Functions of the small intestine
Mechanical digestion and mixing chyme from stomach with various secretions – segmentation
Secretions – water, ions, mucus (and receives bile and pancreatic juice)
Chemical digestion of nearly all nutrients (enzymes from pancreas and small intestine, bile emulsifies fats and solubilises hydrophobic digestion products)
Absorption of nearly all nutrients, most water, ions
- Move chyme along to large intestine – peristalsis
Folds of the SI
- Circular folds= plicae circulares (mucosa and submucosa)
- Villi (mucosa)
- Brush border = microvilli (on epithelial cells)
Parts of the SI
- Duodenum–first 20-25cm
- Jejunum – proximal 2/5 of small intestine after duodenum
- Ileum–dista l3/5
Anatomy of the duodenum
- From pyloric sphincter to duodenojejunal flexure, where the suspensory muscle of the duodenum (ligament of Treitz) is located
- 4 parts of duodenum: superior, descending, horizontal, ascending
- Descending part receives bile duct and pancreatic duct at major duodenal papilla on left (medial) side
Functions of the duodenum
- Receives chyme from stomach – highly acidic
- Neutralised by duodenal gland secretions, bile from liver & gallbladder, pancreatic juice from exocrine pancreas
- Involved in regulating gastric emptying, gallbladder contraction, secretion by pancreas and liver (endocrine, neural)
- Digestion and absorption
Functions of jejunum
- Mixes chyme with bile and pancreatic juice and small intestine secretions – segmentation
- Peristalsis–moves contents along
- Most chemical digestion
- Most absorption
Functions of the ileum
- Segmentation, peristalsis
- Some chemical digestion
- Some absorption
Surface anatomy of the small intestine
- the right upper and left upper quadrants.
- epigastric, right and left hypochondriac and umbilical.
Jejunum and ileum
- All quadrants
- All regions
Movements in the SI
Segmentation serves to mix digestive tract contents. This movement consists of contraction and relaxation of short lengths of inner circular muscle (1-5cm), and is controlled by the enteric nervous system. Slow wave potentials control segmentation, and consist of depolarization/repolarization, at about 12-15 (in duodenum) and 6-9 (in ileum) cycles per minute – action potentials cause contraction (segmentation).
Peristalsis is waves of relaxation and contraction of circular muscle, with leading wave of contraction of longitudinal muscle, which propels contents along the length of the small intestine. Peristalsis occurs over 10-70 cm of the small intestine.
Regulation of motility in the SI
Regulation of motility of the small intestine is via local mechanisms and chemical stimuli e.g. distension, low pH, digestion products etc. which are detected by the nervous system, which stimulate smooth muscle contraction (local reflexes). 3-5 hours is the typical transit time through the small intestine.
The ileocaecal sphincter is located at the junction of the ileum and caecum of the large intestine. It relaxes to allow chyme from the ileum to the caecum and tonically contracts to prevent reflex, with stronger contractions as the caecum distends.
Functions of the large intestine
The large intestine has several functions:
- It provides important secretions (primarily mucus)
- It absorbs most of the remaining water and ions, as well as some nutrients, remaining from the small intestine.
- Bacteria in the large intestine perform some digestion
- Compaction of remaining luminal contents
- Peristalsis to move faeces along
- Storage and defecation (elimination) of faeces
Describe the parts of the LI
Structurally, there are four major regions of the large intestine: the cecum, colon (ascending, transverse, descending, sigmoid), rectum and anal canal. Additionally, there is the appendix which is a twisted, coiled tube, measuring about 8cm in length, that is attached to the caecum.
Describe the muscular layers of the LI
The large intestine has three discontinuous bands of outer longitudinal muscle termed the taeniae coli and a complete inner circular layer. These bands cause sacculations in between each other, which are termed haustra.
The epiploic (omental) appendices are small pouches of the peritoneum filled with fat and situated along the colon and upper part of the rectum. They are chiefly appended to the transverse and sigmoid parts of the colon, however, their function is unknown.
LI embryological origin
The large intestine is a midgut and hindgut derivative: the junction is just proximal to the splenich flexure.
The caecum is usually anterior to the iliacus muscle and covered on all sides by the peritoneum. The cecum is continuous with the ascending colon at the entrance of the ileum and is usually in contact with the anterior abdominal wall. The root of the appendix is 2cm inferior to the ileocaecal vale, at intersection of taeniae colil. The position of the appendix is variable (anterior, posterior, inferior, medial, lateral to caecum). The appendix is suspended form the terminal ileum by the mesoappendix.
The rectum is the distal part of the intestine, extending from the sigmoid colon. It has no haustra, no teniae coli (compete layer of longitudinal muscle instead), nor mesentery. It stores faeces prior to defecation. The perineal flexure is a pelvic diaphragm (muscular floor of pelvis) that pulls rectoanal junction anteriorly – contributing to fecal continence. The rectum has transverse rectal folds (incomeplete ‘valves’).
