Anatomy and Physiology of the Small and Large Intestines

Development of the Intestines

The mature intestine develops during embryogenesis from all three germ layers: endoderm, mesoderm, and ectoderm. The endoderm gives rise to the simple columnar epithelial cell lining of the surface of the small and large intestines. Cells of the lamina propria and muscularis layers derive from the embryonic mesoderm. The enteric neurons derive from embryonic ectoderm, specifically from migrating neural crest cells. This schema is true for the esophagus, stomach, pancreas, and liver; organs of the digestive system are therefore ontogenically and functionally related. Development of the intestine is covered in detail in Chapter 1 .

Anatomy of the Small Intestine

The small intestine is a convoluted tubular organ, extending from the pylorus to the ileocecal valve, occupying the central and lower parts of the abdominal cavity. Mostly circumscribed by the large intestine, it is divided into three segments: duodenum, jejunum, and ileum. The average length of the small intestine is between 250 and 300 cm in the newborn, increasing to as much as 600 to 800 cm in the adult. ^, The caliber of the small intestine gradually diminishes from its origin to its termination. The duodenum constitutes approximately the first 25 cm of the small intestine in adults; the remaining length is arbitrarily divided into the proximal two fifths, designated as the jejunum, and the distal three fifths, designated as the ileum. The transition from jejunum to ileum is arbitrary, because there are no histologic or gross anatomic demarcations between these segments. The duodenum is derived from the distal foregut during embryologic development. It is largely retroperitoneal. The proximal 2 to 5 cm of the duodenum is occasionally supported on a short mesentery, and the remainder lies firmly fixed in a retroperitoneal position, forming an incomplete circle around the head of the pancreas, where it is devoid of mesenteric cover. The duodenum emerges from this retroperitoneal position at the ligament of Treitz in the left upper quadrant. The duodenum is arbitrarily divided into four segments:

• The first portion of the duodenum, which begins at the pylorus and ends at the neck of the gallbladder, is the most mobile segment. This portion is also referred to as the duodenal bulb.

• The second portion, often referred to as the descending portion, descends from the level of the neck of the gallbladder along the right side of the vertebral column to the level of the third lumbar vertebra.

• The third, horizontal portion courses over the lower border to the third lumbar vertebra, passing from right to left, with a slight inclination upward, lying just inferior to the origin of the superior mesenteric artery in front of the aorta.

• The fourth portion begins its ascending course immediately to the left of the aorta, up to the level of the second lumbar vertebra, where it makes a ventral turn as it becomes the jejunum (duodenojejunal flexure or ligament of Treitz).

The biliary and pancreatic ducts drain into the second, descending portion of the duodenum. In most children, both ducts join approximately 1 to 2 cm from the outer margins of the duodenal wall and thereafter traverse the posteromedial aspect of the duodenal wall, through the sphincter of Oddi, to empty into the lumen of the second part of the duodenum at the ampulla of Vater. In 5% to 10% of individuals, an accessory pancreatic duct also enters 1 to 2 cm proximal to the ampulla of Vater, as the duct of Santorini.

The jejunum and ileum are derived from the endodermal midgut. There is no distinct demarcation between them, but progressive structural differences are present from the proximal jejunum to the distal ileum. The jejunal wall is thicker and more vascular than the ileum and diminishes in size with distal progression. The intestinal luminal diameter is also greatest in the jejunum, shrinking in diameter as it progresses distally.

The jejunum and ileum, attached to and loosely suspended from the posterior abdominal wall by the mesentery, are freely mobile, enabling each coil to accommodate easily to changes in form and position with propulsive peristaltic contractions.

The mesentery begins as an anterior reflection of the posterior peritoneum, attached to the posterior abdominal wall along a line extending from the left side of the lumbar vertebral bodies to the right sacroiliac joint, where it crosses over the duodenum along with other retroperitoneal structures, enveloping the jejunum, ileum, jejunal and ileal branches of the superior mesenteric blood vessels, nerves, lacteals, lymph nodes, and a variable amount of fat. The mesentery is fan shaped, with the breadth greater in the middle than at its upper and lower ends. The entire length of the jejunum and the ileum hangs suspended in the mesentery, with the exception of the retroperitoneal terminal ileum and cecum.

The duodenum derives its arterial supply from the right gastric, supraduodenal, right gastroepiploic, and superior and inferior pancreaticoduodenal arteries, whereas the venous drainage is via the superior mesenteric, splenic, and portal veins. The jejunal and ileal branches of the superior mesenteric artery, which itself courses over the third portion of the duodenum, form the arterial arcade that supplies the jejunum and ileum. The main venous drainage of the jejunum and ileum is through the portal and superior mesenteric veins to the liver.

Lymphatic drainage, coursing through the mesentery from the villous lacteals and the lymph follicles, converges to the preaortic lymph nodes around the superior mesenteric and celiac arteries. Approximately 70% of the lymph passes via the intestinal trunk and about 25% via the thoracic duct to the main subclavian vein. The intestines are overall quite rich in lymphoid tissue; the Peyer patches are small aggregates of lymphoid tissue located along the antimesenteric border of the small intestine. They are most abundant in the region of the midileum to the ileocecal valve. The Peyer patches are more prominent during childhood and regress in size and number with advancing age.

