Small & Large Intestine Flashcards
What is the embryological origin of the Small Intestine?
Derived from the foregut (proximal part of duodenum) & midgut (from apex of duodenal loop onwards)
What are the 5 main functions of the SI?
• Mechanical digestion & mixing chyme from stomach with various secretions – segmentation
• Secretions – water, ions, mucous (receives bile & pancreatic juice)
• Chemical digestion of nearly all nutrients
o Enzymes from pancreas & small intestine, bile emulsifies fats
• Absorption of nearly all nutrients, most water, ions
• Move chyme along to large intestine (peristalsis)
What are the parts of the SI?
- Duodenum – first 20-25cm (above the level of the umbilicus)
- Jejunum – proximal 2/5 of small intestine after duodenum
- Ileum – distal 3/5
What are the 4 parts of the duodenum?
• From pyloric sphincter to duodenojejunal flexure
• C shaped structure above the level of the umbilicus
• 4 parts: superior, descending, horizontal, ascending
o Descending part contains the major duodenal papilla which is the common entry point for the bile duct & pancreatic duct. It also contains the minor duodenal papilla, which is the entrance for the accessory pancreatic duct
Name the Functions of the Duodenum
- Receives chyme from stomach => highly acidic
- Neutralised by duodenal gland secretions (mucous & bicarbonate) bile from liver & gallbladder, as well as pancreatic juice from the exocrine pancreas
- Involved in regulating gastric emptying, gallbladder contraction, secretion by pancreas & liver (endocrine, neural)
- Because the duodenum is quite short there is some (but limited) digestion & absorption
What are some of the substances secreted in the duodenum?
o Duodenum secretes CCK, GIP & Secretin which act on chief & parietal cells in the stomach to inhibit gastric secretions
What is the function of the Jejunum?
- Mixes chyme with bile & pancreatic juice & small intestine secretions – segmentation
- Peristalsis – moves contents along
- Most chemical digestion & most absorption occurs here
What are the functions of the Ileum?
• Segmentation, peristalsis
• Processes continue but largely finish up
o Some chemical digestion
o Some absorption
• Vitamin B12 complexes with intrinsic factor
• Iron complexes
Describe the surface anatomy of the small intestine:
Duodenum
Jejunum
Ileum
• Duodenum:
o Umbilicus or epigastric regions (or both)
o Right upper quadrant & extends a little bit into the left upper quadrant
• Jejunum & ileum
o All 9 regions; dominates the umbilicus & hypogastric regions
Describe Segmentation in the SI.
What is it controlled by?
How many slow wave potentials and cycles per minute?
• Serves to mix digestive tract contents – brings chyme into contact with the intestinal wall
• Contraction & relaxation of short lengths of smooth, inner circular muscle
o Work over short segments (1-5cm) of the small intestine
• Controlled locally by the enteric nervous system (pacemakers)
• Slow wave potentials; depolarization/repolariesation, at about 12-15 (in duodenum) to 6-9 (ileum) cycles per minute – action potentials cause contraction (segmentation
Describe Peristalsis
- Peristalsis – waves of relaxation & contraction of circular muscle, with leading wave of contraction of longitudinal muscle
- To propel contents along length of SI
- Occur over 10-70cm of SI
- Migrating motility complex – during fasting, intense peristaltic contractions progressing from duodenum to ileocaecal junction, once every 1.5 hours
How is motility of the Small Intestine Regulated?
• Local mechanical & chemical stimuli (eg distension, low pH, digestion products etc) are detected by enteric nervous system (local reflexes)
• Smooth muscle contraction controlled by enteric nervous system (local reflexes)
• 3-5 hours is typical transit time through small intestine
o Most digestion & absorption occurs here
Describe the ileocaecal Sphincter
- At junction of ileum & caecum of large intestine
- Relaxes to allow chyme from ileum to caecum
- Tonically contracts to prevent reflux
- Stronger contraction as caecum distends
What are the functions of the Large Intestine?
• Secretions – primarily mucous
• Absorption of most remaining water & ions, some nutrients
o Less amounts of water & ions which were protective against the compacted& dehydrated faeces
• Bacteria perform some digestion
• Compaction of remaining luminal contents
• Peristalsis to move faeces along
• Storage & defecation (elimination) of faeces
What are the parts of the Large Intestine?
