Medical School · Year 1 · Anatomy Thorax Abdomen · includes a quiz and discussion video

Lecture 10: Small and Large Intestine

Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the gross anatomy of the duodenum, jejunum, and ileum
2. Identify distinguishing features between jejunum and ileum
3. Describe the gross anatomy of the large intestine including its parts and features
4. Explain the blood supply and venous drainage of the intestines
5. Describe the lymphatic drainage and nerve supply
6. Correlate anatomical features with common intestinal pathologies

Small Intestine Overview

The small intestine is the primary site of digestion and nutrient absorption in the gastrointestinal tract. It extends from the pylorus of the stomach to the ileocecal junction, measuring approximately 6-7 meters in length. The small intestine is divided into three parts: the duodenum, jejunum, and ileum.

Beyond its digestive function, the small intestine plays an important role in immune surveillance. Gut-associated lymphoid tissue, including the aggregated lymphoid nodules known as Peyer's patches, samples antigens from the intestinal lumen and initiates appropriate immune responses.

<image>Panel A: Overview of the abdominal cavity with the C-shaped duodenum in tan wrapping around the pancreatic head. Panel B: The jejunum as pink-red coiled loops in the upper left abdomen transitioning from the duodenum. Panel C: The ileum as paler pink coiled loops in the lower right abdomen terminating at the ileocecal junction with the cecum. Panel D: The mesentery as a fan-shaped pink membrane with vessels suspending the jejunum and ileum from the posterior abdominal wall.</image>

Duodenum

The duodenum is the first and shortest part of the small intestine, measuring approximately 25 centimeters in length—a distance that corresponds to the breadth of approximately 12 fingers, hence its name. It has a characteristic C-shape that wraps around the head of the pancreas and is mostly retroperitoneal in position.

The first part of the duodenum, also called the superior portion or D1, is approximately 5 centimeters long. It is the only intraperitoneal portion and is therefore mobile. On radiographic imaging with contrast, it appears as the "duodenal cap" or bulb. Important structures lie posterior to D1, including the common bile duct, portal vein, and gastroduodenal artery. The anterior and superior surfaces of D1 are in contact with the gallbladder and liver. This first portion is the most common site for duodenal ulcers, particularly on the anterior wall where perforation can lead to peritonitis, or on the posterior wall where erosion into the gastroduodenal artery can cause massive hemorrhage.

The second part, the descending portion or D2, is approximately 7-10 centimeters long and is retroperitoneal. This segment receives the common bile duct and main pancreatic duct at the major duodenal papilla (ampulla of Vater) on its posteromedial wall. The accessory pancreatic duct opens at the minor duodenal papilla approximately 2 centimeters above the major papilla. The right kidney lies posterior to D2.

The third part, the horizontal or inferior portion (D3), is approximately 10 centimeters long and crosses horizontally at the level of the L3 vertebra. It is retroperitoneal. The superior mesenteric artery and vein cross anterior to D3, which can cause compression in superior mesenteric artery syndrome when the angle between the aorta and SMA becomes narrowed. Posterior to D3 lie the aorta, inferior vena cava, and right ureter.

The fourth part, the ascending portion (D4), is approximately 2.5 centimeters long and ascends to the left of the L2 vertebra. It ends at the duodenojejunal flexure, where the suspensory muscle of the duodenum (ligament of Treitz) attaches the duodenum to the right crus of the diaphragm. This ligament, composed of both smooth muscle and fibrous tissue, serves as an important clinical landmark separating upper from lower gastrointestinal bleeding.

