Large Intestine Flashcards
Large intestine anatomy Consists of? Where is the cecum? Appendix? Small intestine turns to large intestine at what point? Ascending colon runs from? Transverse colon runs from? Attached by? Descending colon runs from? Sigmoid colon runs from? General anatomy of colon itself?
Colon, cecum, appendix, rectum and anal canal.
The cecum is a blind pouch just distal to ileocecal valve- larger in herbivores.
The appendix is a thin, finger-like extension of the cecum- not physiologically relevant in humans.
Ileocecal valve
Ascending colon = right side of the abdomen, runs from cecum to the hepatic flexure (turn of the colon by liver).
Transverse colon runs from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). Hangs off stomach, attached by wide band of tissue called the greater omentum (posterior side= mesocolon).
Descending colon runs from the splenic flexure to the sigmoid colon.
Sigmoid (s-shaped) colon runs from descending colon to rectum.
Peritoneum= fatty tags
Muscle coat= taeniae coli= 3 thick longitudinal bands
Pouched in appearance (haustra)
Solitary nodules (lymphoid tissue) in walls (interaction with immune system)
Principle function of colon
Reabsorption of electrolytes + water
Elimination of undigested food and waste
Length+ diameter of colon?
1.5m long, 6cm diameter.
Blood supply to colon?
Ascending colon supplied by right colic artery + Ileocolic artery (branches of superior mesenteric artery)
Proximal transverse colon (first 2/3 of transverse) supplied by middle colic artery (branch of superior mesenteric artery)
Last 1/3 supplied by inferior mesenteric artery
Region between the two= sensitive to ischaemia
Function of taenia coli
Structure compared to circular muscle? Implication?
Continuous?
Reason?
Large intestine motility
Taenia coli= shorter than circular muscle layer, ovoid segments called haustra- can contract individually.
Apart from rectum and anal canal- substantial and continuous.
Movements of large intestine more complicated than small intestine
Haustra
Formation?
Also affected by?
Where it’s tighter+ narrower
Pouched ovoid segments
Taenia Coli= shorter than large intestine
Muscular movement changes place of them a little
How much water can large intestine reabsorb
Up to 4.5 litres
After this, diorrhoea (with cholera)
Rectum Difference to colon? Terminal portion? Surrounded by? Adaptations?
Similar histology, but distinguished by:
Transverse rectal folds in submucosa
Absence of taenia coli in muscularis externa.
Terminal portion= in anal canal.
Surrounded by:
Internal anal sphincters (circular muscle) (not under conscious control)
External (striated muscle) anal sphincters (under conscious control)
Transverse Rectal Folds= allow faeces to be held there before rest
Gut tube structure
Similarities?
Differences? Enterocytes? Crypts? Goblet cells? Goblet cells stimulated by? Paneth cells? Enteroendocrine cells? Glycocalyx?
Enterocytes+ Goblet cells= abundant Abundant crypts with stem cells (have memory of what they are meant to differentiate to)
Adaptated longitudinal muscle (taenia coli)
No villi= smaller SA than small intestine= smooth at gross level
Enterocytes= short+ irregular microvilli compared to small intestine: primary function= reabsorption of salts (water= passively absorpted) not macronutrients (should have been done in small intestine)
Crypts dominated by goblet cells: higher no. of goblet cells that in small intestine, no. increases distally towards rectum because:
Absorbing more water= contents becomes more solid= need more mucous to help passage
Covers bacterial+ particulate matter
Stimulated by Acetylcholine (parasympathetic+ enteric nervous system= Goblet cell secretion
No Paneth cells because anti-bacterial function may reduce gut flora+ most bacteria already eliminated by the time they get to the colon
Rarer enteroendocrine cells
Glycocalyx= no digestive enzymes
Muscle layers of large intestine Layers? Between taenia coli? Relationship between layers? Haustra?
Muscularis externa= inner circular + outer longitudinal layer
Circular muscles= segmentally thickened between haustra
Longitudinal layer concentrated in three bands: taenia coli
Between the taenia, longitudinal layer =thin
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals (anchoring outside muscle layer to rest of colon)
Large intestine motility types+ functions?
What promotes it?
Colonic contractions- kneading process (for mixing)- minimally propulsive- 5-10cm/hr at most.
Promotes absorption of electrolytes +water.
Antipropulsive patterns in proximal colon, ‘antipropulsive’ patterns dominate to retain chyme- because proximal colon does more absorption
Haustral contractions: localised segmental contractons of circular muscle in transverse + descending colon, cause back and forth mixing (pushing food from one haustra to another and back again)
Short propulsive movements every 30 mins.
Increase in frequency following a meal (to clear out colon for next food)
Mass movement: 1-3 times daily- resembles peristaltic wave (much larger).
Can propel contents 1/3-3/4 of length of large intestine in few seconds.
Foods that containing fibre (indigestible material) + caffeine+ smoking+ eating again promotes rapid transport through colon
Control of large intestine Parasympathetic? Sympathetic? External anal sphincter? Location of nerves? Hormonal/ Paracrine control?
Parasympathetic: ascending colon and most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves. Tends to promote gut function
Sympathetic- lower thoracic and upper lumbar spinal cord (inhibitory function of blood supply+ motility)
External anal sphincter controlled by somatic motor fibres in the pudendal nerves.
Myenteric plexus ganglia concentrated below taenia coli
Aldosterone promotes Na+ H2O
Vitamin D promotes Ca absorption
Lack of enteric nervous system?
Hirschsprung’s disease
No enteric intramural ganglia
Defecation= stored?
Defecation reflex controlled by?
Process? Augmented by?
Urge resisted?
Sigmoid colon
Sacral-spinal cord- has reflex+ voluntary actions
Reflex to sudden distension of walls of rectum.
Pressure receptors send signals via myenteric plexus to initiate more peristaltic waves in descending, sigmoid colon and rectum. Internal anal sphincter inhibited.
Weak intrinsic signal (if you cut all nerves to gut you still get a signal but its weaker) augmented by autonomic reflex.
Urge resisted, sensation subsides
Rectum?
What can/’t it distinguish between? Importance?
Can distinguish between solid, liquid and gas. Important in knowing what can be passed appropriately in what circumstance.
Can’t differentiate between air+ oil, problem for Orlistat= inhibits lipase, steatorrhoea,