- Caecum, appendix (hangs off inferior part of caecum), ascending colon, transverse colon, descending colon, sigmoid colon (s shaped), rectum, anal canal
- Right colic (hepatic) flexure (junction between ascending & transverse colon), left colic (splenic)
What are the taeniae coli?
3 discontinuous bands of outer longitudinal muscle
What is the Haustra?
o small pouches caused by sacculation, give the colon its segmented appearance
o Contraction of outer longitudinal layer causes this sacculation
What are the epiploic (omental) appendices
peritoneal covered accumulations of fat associated with the colon
What are the Midgut and Hindgut derivatives?
junction just proximal to splenic flexure
What is the location of the Caecum?
Is the caecum intra or retro-peritoneal
• Caecum – usually anterior to iliacus muscle & covered on all sides by peritoneum
o Can vary in location; left lumbar region or the iliac fossa region.
o Caecum is intraperitoneal but doesn’t have a mesentery
What is the location of the the root of the appendix
• Root of appendix – 2cm inferior to ileocaecal valve, at intersection of taeniae coli; position of appendix is variable (anterior, posterior, inferior, medial, lateral to caecum)
o Mesoappendix – mesentery of appendix
What is different about the rectum vs. the rest of the Large Intestine?
The rectum is the distal part of the intestine
• Distal part of large intestine
• No haustra, no teniae coli (complete layer of longitudinal muscle instead)
• Lacks a mesentery (inferior part of rectum is infraperitoneal).
• Stored faeces
• Perineal flexure – pelvic diaphragm (muscular floor of pelvis) pulls rectoanal junction anteriorly –contributes to faecal continence
o The band = puborectalis, and when contracted this pulls the junction anteriorly, so when it relaxes the junction straightens out & sphincters relax, allowing defacation to occur
• Transverse rectal folds (valves)
o Help break up the column of faeces, promote faecal continence
• Internal anal sphincter (smooth muscle) & external anal sphincter (voluntary control)
Describe the intestines, with relation to the peritoneum: Duodenum Jejunum, Ileum Ascending, descending colon Transverse, sigmoid colon Transverse mesocolon Right, left paracolic gutters
• Duodenum: secondarily retroperitoneal
o Had a mesentery during development, but later positioned against posterior abdominal wall
• Jejunum, ileum: intraperitoneal, have mesentery
• Ascending, descending colon: secondarily retroperitoneal
• Transverse mesocolon divides the greater sac of the peritoneal cavity into supracolic & infracolic compartments
• Right, left paracolic gutters lateral to the ascending, descending colon
o Ascites: fluid collects in these paracolic gutters
Describe the Greater Omentum
• Transverse mesocolon – mesentery (2 layers of peritoneum)
• Greater omentum – 4 layers of peritoneum
• Transverse mesocolon & transverse colon often fuse to the greater omentum
o Greater omentum comes back up to the posterior abdominal wall where it splits to come around the transverse colon.
o Transverse colon visceral peritoneum disappears (continuous with lesser sac/omental bursa) as it fuses to the greater omentum of stomach.
Describe the surface anatomy of the Large Intestine
- Caecum: right lumbar region (can also sit in right iliac fossa)
- Root of appendix: usually at McBurney’s point (1/3 of the way from ASIS to umbilicus)
- Ascending colon: right lumbar
- Transverse colon: may be in umbilical or epigastric region
- Descending colon: left lumbar
- Sigmoid colon: left inguinal, hypogastric
- Right colic flexure is slightly inferior to the left colic flexure due to the size of the liver above it.
SMALL INTESTINE: Which ARTERY supplies the midgut derivatives?
And what are the midgut derivatives!
• Distal half of duodenum, all of jejunum & ileum supplied by the superior mesenteric artery (midgut derivatives)
What are the branches of the superior mesenteric artery?
o 12-20 jejunal & ileal branches, ileocolic arterial branch
• Arcades (the anastomotic loops – less in jejenum, more in ileum)
• Vasa recta (longer in jejunum)
SMALL INTESTINE: Which ARTERY supplies the foregut derivatives?
And what are the foregut derivatives?
• Proximal duodenum (foregut) = coeliac artery branches
What are the branches of the colic artery?
o Superior pancreaticoduodenal
o Supraduodenal
LARGE INTESTINE: Which ARTERY supplies the midgut derivatives of the Large Intestine? What are the midgut derivatives?