<image>Panel A: D1 (superior portion, 5 cm, mobile) connecting to the pylorus with the portal vein, common bile duct, and gastroduodenal artery shown posteriorly through a transparent wall. Panel B: D2 (descending portion, 7-10 cm, retroperitoneal) with the major and minor duodenal papillae visible on the opened medial wall. Panel C: D3 (horizontal portion, 10 cm) crossing L3 with the SMA and SMV crossing anteriorly and the pancreatic head filling the C-curve. Panel D: D4 (ascending portion, 2.5 cm) terminating at the duodenojejunal flexure with the ligament of Treitz as a white fibrous band attached to the diaphragm.</image>

Jejunum and Ileum

The jejunum and ileum together comprise the mobile, mesentery-suspended portion of the small intestine. The jejunum occupies primarily the upper left portion of the abdomen, while the ileum lies predominantly in the lower right. Both are completely intraperitoneal, suspended from the posterior abdominal wall by the mesentery.

The mesentery of the small intestine is a broad, fan-shaped fold of peritoneum that contains the superior mesenteric vessels, lymphatics, nerves, and fat. Its root is approximately 15 centimeters long, extending obliquely from the left side of the L2 vertebra to the right iliac fossa.

Several anatomical features distinguish the jejunum from the ileum, and these differences are useful during surgery. The jejunum has a thicker wall and wider lumen than the ileum. It appears more vascular and therefore redder because of its richer blood supply. The jejunal mesentery contains less fat than the ileal mesentery, making the jejunal vessels more visible. The arterial arcades in the jejunal mesentery are simpler, typically with only 1-2 tiers of arches, while the ileal arcades have 4-5 tiers. The vasa recta (straight arteries that pass from the arcades to the intestinal wall) are longer in the jejunum and shorter in the ileum. The circular folds (plicae circulares) are prominent and numerous in the jejunum but become sparse and eventually absent in the ileum. Peyer's patches, the aggregated lymphoid nodules important for immune function, are few in the jejunum but abundant in the ileum, particularly in the terminal portion.

<image>Panel A: Jejunum with a thicker, redder wall, wider lumen, and prominent circular folds as wavy internal projections (plicae circulares). Panel B: Jejunal mesentery showing 1-2 tiers of arterial arcades with long vasa recta and minimal mesenteric fat allowing vessel visibility. Panel C: Ileum with a thinner, paler wall, narrower lumen, and sparse or absent circular folds. Panel D: Ileal mesentery showing 4-5 tiers of arterial arcades with short vasa recta, abundant mesenteric fat obscuring vessels, and multiple Peyer's patches as oval lymphoid aggregates on the antimesenteric border.</image>

Blood Supply of the Small Intestine

The duodenum receives blood from two arterial systems, reflecting its embryological origin at the junction of the foregut and midgut.

The superior pancreaticoduodenal artery arises from the gastroduodenal artery, which is a branch of the common hepatic artery from the celiac trunk. It supplies D1 and the proximal portion of D2. The inferior pancreaticoduodenal artery arises from the superior mesenteric artery and supplies D2 through D4. These vessels form anterior and posterior pancreaticoduodenal arcades that anastomose around the head of the pancreas.

The jejunum and ileum receive their entire blood supply from the superior mesenteric artery. This vessel gives off 15-18 jejunal and ileal branches that enter the mesentery and form progressively smaller arterial arcades. From the terminal arcades, vasa recta extend to the intestinal wall. These arteries are functionally end arteries at the level of the intestinal wall, meaning that obstruction of a vas rectum causes ischemia of the corresponding intestinal segment.

Venous drainage from the duodenum flows through the pancreaticoduodenal veins to the superior mesenteric vein or, in some cases, directly to the portal vein. The jejunum and ileum drain through jejunal and ileal veins that parallel the arterial supply, ultimately emptying into the superior mesenteric vein. The superior mesenteric vein joins the splenic vein behind the neck of the pancreas to form the portal vein.

