The Small Intestine: Six Metres Folded Into a Handspan
Everything you have ever eaten has passed through this tube. Six to seven metres of it, coiled into a space no bigger than a shoebox, and yet its inner lining is not six metres of smooth pipe but something closer to the floor area of a small apartment — around two hundred square metres of absorbing surface, folded three separate times over, each fold smaller than the last. That is the whole design problem of the small intestine stated in one sentence: pack the largest possible surface into the smallest possible volume, keep it moving, keep it alive, and hang the entire arrangement from a stalk of tissue only fifteen centimetres wide. Every calorie you have ever burned, every vitamin in your blood, every gram of protein in your muscles crossed a membrane one cell thick somewhere along this tube. It is the busiest border in the body — and the most vulnerable.
The surgeon has the abdomen open and a problem to solve: somewhere in this glistening heap of bowel there is a hole, and she needs to know where she is. She lifts a loop and looks at it the way a farmer looks at soil. This one is thick-walled and wide, deep red, and when she stretches it against the light she sees the mucosal folds standing close together inside it like the pages of a book; the mesentery attached to it is almost translucent, with one great arch of artery giving off long straight vessels to the wall. She lets it fall and picks up a loop from lower down and to the right. This one is narrow and pale and floppy, its folds faint or gone, its mesentery yellow and heavy with fat that creeps up onto the bowel itself, and its blood supply a dense thicket of short little arches. She has no map and no labels, and yet she knows precisely how far along the six metres she is standing — because the small intestine tells you where you are, if you have learned to read it.
Six metres in a shoebox
The small intestine is one continuous tube in three named parts — and only the last two are free. It begins at the pylorus and ends at the ileocaecal valve, and along the way it is called duodenum, jejunum and ileum. The duodenum is a special case in almost every respect: it is short (about twenty-five centimetres), C-shaped, mostly RETROPERITONEAL, clamped around the head of the pancreas and dealt with separately in the duodenum and pancreas. Everything after the duodenojejunal flexure — the sharp turn held up by the suspensory ligament of Treitz — is different: intraperitoneal, mobile, slung on a mesentery, and free to coil. That mobile portion is the six to seven metres people mean when they say "small bowel": roughly the proximal two-fifths jejunum and the distal three-fifths ileum, with no line, no valve and no landmark between them. The transition is gradual and the two are told apart by character rather than by boundary. Measured lengths vary enormously — in the living, relaxed body the tube is shorter than the classic cadaveric figures because muscle tone holds it contracted — but the proportions hold, and so does the astonishing fact that all of it fits below the transverse colon and inside the frame of the abdominal wall described in the anterior abdominal wall.
Three tricks that make an apartment out of a tube
A smooth cylinder six metres long would offer about half a square metre. The gut gets two hundred. The amplification happens in three stages, each an order of magnitude smaller than the one before, and each multiplying the surface again. FIRST, the PLICAE CIRCULARES (valves of Kerckring, or valvulae conniventes): permanent circular shelves of mucosa and submucosa that project into the lumen and run transversely, often two-thirds or more of the way around. They are permanent — unlike the rugae of the stomach they do not flatten when the bowel is distended, which is exactly why the radiologist can identify small bowel on a plain film by the fine lines crossing the FULL width of a dilated loop. They multiply the surface roughly THREEFOLD. SECOND, the VILLI: finger-like projections about half a millimetre to one millimetre tall, covering every square millimetre of mucosa including the surfaces of the plicae, giving the lining the velvet texture that anyone who has handled fresh bowel remembers. They multiply the surface about TENFOLD. THIRD, the MICROVILLI: on the apical membrane of every single absorptive enterocyte stand thousands of tiny cytoplasmic projections, so densely packed that under the light microscope they blur into a fuzzy line — the BRUSH BORDER. They multiply the surface roughly TWENTYFOLD, and their membrane carries the digestive enzymes themselves: lactase, sucrase-isomaltase, maltase, the peptidases. Three by ten by twenty is six hundred, and half a square metre multiplied six hundred times is about TWO HUNDRED SQUARE METRES — an area comparable to a tennis court, wrapped around a space you could hold in two hands.
Think of a road map. Opened out it covers a table; folded along its creases it slips into a pocket, and not a single square centimetre of printed road has been lost. Now imagine that each of the folded panels is itself pleated, and that the paper fibres on every pleat are themselves raised into a nap like velvet. That is the small intestine: plicae circulares are the big creases, villi are the pleats, microvilli are the nap. Nothing has been added — the tube is still only a few centimetres wide — but the readable surface has been multiplied six hundred times. And like a map, the folding has a cost: the more intricate the surface, the more there is to damage. Flatten the nap and the pleats, as infection or coeliac disease can, and the tube is still six metres long while the map has become unreadable.
