Small & Large Intestine Flashcards
(46 cards)
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.