<image>Panel A: The duodenum receiving the superior pancreaticoduodenal artery from the gastroduodenal artery in bright red and the inferior pancreaticoduodenal artery from the SMA in darker red. Panel B: Anastomotic arcades formed by the pancreaticoduodenal arteries around the pancreatic head. Panel C: The SMA as a large red vessel giving off multiple jejunal and ileal branches forming arcades in the mesentery with vasa recta supplying the intestinal wall. Panel D: The SMV in blue paralleling the SMA with an inset showing the portal vein forming from the SMV and splenic vein.</image>

Large Intestine Overview

The large intestine extends from the ileocecal junction to the anus, measuring approximately 1.5 meters in length. Its primary functions are absorption of water and electrolytes and formation of feces.

The large intestine consists of several named segments: the cecum with its vermiform appendix, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, the rectum, and the anal canal.

Several external features distinguish the large intestine from the small intestine. The taeniae coli are three longitudinal bands of smooth muscle that run along the length of the colon from the cecum to the rectosigmoid junction. These bands represent the outer longitudinal muscle layer concentrated into strips rather than forming a complete covering. The taeniae are shorter than the colon itself, causing the intestinal wall to pucker into sacculations called haustra. Epiploic appendages (appendices epiploicae) are small fat-filled pouches of peritoneum attached along the taeniae, particularly prominent in the sigmoid colon. The large intestine also has a greater diameter than the small intestine and lacks the circular folds that characterize the jejunum.

<image>Panel A: Anterior view of the colon framing the peritoneal cavity with three taeniae coli as white longitudinal bands labeled taenia libera, taenia mesocolica, and taenia omentalis. Panel B: Haustral sacculations as pouched segments between the taeniae giving the colon its characteristic appearance. Panel C: Epiploic appendages as small yellow fatty tags attached along the taeniae. Panel D: Inset cross-section showing the taeniae as concentrated bands of the outer longitudinal muscle layer with haustra as outpouchings between them.</image>

Cecum and Appendix

The cecum is a blind pouch located below the ileocecal junction in the right iliac fossa. It is typically intraperitoneal, though it may be partially retroperitoneal in some individuals. The ileocecal valve, formed by two horizontal folds of mucosa at the ileal opening, controls the flow of ileal contents into the cecum and helps prevent reflux of cecal contents back into the ileum.

The vermiform appendix is a narrow, worm-like tube attached to the posteromedial aspect of the cecum, approximately 2-3 centimeters below the ileocecal junction. Its base can be located by following the taeniae coli, which converge at the appendiceal attachment. The appendix is variable in length, typically measuring 6-10 centimeters. It is intraperitoneal and suspended by the mesoappendix, a small triangular fold of mesentery.

The position of the appendix is highly variable. The retrocecal position, with the appendix lying behind the cecum, is most common (approximately 65%). The pelvic position, with the appendix hanging over the pelvic brim, occurs in approximately 30% of individuals. Less common positions include subcecal (2%), pre-ileal (1%), and post-ileal (0.5%).

The appendicular artery, a branch of the ileocolic artery (from the SMA), supplies the appendix. This artery is an end artery with no significant anastomoses, making the appendix vulnerable to ischemia if the artery is thrombosed. This contributes to the rapidity with which acute appendicitis can progress to gangrene and perforation.

McBurney's point, located one-third of the distance from the anterior superior iliac spine to the umbilicus, marks the surface projection of the appendix base. This is the site of maximal tenderness in classic acute appendicitis and the location of the McBurney incision for open appendectomy.

<image>Panel A: The cecum as a blind tan pouch in the right iliac fossa with the terminal ileum entering through the ileocecal valve shown in an inset cross-section as two horizontal mucosal folds. Panel B: The taeniae coli converging at the appendix base with the appendix as a thin pink tube (6-10 cm) and the mesoappendix containing the appendicular artery from the ileocolic artery. Panel C: Inset showing variable appendix positions including retrocecal (65%), pelvic (30%), and other positions. Panel D: McBurney's point marked on the anterior abdominal wall at one-third the distance from the ASIS to the umbilicus.</image>

Colon

The ascending colon extends from the cecum to the right colic (hepatic) flexure, which lies just below the right lobe of the liver. This segment is secondarily retroperitoneal, with only its anterior surface covered by peritoneum. The right paracolic gutter lies lateral to the ascending colon.