Intestines and peritoneum
Duodenum is secondarily retroperitoneal (had a mesentery during development, but later positioned against posterior abdominal wall).
Jejunum, ileum: are intraperitoneal, have mesentery
Ascending, descending colon: secondarily retroperitoneal
Transverse, sigmoid colon: intraperitoneal, have a mesentery (mesocolon)
The transverse mesocolon and transverse colon divide the greater sac of the peritoneal cavity into the supracolic and infracolic compartments. Immediately lateral to the ascending and descending colons are the right and left paracolic gutters. These depressions are formed between the lateral margins of the ascending and descending colon and the posterolateral abdominal wall.
Surface anatomy of the LI
- Cecum: right flank
- Root of appendix: usually at McBurney’s point (1/3 of the way from ASIS to umbilicus)
- Ascending: right lumbar
- Descending: left lumbar
- Sigmoid: variable in size and region, left inguinal.
More superior location of left vs right colic flexures – liver vs. spleen.
Arteries of the SI
The proximal duodenum (foregut) is supplied by the coeliac artery branches, including:
- Superior pancreaticoduodenal
The distal half of the duodenum, all of the jejnum and ileum, is supplied by the superior mesenteric artery (midgut derivatives), including:
- Inferior pancreaticoduodenal arteries
- 15-20 jejunal and ileal branches, ileocoloc (arcased, vasa recta)
Arteries of the LI
The proximal half (nearly to splenic flexure) is upplied by the superior mesenteric artery (midgut derivative)
- Ileocolic a
- Right colic a (usually a branch of the middle colic or ileocolic)
- Middle colic a
The distal half is supplied by the inferior mesenteric artery (hindgut derivate)
- Left colic artery
- Sigmoid arteries
- Superior rectal artery
- Anastamoses along large intestine: marginal artery.
Veins of the small and large intestine
Veins have the same names as arteries. The superior and inferior mesenteric (and splenic) veins drain to the portal vein (hepatic portal vein), then through the liver sinusoids (capillaries), to the hepatic vv,, IVC.
Lymphatics of the small and large intestine
The lymphatic system transports lipids absorbed from the lumen of the GI tract (milky lymph = chyle) and excess ECF. Nodes are located in mesenteries and retroperitoneal along blood vessels.
- The midgut drains to inferior mesenteric nodes, lumbar lymph trunks, cisterna chili, thoracic duct.
- The hindgut drains to inferior mesenteric nodes, lumbar lymph trunks, cisterna chili, thoracic duct.
Parasympathetic innervation of the intestines
The midgut (distal duodenum to left colic flexure): medulla, vagus nn and trunks, through SM ganglia, along SMA, to synapse in intramural ganglia.
The hindgut (left colic flexure to anal canal): lateral horn of S2-S4 spinal cord, ventral root, spinal nerve, ventral rami, pelvic splanchnics, pelvic (inferior hypogastric) plexus, along IMA branches and sigmoid mesocolon, to synapse in intramural ganglia.
Large intestine motility
Typical transit time through the large intestine is about 1 day, but varies with diet. There are 2 types of movements in the large intestine, haustral churning and mass movements, and they are controlled by the enteric nervous system. Haustral churning is similar to segmentation in the small intestine, and serves to mix the contents. Mass movements are peristaltic movements. These occur about 1-3 times per day, and propel contents towards the rectum.
Long reflexes include the gastroileal and gastrocolic reflexes. Food entering the stomach causes distension, which triggers ileal peristalsis to move ileal contents to the large intestine (gastroileal reflex), and large intestine peristalsis (mass movement) to move faecal contents towards the rectum. This may trigger a defecation reflex.
Stretch receptors in the wall of the rectum are activated by distension of the rectal wall as it fills with faeces, usually as a result of a mass movement. This initiates a local (enteric nervous system) reflex response: weak contraction of the inner circular and outer longitudinal smooth muscle layers of the rectum, and relaxation of the internal anal sphincter (also smooth muscle). In addition, a long reflex involving the sacral spinal cord is triggered: afferent neurons to S2-S4 of the spinal cord activate sacral parasympathetic neurons, which cause stronger contraction of the rectum and more relaxation of the internal anal sphincter.
The distension of the rectum is also consciously perceived, and there is an urge to defecate. If not a convenient time & place to defecate, contraction of the external anal sphincter is maintained (it is tonically contracted, and is a skeletal muscle under voluntary control). Over time, the stretch receptors in the rectum accommodate and rectal contractions decrease, the internal anal sphincter contracts, and the urge to defecate passes. If it’s the right time & place, the external anal sphincter and pelvic diaphragm relax (skeletal muscles under voluntary control), and defecation occurs, aided by increased intraabdominal pressure through the Valsalva manoeuvre, including contraction of abdominal wall muscles and the diaphragm.