Jejunum or ileum? The surgeon's table of differences
No boundary exists, so you identify the bowel by its habits — and every habit follows from one fact: the jejunum absorbs, the ileum defends and salvages. WALL AND CALIBRE: the jejunum is thicker-walled and wider — a loop you can feel the substance of between finger and thumb; the ileum is thinner and narrower. COLOUR: the jejunum is a deeper, redder, more vascular colour; the ileum is paler. MUCOSA: the jejunum has tall, closely set plicae circulares — the reason its wall feels thick — while in the ileum they become sparse, low and finally absent in the terminal segment. VASCULAR PATTERN, the single most reliable sign at operation: the jejunum is supplied by FEW, LARGE arterial ARCADES — often only one or two tiers — from which LONG, straight vasa recta run to the bowel wall; the ileum is supplied by MANY tiers of short arcades, three, four or five deep, from which SHORT vasa recta arise. MESENTERIC FAT: the jejunal mesentery is relatively fat-free, so the arcades are easy to see through it as "windows"; the ileal mesentery is loaded with fat that creeps out along the vessels and onto the bowel wall itself. LYMPHOID TISSUE: the ileum carries PEYER'S PATCHES — visible aggregates of lymphoid follicles on the antimesenteric border, part of the gut-associated lymphoid tissue introduced in lymphatics and the body cavities — while the jejunum has only scattered solitary follicles. POSITION: the jejunum occupies mainly the UPPER LEFT of the infracolic compartment, the ileum the LOWER RIGHT and the pelvis. FUNCTION: most digestion and absorption of sugars, amino acids, fats, iron and calcium happen in the jejunum; the terminal ileum alone absorbs vitamin B12 (with intrinsic factor) and reclaims bile salts.
The wall, from lumen outwards
MUCOSA. Between the villi the epithelium dips down into tubular CRYPTS OF LIEBERKÜHN, and the crypt is the factory. At its base sit the stem cells that renew the entire lining every three to five days — the fastest-turning tissue in the body, and the reason the gut is the first casualty of any drug or radiation aimed at dividing cells. Beside them, PANETH CELLS release lysozyme, defensins and phospholipase, sterilising the crypt so the stem cells are never colonised. Migrating up the villus are the absorptive ENTEROCYTES with their brush border, the GOBLET CELLS secreting protective mucus (progressively more numerous towards the ileum), and the ENTEROENDOCRINE CELLS that release secretin, cholecystokinin, GIP, motilin and serotonin into the blood — the chemical control system that tells the stomach, pancreas and gallbladder that a meal has arrived. Beneath the epithelium lies the lamina propria, packed with lymphocytes and plasma cells pouring out IgA, and a thin muscularis mucosae that twitches the villi. SUBMUCOSA. Loose connective tissue carrying the arteries, veins, lymphatics and MEISSNER'S SUBMUCOSAL PLEXUS, which governs secretion and local blood flow. In the DUODENUM ONLY it also contains BRUNNER'S GLANDS, secreting alkaline mucus to neutralise the acid arriving from the stomach — nowhere else in the gut are they found, so their presence on a slide names the segment instantly. MUSCULARIS EXTERNA. An inner CIRCULAR and an outer LONGITUDINAL layer, and squeezed between the two, AUERBACH'S MYENTERIC PLEXUS, which sets the rhythm of movement. SEROSA. The visceral peritoneum, covering the whole circumference of the mobile bowel and continuing as the two leaves of the mesentery, as described in the peritoneum.
The entire lining of your small intestine is younger than the milk in your fridge. Stem cells at the base of each crypt of Lieberkühn divide continuously, and their daughters migrate up the villus, work as enterocytes for three to five days, and are shed from the tip into the lumen — where they are digested and their protein reabsorbed a little further down. You shed and rebuild something like two hundred grams of intestinal lining every day. This is why the gut is the tissue that suffers first whenever anything attacks dividing cells: the mouth ulcers and diarrhoea of chemotherapy, the mucositis of radiotherapy, the villous atrophy of untreated coeliac disease. It is also why recovery, once the insult stops, can be so complete: the factory in the crypts was never the target, only its products.