The right colic (hepatic) flexure marks the sharp, approximately 90-degree angle where the ascending colon turns to become the transverse colon.

The transverse colon extends from the hepatic flexure to the left colic (splenic) flexure. It is intraperitoneal, suspended from the posterior abdominal wall by the transverse mesocolon, and is the most mobile portion of the colon. The transverse mesocolon attaches along the anterior surface of the pancreas.

The left colic (splenic) flexure is positioned higher and more posterior than the hepatic flexure. It is anchored to the diaphragm by the phrenicocolic ligament, which creates a partial shelf that supports the spleen and limits the upward spread of infection along the left paracolic gutter. The splenic flexure represents a watershed area between the superior mesenteric artery and inferior mesenteric artery territories, making it vulnerable to ischemia in low-flow states.

The descending colon extends from the splenic flexure to the sigmoid colon. Like the ascending colon, it is secondarily retroperitoneal with only its anterior surface covered by peritoneum. It is narrower than the ascending colon. The left paracolic gutter lies lateral to it.

The sigmoid colon is an S-shaped segment extending from the descending colon to the rectum at approximately the S3 vertebral level. It is intraperitoneal, suspended by the sigmoid mesocolon, which has an inverted V-shaped attachment to the posterior pelvic wall. The sigmoid colon is highly mobile.

<image>Panel A: The ascending colon (retroperitoneal, right side) extending from the cecum to the hepatic flexure as a sharp angle under the liver. Panel B: The transverse colon (intraperitoneal) suspended by the transverse mesocolon from the pancreas crossing to the splenic flexure attached to the diaphragm by the phrenicocolic ligament. Panel C: The descending colon (retroperitoneal, left side, narrower) continuing to the sigmoid colon (S-shaped, intraperitoneal, with sigmoid mesocolon). Panel D: The paracolic gutters as shaded areas lateral to the ascending and descending colon with the watershed area at the splenic flexure marked.</image>

Blood Supply of the Large Intestine

The blood supply to the large intestine derives from both the superior mesenteric artery (for midgut-derived structures) and the inferior mesenteric artery (for hindgut-derived structures).

The superior mesenteric artery supplies the cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon through three branches. The ileocolic artery supplies the terminal ileum, cecum, and appendix; it is the only constant branch of the SMA to the colon. The right colic artery supplies the ascending colon; this vessel is variable and may arise from the ileocolic or middle colic rather than directly from the SMA. The middle colic artery supplies the transverse colon.

The inferior mesenteric artery supplies the distal one-third of the transverse colon, descending colon, sigmoid colon, and upper rectum. The left colic artery supplies the descending colon and splenic flexure. Multiple sigmoid arteries supply the sigmoid colon. The superior rectal artery, the terminal branch of the IMA, supplies the upper rectum.

The marginal artery of Drummond is a continuous anastomotic arcade that runs along the mesenteric border of the colon, connecting the SMA and IMA territories. This vessel provides collateral circulation and can maintain viability of the colon when a major vessel is occluded, though this collateral supply may be inadequate in acute occlusions.

The arc of Riolan (meandering mesenteric artery) is an inconstant, more central anastomosis between the middle colic and left colic arteries. When present, it provides an important collateral pathway.

Watershed areas, located at the splenic flexure (junction of SMA and IMA territories) and the rectosigmoid junction, are vulnerable to ischemia in states of hypoperfusion because they lie at the margins of arterial territories.

