Large Bowel Flashcards
What does the large bowel consist of? (5)
Colon Caecum Appendix Rectum Anal canal
How long is the large bowel and what is its diameter?
1.5m
6cm
What is the caecum?
The caecum is a blind pouch just distal to the ileocecal valve- larger in herbivores.
what is the appendix?
The appendix is a thin, finger-like extension of the caecum - not physiologically relevant in humans.
What are the principle functions of the large bowel?
the reabsorption of electrolytes & water, and the elimination of undigested food and waste.
What structures can the colon be divided into?
Ascending colon
Transverse colon
Descending colon
Sigmoid colon
What is the ascending colon?
on the right side of the abdomen, runs from the caecum to the hepatic flexure (the turn of the colon by the liver).
what is the transverse colon?
runs from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). Hangs off the stomach, attached by a wide band of tissue called the greater omentum (posterior side, mesocolon).
Where is the descending colon?
colon runs from the splenic flexure to the sigmoid colon.
Where is the sigmoid colon?
colon (s-shaped) colon runs from descending colon to the rectum.
Describe the blood supply to the parts of colon?
Proximal T colon : middle colic artery
Distal third T colon : inferior mesenteric artery
- Reflects embryological division between the midgut and hindgut.
Region between the two is sensitive to ischemia.
What does the middle colic artery branch off?
Superior mesenteric artery
What are fatty tags? ( appendices epiploicae)
The peritoneum carries fatty tags
Purpose unknown : suggested it is protective against intra-abdominal infections
What are taenia coli needed for?
These are a coat of muscle with 3 thick longitudinal bands
Large intestine motility
The gut wall is pouched in appearace
What are solitary nodules?
Similar to peyer’s patches in s.bowel
- nodules of lymphoid tissue
What are haustra?
The pouched ovoid segments caused by teania coli
where does the colon absorb electrolytes and water?
Most in the proximal colon
Na+ and Cl- absorbed by exchange mechanisms and ion channels.
Water follows by osmosis.
K+ moves passively into lumen.
Large intestine can reabsorb approx 4.5 litres water (usually 1.5 litres). Above this threshold diarrhoea.
What is the Dilated distal portion of the alimentary canal?
Rectum
How is the histology of the rectum different to the colon?
Histology similar to the colon, but distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa.
What muscles make up the anal canal?
Terminal portion is anal canal.
Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.
Describe the mucosal structure of the large bowel?
Like the small intestine
Enterocytes and goblet cells are abundant.
Abundant crypts
Stem cells are found in the crypts.
Why does the large intestine have a smaller surface area than small?
Has no villi
So mucosa appear smooth
Describe the mucosal organisation of the large bowel?
Enterocytes have short, irregular microvilli - primarily concerned with resorption of salts.
(Water is absorbed as it passively follows the electrolytes, resulting in more solid gut contents)
Crypts dominated by goblet cells.
Where are goblet cells most found in the large bowel?
Higher number of goblet cells than small bowel
In cypts than along the surface
Increases towards rectum
Apical ends are packed with mucus-filled secretion granules awaiting release.
Mucus - facilitates the passage of the increasingly solid colonic contents and covers bacteria & particulate matter.
What stimulates goblet secretion?
Acetylcholine (parasympathetic and enteric nervous system) stimulates Goblet Cell secretion.
Does the glycolcalyx have digestive enzymes?
Digestive enzymes
- the surface of microvilli is covered with glycocalyx
rich carbohydrate layer on apical membrane
serves as protection from digestional lumen
yet allows for absorption.
traps a layer of water & mucous known as “unstirred layer”
regulates rate of absorption from intestinal lumen
Describe the muscular layers in the large bowel?
Like the small bowel, muscularis externa consists of an inner circular and outer longitudinal layer.
Circular muscles segmentally thickened.
Longitudinal layer concentrated in three bands- taenia coli
Between the taenia, longitudinal layer is thin
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.
Shorter than circular muscle layer, ovoid segments called haustra- can contract individually.
Apart from rectum and anal canal- substantial and continuous.
How do contractions change across large bowel?
Proximal colon : antipropulsive patterns to retain chyme
Transverse and decesnding colon : localised segmental contractions of haustral muscle cause back anf forth mixing
Short propulsive movements every 30 mins
What mass movement occurs?
Peristaltic wave
Can propel 33-75% of large intestine in seconds
Fibrous foo promotes rapid transport
Describe the innervation to the ascending and transverse column?
Parasympathetic vagus nerve
Sympathetic- lower thoracic and upper lumbar spinal cord.
- More distal innervated by pelvic nerves.
What innervation controls external anal sphincter?
somatic motor fibres in the pudendal nerves.
Afferent sensory neurons detect pressure.
What is Hirschsprung’s disease?
Enteric nervous system also important- Hirschsprung’s disease (no enteric intramural ganglia).
The absence of ganglion cells causes the muscles in the bowels to lose their ability to move stool through the intestine (peristalsis).
What plexus is concentrated below taenia coli?
Myenteric plexus ganglia
Describe paracrine control?
aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)
What is the defecation reflex controlled by?
controlled primarily by the sacral spinal cord- both reflex and voluntary actions.
Why is defecation a reflex of distension of the walls of the rectum?
Pressure receptors send signals via myenteric plexus to start peristaltic eaves down the sigmoid colon and rectum
Internal and spincter inhibited
Weak intrinsic signal increased by autonomic reflex.
External anal sphincter under voluntary control.
Urge resisted, sensation subsides
Why are the last few cm of the rectum known as the social part?
Can distinguish between solid, liquid and gas.
That perceptual ability is important in knowing what can be passed appropriately in what circumstance.
What makes up faeces?
Two thirds water.
Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+).
Bile pigments give colour.
Bacterial fermentation gives odour.
What are the roles of intestinal flora? (7)
Synthesize and excrete vitamins e.g. Vitamin K- germ-free animals can have clotting problems.
Prevent colonization by pathogens by competing for attachment sites or for essential nutrients.
Antagonize other bacteria through the production of substances which inhibit or kill non-indigenous species.
Stimulate the production of cross-reactive antibodies. Antibodies produced against components of the normal flora can crossreact with certain related pathogens, and thereby prevent infection or invasion.
Stimulate the development of certain tissues, including caecum and lymphatic tissues
Fibre (indigestible carbohydrate) can be broken down by colonic bacteria.
Produces short chain fatty acids which can regulate gut hormone release, or be absorbed to be used as an energy source or to influence functions such as food intake or insulin sensitivity directly.
What are the most prevalent bacteria?
Bacteroides- Gram-negative, anaerobic, non-spore forming bacteria. Implicated in the initiation colitis and colon cancer.
What are Bifidobacteria?
Gram-positive, non-sporeforming, lactic acid bacteria. Have been described as “friendly” bacteria. Thought to prevent colonization by potential pathogens
There are linked between the gut bacteria and?
1) Drug metabolism
2) Insulin resistance
3) Bile acid metabolism
4) Lipid metabolism
5) Obesity