• Proximal half (nearly to splenic flexure) supplied by superior mesenteric artery (midgut derivative)
What are the branches of the Superior Mesenteric Artery?
o Ileocolic, right colic, middle colic aa
• R colic often missing
What is the distal half of the Large Intestine supplied by?
• Distal half supplied by inferior mesenteric artery (hindgut derivative)
What are the branches of the inferior mesenteric a?
o Left colic, sigmoid, superior rectal aa
Anastomoses along the large intestine are called what?
Marginal Artery
What are the veins which drain the Large intestine and to where do they drain?!
• Superior & Inferior mesenteric (and splenic) veins drain to the hepatic portal vein (all nutrients except fats) then through liver sinusoids to hepatic vein, IVC
Where are the lymph nodes?
- Transport lipids absorbed from lumen of GI tract (milky lymph = chyle) & excess ECF
- Nodes in mesenteries & retroperitoneal along blood vessels
Where does the midgut drain?
• Midgut drains to superior mesenteric nodes, intestinal lymph trunk, cisterna chyli, thoracic duct
Where does the hindgut drain?
• Hindgut drains to inferior mesenteric nodes => lumbar lymph trunk (also receives lymph from lower limbs & posterior abdo wall) => cisterna chyli => thoracic duct
Describe the parasympathetic innervation of the midgut!
• Midgut (distal duodenum to left colic flexure – ie SI plus first part of LI)
o Medulla, vagus nn & trunks, through superior mesenteric ganglia, along superior mesenteric artery, to synapse in intramural ganglia
Describe the parasympathetic innervation of the hindgut!
• Hindgut (left colic flexure to anal canal)
o Lateral horn of S2-S4 spinal cord, ventral root, spinal nerve, ventral rami, pelvic splanchnics, pelvic (inferior hypogastric) plexus, along inferior musculophrenic artery & sigmoid mesocolon to synapse in intramural ganglia
o Supplied by sacral spinal cord (S2, S3, S4) via pelvic splanchnic nerves
Describe the SYMPATHETIC innervation of the Midgut
- Lateral horn T9-T11 (midgut)
- Through sympathetic chain
- Thoracic & lumbar splanchnic nn
- Synapse in superior mesenteric ganglion (for midgut) & inferior mesenteric ganglion (for hindgut)
- Follow blood vessels
*NOTE: the foregut is T6-T9
Describe the SYMPATHETIC innervation of the Hindgut
Lateral horn T12-L2 (hindgut) of spinal cord
o Note T6-T9 is foregut
• Through sympathetic chain
• Thoracic & lumbar splanchnic nn
• Synapse in superior mesenteric ganglion (for midgut) & inferior mesenteric ganglion (for hindgut)
• Follow blood vessels
Afferent Innervation of Intestines and Referred Pain
- Pain afferents travel with sympathetic fibers (except to dorsal root not lateral horn) to spinal segments T9-T11 (midgut) & T12-L2 (hindgut)
- Pain is referred to umbilical (midgut), or hypogastric regions (corresponding dermatomes)
Describe Large Intestine Motility- what are the 2 types of movements in the LI?
• There are 2 types of movements in the LI; haustral churning & mass movements – both controlled by the enteric nervous system
o Haustric churning is similar to segmentation in the SI, and serves to mix the contents
o Mass movements are peristaltic movements – these occur about 1-3 times per day, and propel contents towards the rectum
Describe a Long Reflex
• Long reflexes include the gastroileal & gastrocolic reflexes – controlled by ENS
o Gastroileal reflex: when food is present in the stomach, motility increases in the ileum, as the ileocaecal sphincter relaxes. Mainly controlled by ENS, may also be stimulated by Gastrin.
o Gastrocolic reflex: an increased motility in LI, in response to increased food in stomach after a meal; depends on PSNS innervation of the colon, but the hormones gastrin & cholecystokinin (CCK) may be involved
• Food entering the stomach causes distention, 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.
Describe the defacation 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 (ENS) reflex response: weak contraction of the inner circular & 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 S2-S4 of the spinal cord activate sacral parasympathetic neurons, which cause stronger contraction of the rectum & 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 & 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 & pelvic diaphragm relax (skeletal muscles under voluntary control) & defecation occurs, aided by increased intrabdominal pressure through the Valsalva manoeuvre, including contraction of abdominal wall muscles & the diaphragm