<image>Panel A: The SMA as a large red vessel giving off the ileocolic artery to the terminal ileum, cecum, and appendix, the right colic artery to the ascending colon, and the middle colic artery to the transverse colon. Panel B: The IMA as a smaller red vessel from the aorta at L3 giving off the left colic artery to the splenic flexure and descending colon, sigmoid arteries to the sigmoid, and superior rectal artery to the upper rectum. Panel C: The marginal artery of Drummond as a continuous red arcade along the mesenteric border connecting all colonic arterial territories. Panel D: The watershed areas at the splenic flexure and rectosigmoid junction highlighted as vulnerable zones between arterial territories.</image>

Venous Drainage

Venous drainage of the large intestine parallels the arterial supply and ultimately reaches the portal venous system.

The superior mesenteric vein drains the cecum, appendix, ascending colon, and transverse colon. It receives the ileocolic, right colic, and middle colic veins. The SMV joins the splenic vein behind the neck of the pancreas to form the portal vein.

The inferior mesenteric vein drains the descending colon, sigmoid colon, and upper rectum through the left colic, sigmoid, and superior rectal veins. The IMV usually joins the splenic vein near its junction with the SMV, though it may join the SMV directly or at the SMV-splenic confluence.

An important portosystemic anastomosis exists in the rectal region. The superior rectal vein (draining to the portal system via the IMV) communicates with the middle and inferior rectal veins (draining to the systemic circulation via the internal iliac and internal pudendal veins). In portal hypertension, these anastomoses can dilate, though clinically significant hemorrhoids from portal hypertension are less common than esophageal varices.

<image>Panel A: The SMV in blue receiving the ileocolic, right colic, and middle colic veins from the right colon and transverse colon. Panel B: The IMV in blue receiving the left colic, sigmoid, and superior rectal veins from the left colon. Panel C: The IMV joining the splenic vein which then joins the SMV to form the portal vein behind the pancreatic neck. Panel D: Inset showing the rectal portosystemic anastomosis between the superior rectal vein draining to the portal system and the middle and inferior rectal veins draining to the systemic circulation.</image>

Lymphatic Drainage

Lymphatic drainage of the large intestine follows the arterial supply through multiple node groups.

Epicolic nodes lie on the colonic wall itself. Paracolic nodes lie along the marginal artery. Intermediate nodes are located along the named colic arteries (ileocolic, right colic, middle colic, left colic, sigmoid). Principal nodes are situated at the origins of the SMA and IMA. All lymph eventually drains to the cisterna chyli and thoracic duct.

This pattern of lymphatic drainage is clinically important because colorectal cancer spreads along lymphatic channels. Surgical resection of colorectal cancer includes removal of the regional lymph nodes along with the primary tumor, with the extent of resection determined by the arterial supply to the involved segment.

Nerve Supply

The innervation of the intestines follows the division between midgut and hindgut.

Parasympathetic innervation to the midgut (small intestine and colon to the splenic flexure) comes from the vagus nerve, which reaches the intestines through the celiac and superior mesenteric plexuses. Parasympathetic innervation to the hindgut (splenic flexure to rectum) comes from the pelvic splanchnic nerves (S2-S4), which ascend retroperitoneally to reach the distal colon.

Sympathetic innervation reaches the intestines through the splanchnic nerves. The lesser and least splanchnic nerves (from T10-T12) synapse in the superior mesenteric ganglion and supply the midgut. The lumbar splanchnic nerves (from L1-L2) synapse in the inferior mesenteric ganglion and supply the hindgut.

The enteric nervous system, consisting of the myenteric (Auerbach's) plexus between the muscle layers and the submucosal (Meissner's) plexus, controls local reflexes, peristalsis, and secretion. This intrinsic nervous system can function independently of CNS input, earning it the designation of "the brain of the gut."