A fifteen-centimetre root carrying six metres
The mesentery is the single most consequential fold of peritoneum in the abdomen. The mesentery proper is a fan of two peritoneal leaves enclosing fat, arteries, veins, lymphatics, nodes and autonomic nerves. Its free border — the edge attached to the bowel — is six or seven metres long, gathered and ruffled like a curtain heading. Its attached border, the ROOT, is only about fifteen centimetres long, and it runs obliquely across the posterior abdominal wall from the DUODENOJEJUNAL FLEXURE at the left side of the body of L2, downwards and to the right, to the RIGHT SACROILIAC JOINT and the ileocaecal junction. On its way the root crosses, from above downwards, the third part of the duodenum, the abdominal aorta, the inferior vena cava, the right ureter and the right psoas major — which is why an inflamed loop of ileum can irritate the ureter and mimic renal colic, and why the psoas sign has any meaning at all. Between the leaves, the average distance from root to bowel is fifteen to twenty centimetres, so the fan is deep as well as long. That geometry is the anatomical basis of VOLVULUS: six metres of heavy, mobile bowel hanging from a narrow stalk can rotate about that stalk, kinking the vessels inside it before it even obstructs the lumen — which is why a volvulus threatens the blood supply first and the bowel contents second. The same geometry explains why the mesentery must be handled with such respect in surgery, and why a mesenteric tear in blunt trauma can devascularise a metre of gut that looks, at first glance, entirely intact. The wider family of peritoneal folds is set out in mesenteries, omenta and peritoneal ligaments.
Blood, chyle and pain
ARTERIES. The jejunum and ileum are pure MIDGUT, and the midgut has exactly one artery: the SUPERIOR MESENTERIC ARTERY, arising from the front of the aorta at L1, just below the coeliac trunk, and running down between the leaves of the mesentery. From its LEFT side it gives off fifteen to eighteen JEJUNAL AND ILEAL BRANCHES, and these are the vessels that join one another end to end to form the arcades — few and large proximally, many and small distally — from which the vasa recta pass to the bowel. From its RIGHT side come the middle colic, right colic and ILEOCOLIC arteries; the ileocolic supplies the terminal ileum, caecum and appendix and marks the end of the small bowel's territory. VEINS. Blood returns through the SUPERIOR MESENTERIC VEIN, which lies to the RIGHT of the artery throughout, and which joins the splenic vein behind the neck of the pancreas to form the PORTAL VEIN — so every absorbed nutrient goes first to the liver and not to the general circulation, the arrangement traced in the portal system and the IVC. LYMPHATICS. The villi contain a special central lymphatic, the LACTEAL, and it exists for one reason: long-chain fats, absorbed as chylomicrons, are too large for the blood capillaries. They enter the lacteals instead, turning the lymph a milky white called CHYLE — visible to the naked eye in the mesentery after a fatty meal. The chyle passes to mesenteric nodes, then to superior mesenteric and coeliac nodes, then to the intestinal trunk and the CISTERNA CHYLI in front of L1–L2, and up the thoracic duct into the venous system at the root of the neck, as in the thoracic duct. Fat, uniquely, bypasses the liver on its first pass.
Nerves explain a symptom that puzzles every student: why does small-bowel pain sit around the navel, wherever the disease actually is? PARASYMPATHETIC fibres reach the small intestine through the VAGUS, synapsing in the myenteric and submucosal plexuses in the wall; they increase motility, increase secretion and relax sphincters. SYMPATHETIC fibres arise from spinal segments T9 and T10 (the lesser splanchnic nerve principally), synapse in the superior mesenteric ganglion, and reach the bowel along the artery in the SUPERIOR MESENTERIC PLEXUS; they reduce motility and secretion and constrict vessels. Crucially, VISCERAL PAIN fibres travel back with the sympathetics, so pain from the jejunum and ileum enters the cord at T9–T10 — and the brain, which cannot localise visceral input, refers it to the body wall of those segments: the PERIUMBILICAL region. That is why the colic of small bowel obstruction, the early pain of appendicitis before the peritoneum is involved, and the ache of mesenteric ischaemia all begin around the navel, and why the pain moves and sharpens only when the inflamed organ touches the somatically innervated PARIETAL peritoneum. It is the midgut's signature, and it belongs to the wider embryological logic set out in foregut, midgut and hindgut and in the autonomic nerves of the abdomen.
