative to body size in herbivores and in mammals and birds than in amphibians or reptiles, reflecting higher metabolic demands. Microscopic folds to increase surface area appear in all vertebrates, but true villi, the projections that define so much of the human organ's absorptive power, develop only in mammals.