<image>Panel A: Parasympathetic fibers in yellow from the vagus nerve reaching the midgut to the splenic flexure via the celiac and superior mesenteric plexuses. Panel B: Pelvic splanchnic nerves from S2-S4 supplying the hindgut from the splenic flexure to the rectum ascending retroperitoneally. Panel C: Sympathetic fibers in green from splanchnic nerves synapsing in the superior and inferior mesenteric ganglia before reaching the intestines. Panel D: Inset of the enteric nervous system in the intestinal wall showing the myenteric plexus between the circular and longitudinal muscle layers and the submucosal plexus in the submucosa.</image>

Clinical Correlations

Appendicitis is the most common abdominal surgical emergency. The classic presentation involves initial vague, poorly localized periumbilical pain (visceral pain from the midgut, referred to the T10 dermatome) that later migrates to the right lower quadrant as inflammation involves the parietal peritoneum overlying the appendix. Physical examination signs include tenderness at McBurney's point, Rovsing's sign (pressure in the left lower quadrant causing right lower quadrant pain), psoas sign (pain with hip extension, suggesting retrocecal appendix), and obturator sign (pain with internal rotation of the flexed hip, suggesting pelvic appendix).

Meckel's diverticulum is a remnant of the vitelline (omphalomesenteric) duct, present in approximately 2% of the population. It follows the "rule of 2's": found in 2% of people, located within 2 feet (60 cm) of the ileocecal valve, approximately 2 inches (5 cm) long, and may contain 2 types of ectopic tissue (gastric and pancreatic). When gastric mucosa is present, acid secretion can cause ulceration and bleeding.

Intestinal obstruction may affect either the small or large bowel. Small bowel obstruction is most commonly caused by adhesions from prior surgery or by hernias. Large bowel obstruction is most commonly caused by colorectal cancer, volvulus (twisting of the bowel), or diverticular disease.

Ischemic colitis occurs when blood flow to the colon is inadequate, typically at watershed areas. The splenic flexure is particularly vulnerable because it lies at the junction of the SMA and IMA territories.

Diverticular disease involves outpouchings of the colonic mucosa through weak points in the muscular wall, typically where the vasa recta penetrate. The sigmoid colon is most commonly affected due to its smaller diameter and higher intraluminal pressures. Complications include diverticulitis (inflammation) and diverticular bleeding.

Colorectal cancer is most common in the sigmoid colon and rectum. Lymphatic spread follows the arterial supply, which determines the extent of surgical resection required.

<image>Panel A: Appendicitis with pain pattern showing initial periumbilical pain as a shaded central area migrating to McBurney's point circled in the right lower quadrant. Panel B: Meckel's diverticulum as an outpouching on the antimesenteric border of the ileum approximately 60 cm from the ileocecal valve with ectopic gastric mucosa causing ulceration. Panel C: Diverticulosis with multiple outpouchings as tan pouches through the muscle layer at sites where vasa recta penetrate most numerous in the sigmoid colon. Panel D: Distribution of colorectal cancer with the highest frequency in the sigmoid colon and rectum shown darkly shaded with arrows indicating lymphatic spread along arterial supply.</image>

Summary

The duodenum has four parts, with D1 being intraperitoneal and D2-D4 being retroperitoneal. The ligament of Treitz at the duodenojejunal flexure serves as the landmark separating upper from lower gastrointestinal bleeding. The jejunum and ileum can be distinguished by wall thickness, vascularity, number of arterial arcades, length of vasa recta, prominence of circular folds, and abundance of Peyer's patches.

The colon has distinguishing features including the taeniae coli, haustra, and epiploic appendages. The ascending and descending colon are secondarily retroperitoneal, while the transverse and sigmoid colon are intraperitoneal with mesenteries.

The SMA supplies the midgut (cecum to proximal two-thirds of the transverse colon) while the IMA supplies the hindgut (distal transverse colon to upper rectum). The marginal artery of Drummond provides collateral circulation, but watershed areas at the splenic flexure and rectosigmoid junction remain vulnerable to ischemia.

The appendix is most commonly retrocecal (65%) in position, and McBurney's point marks its surface projection. The appendicular artery is an end artery, contributing to the rapidity of gangrenous complications in appendicitis.

Key Terms

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