How it moves: mixing, pushing, and sweeping
Two patterns of movement serve two different purposes. SEGMENTATION is the dominant activity after a meal: rings of circular muscle contract at intervals along a loop, chop the chyme into segments, then relax while new rings form between the old ones — so the contents are divided, mixed with enzymes and bile, and pressed repeatedly against the absorbing surface without travelling far. PERISTALSIS is propulsive: a ring of circular contraction behind the bolus with relaxation ahead of it, a wave that moves the contents onwards at a few centimetres a minute, so that a meal takes roughly three to five hours to traverse the small bowel. Between meals a third pattern appears — the MIGRATING MOTOR COMPLEX, a strong sweeping wave beginning in the stomach or duodenum and travelling the full length of the small intestine roughly every ninety to a hundred and twenty minutes, driven by motilin. It is the housekeeper: it clears residual debris, desquamated cells and bacteria towards the colon, and when it fails — in some cases of scleroderma, diabetes or intestinal pseudo-obstruction — bacteria colonise the small bowel and cause bloating, diarrhoea and malabsorption. Both patterns are generated locally by the interstitial cells of Cajal and coordinated by the enteric nervous system: the gut can continue to segment and to peristalse in an isolated loop with every nerve to the body cut, which is why it is sometimes called the second brain, and why bowel wakes up after surgery whether you tell it to or not.
Meckel's diverticulum and the rule of 2s
In the embryo the midgut loop communicates with the yolk sac through the VITELLOINTESTINAL (omphalomesenteric) DUCT. It normally disappears. When its intestinal end persists, the result is MECKEL'S DIVERTICULUM — the commonest congenital anomaly of the gastrointestinal tract, and a TRUE diverticulum, meaning its wall contains all the layers of the bowel, including muscle. It arises from the ANTIMESENTERIC border of the ileum, which distinguishes it at a glance from an acquired diverticulum. The classic teaching is the RULE OF 2s: present in about 2 per cent of people, situated about 2 feet (60 cm) from the ileocaecal valve, about 2 inches (5 cm) long, commonly presenting before the age of 2, and containing up to 2 types of ectopic tissue — GASTRIC mucosa and PANCREATIC tissue. That ectopic tissue is the whole clinical story: gastric mucosa secretes acid into a segment of ileum with no defence against it, and the result is ulceration of the adjacent normal mucosa with painless, brisk rectal bleeding in a young child. It can also inflame and mimic appendicitis exactly, act as the lead point of an intussusception, or become the fixed point around which a volvulus turns. A technetium-99m pertechnetate scan — the "Meckel's scan" — works because the tracer is taken up by gastric mucosa wherever it happens to be sitting.
The colic and the silence: a man who had an appendicectomy twenty years ago arrives with waves of central pain every few minutes, a distending abdomen, vomiting, and — the question that decides everything — no flatus passed for a day. Adhesions from that old operation have kinked a loop. Early on the bowel fights the obstruction and the abdomen is noisy with high-pitched sounds; when the fight is lost the abdomen goes quiet, and a silent distended abdomen is far more frightening than a loud one. The level tells you the story: a HIGH obstruction vomits early, copiously and with little distension, and the vomit is bile-stained; a LOW obstruction distends greatly first, vomits late, and the vomit becomes feculent — brown and foul from bacterial overgrowth in stagnant contents. The flattened map: a woman with years of bloating, iron-deficiency anaemia and a strange rash is found to have coeliac disease; her jejunal biopsy shows villous atrophy with crypt hyperplasia — the villi worn away, the surface reduced from a tennis court towards a smooth tube. The terminal ileum's revenge: a young man with Crohn's disease, whose favourite territory is exactly this segment, becomes anaemic from B12 deficiency and passes fatty stools because the bile salts that should have been reclaimed there are lost into the colon — a pattern managed with the drugs in inflammatory bowel disease. The pain that is worse than the abdomen looks: an elderly woman in atrial fibrillation describes agonising central pain, yet her abdomen is soft and almost tender-free. Pain out of proportion to the signs, in that clinical setting, means embolic occlusion of the superior mesenteric artery until proven otherwise, and the window in which the bowel can still be saved is measured in hours.
- The small intestine runs from pylorus to ileocaecal valve: duodenum (25 cm, retroperitoneal), then six to seven metres of mobile jejunum and ileum slung on a mesentery, beginning at the duodenojejunal flexure held by the ligament of Treitz.
- Surface area is amplified three times over: plicae circulares (permanent, ×3), villi (×10) and microvilli forming the brush border (×20) — a final absorbing surface of about 200 m².
- JEJUNUM: thicker, wider, redder; tall closely-set plicae; FEW large arcades with LONG vasa recta; little mesenteric fat; upper left. ILEUM: thinner, narrower, paler; sparse or absent plicae; MANY short arcades with SHORT vasa recta; abundant fat creeping onto the wall; Peyer's patches; lower right.
- The wall has four layers: mucosa (crypts of Lieberkühn with stem cells, Paneth, goblet and enteroendocrine cells), submucosa (Meissner's plexus; Brunner's glands in the DUODENUM ONLY), muscularis externa (circular + longitudinal with Auerbach's myenteric plexus between them) and serosa.