The plicae circulares, which are crescentic luminal protrusions of the submucosa covered by mucosa, run almost circumferentially in a circular fashion along the inside diameter of the intestinal wall and are most prominent in the distal duodenum and proximal jejunum, decreasing in number and size with progression through the ileum. They are permanent structures and do not smooth out when the intestine is distended. Together with the continuous reduction in caliber and villus height, these anatomic differences amount to a fourfold reduction in the surface area per length occurring over the course of the small intestine from distal duodenum and jejunum to the ileum.

The junction of the small intestine with the large intestine is referred to as the ileocecal valve, partly because of its structural appearance in most individuals and partly because the end of the terminal ileum (being wedged into the wall of the cecum) functions as a flutter valve. The ileocecal valve (sphincter) opens when a peristaltic wave strong enough to overcome the resistance of the valve arrives at the terminal ileum. The cecum, in concert, will generate reflexive relaxation. Overdistension or peristaltic contraction of the cecum causes a reflexive contraction of the sphincter. This protective mechanism prevents overfilling of the cecum or cecoileal reflux. This is an important factor to be remembered by endoscopists when attempting to intubate the terminal ileum during colonoscopy. Reflexive contraction of the sphincter due to overdistension with air will often thwart successful intubation of the ileum.

Anatomy of the Large Intestine

The large intestine commences at the cecum as a blind pouch below the termination of the small intestine. It hooks around, usually encircling the convolutions of the small intestine, and terminates at the rectum. From proximal to distal (oral to anal), the large intestine consists of the following segments:

• Cecum and vermiform appendix

• Colon, which in turn is composed of four sections—ascending, transverse, descending, and sigmoid colon

• Rectum

• Anal canal

The colon is approximately 60 cm long in the newborn, increasing to approximately 150 cm in the adult. The caliber of the large intestine is greatest at the cecum and gradually diminishes as it approaches the rectum, where it balloons out considerably in size just above the anal canal. The colonic wall remains constant in thickness throughout its entire length and lacks the villi that are a hallmark of the small intestine. The colon, in part, functions as a receptacle and reservoir for fecal matter; periodic high-amplitude contractions propel the contents caudally. Absorption of fluids and electrolytes, which is its main function, takes place along the entire length. The colon is easily distinguished from the small intestine by several distinctive characteristic features:

• It lacks villi.

• It is larger in caliber.

• Its outer longitudinal muscular layer is congregated into three distinct longitudinal bands, or teniae coli, extending from the cecum to the rectum.

• It has a characteristic sacculated and puckered appearance due to outpouchings (termed haustra) of its walls. The luminal surface is interrupted by intermittent creases (called plicae semilunares) that can be visualized on colonoscopy as irregular folds in the luminal surface.

• Fatty projections of the mesentery and the serosa (termed appendices epiploicae) are found scattered over the free surface of the entire large intestine, with the exception of the cecum, vermiform appendix, and rectum.

Extending as a reflection of the peritoneal lining, the mesentery envelops the colon just as it does the small intestine. The transverse colon and sigmoid colon hang fully suspended by the mesentery, whereas only a portion of the cecum is fully suspended. The ascending and descending colon are retroperitoneal. The prominent mesentery of the transverse colon is termed the transverse mesocolon, and the appendix is anchored by a short and well-defined mesentery referred to as the mesoappendix.

Originating from the midgut, the proximal colon, cecum, ascending colon, and proximal two thirds of the transverse colon all derive their blood supply from the superior mesenteric artery. The inferior mesenteric artery supplies the remaining one-third of the transverse colon, descending colon, sigmoid colon, and rectum. In addition to the blood supply from the inferior mesenteric artery, the rectum and anal canal also receive blood from the internal iliac and median sacral arteries. The superior and inferior mesenteric veins drain the same regions of the large intestine supplied by the corresponding arteries. With the exception of the lower half of the anal canal, the large intestine derives its nerve supply from the parasympathetic and sympathetic systems. The nerve distribution pattern closely mimics the arterial supply. The proximal colon receives its sympathetic neuronal innervation from the celiac and superior mesenteric ganglia, whereas the parasympathetic supply is from the vagus nerve. In each case, the nerves are distributed to the proximal colon in plexuses around the branches of the superior mesenteric artery. The distal colon receives its sympathetic nerve supply via branches from the lumbar segments of the sympathetic trunk, and the parasympathetic nerves originate from the pelvic splanchnic nerves. The lymphatic drainage of the large intestine courses through the mesentery in proximity to the arterial and venous supplies. First draining through groups of small pericolic nodes along the right and middle colic arteries, lymph flow from the colon drains into intermediate nodes located within the mesentery. The lymph ultimately terminates in the large colic preaortic nodes surrounding the superior and inferior mesenteric arteries. The rectum and anal canal drain into the inferior mesenteric and iliac nodes via perirectal nodes, which lie in close apposition to the rectal walls.

As stated previously, the primary function of the large intestine is water and electrolyte absorption; however, the large intestine is capable of absorbing small quantities of short-chain fatty acids (SCFAs), which are by-products of the anaerobic bacterial fermentation of oligo- and polysaccharides. The SCFAs absorbed by the colon contribute only about 7% of overall total body energy requirements, with slightly higher amounts being contributed during infancy. ^, More importantly, the colonic epithelium depends on the luminal SCFAs for their energy supply, as evidenced by the development of diversion colitis after surgical diversion of the fecal stream and resolution of the colitis with colonic instillation of n -butyric acid. ^,