- The mesentery has a free border six to seven metres long but a ROOT only 15 cm, running from the duodenojejunal flexure (left of L2) to the right sacroiliac joint — crossing the third part of the duodenum, aorta, IVC, right ureter and psoas. That narrow stalk is the anatomical basis of volvulus.
- Arterial supply is entirely from the SUPERIOR MESENTERIC ARTERY (L1, midgut): 15–18 jejunal and ileal branches from its left side forming the arcades, plus middle colic, right colic and ileocolic from its right side.
- Venous blood drains to the superior mesenteric vein (lying to the RIGHT of the artery) and thence to the portal vein — so absorbed nutrients meet the liver first.
- Lymph: each villus has a central LACTEAL taking absorbed long-chain fat as milky CHYLE → mesenteric nodes → superior mesenteric and coeliac nodes → cisterna chyli → thoracic duct. Fat alone bypasses the portal circulation.
- Nerves: vagal parasympathetic increases motility and secretion; sympathetic T9–T10 via the superior mesenteric plexus carries VISCERAL PAIN — hence PERIUMBILICAL colic, which localises only when parietal peritoneum is involved.
- Motility: segmentation mixes, peristalsis propels (a meal crosses in 3–5 hours), and the migrating motor complex sweeps the empty gut every 90–120 minutes under motilin — its failure allows bacterial overgrowth.
- Meckel's diverticulum: a TRUE diverticulum on the ANTIMESENTERIC border, remnant of the vitellointestinal duct — rule of 2s (2%, 2 feet from the ileocaecal valve, 2 inches long, before age 2, 2 ectopic tissues: gastric and pancreatic).
- Believing there is a landmark between jejunum and ileum. There is none — no valve, no line, no vessel. The transition is gradual and identification rests on wall thickness, colour, plicae, arcade pattern, mesenteric fat and position.
- Reversing the arcade rule. It is the JEJUNUM that has FEW arcades with LONG vasa recta, and the ILEUM that has MANY arcades with SHORT vasa recta — the mnemonic is that the jejunum, being higher up and better perfused, needs fewer, bigger arches.
- Placing Brunner's glands throughout the small intestine. They exist in the submucosa of the DUODENUM ONLY; the crypts of Lieberkühn, by contrast, are found along the whole length. Peyer's patches likewise are a feature of the ILEUM, not of the whole tube.
At laparotomy a surgeon inspects a loop of bowel. Its wall is thin and pale, its mesentery is loaded with fat that extends onto the bowel wall, and the mesenteric vessels form four or five tiers of short arcades giving off short vasa recta. Which segment is she holding, and what does its blood supply pattern tell her?
- The small intestine runs pylorus → ileocaecal valve. Beyond the retroperitoneal duodenum lie six to seven metres of mobile jejunum (proximal two-fifths) and ileum (distal three-fifths), beginning at the duodenojejunal flexure and slung on a mesentery.
- Absorptive surface is amplified ×3 by plicae circulares, ×10 by villi and ×20 by microvilli (the brush border) to about 200 m². The lining is rebuilt every 3–5 days from stem cells in the crypts of Lieberkühn, which is why it is the first tissue to fail in coeliac disease, chemotherapy and radiotherapy.
- Jejunum vs ileum: thicker/wider/redder with tall plicae, FEW large arcades and LONG vasa recta, little fat, upper left — versus thinner/paler with sparse plicae, MANY short arcades and SHORT vasa recta, abundant fat, Peyer's patches, lower right. Only the terminal ileum absorbs B12 and reclaims bile salts.
- Everything is served by the superior mesenteric artery (midgut) and drained by the superior mesenteric vein to the portal vein, with lacteals carrying fat as chyle to the cisterna chyli. Sympathetic T9–T10 pain fibres make small-bowel disease hurt around the umbilicus — the sign that opens the differential of obstruction, Crohn's, Meckel's diverticulum and mesenteric ischaemia.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Abdomen: the jejunum and ileum, the mesentery and the superior mesenteric artery.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Small intestine: distinguishing features of jejunum and ileum; Meckel's diverticulum.
- Netter FH. Atlas of Human Anatomy — Mesenteric vasculature; arterial arcades of the jejunum and ileum.
- Last RJ. Last's Anatomy: Regional and Applied — The small intestine and the root of the mesentery.
- Snell RS. Clinical Anatomy by Regions — Small bowel obstruction, volvulus and intussusception.
- TeachMeAnatomy — The Small Intestine; The Superior Mesenteric Artery.
