Large Bowel Flashcards
What is the large bowel made of?
colon, caecum, appendix, rectum and anal canal
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.
Label this diagram.
What are the principal functions of the large bowel?
The principal functions of the large bowel are the reabsorption of electrolytes & water, and the elimination of undigested food and waste.
How long and wide is the large bowel?
1.5m long
6cm diameter
What are the colons?
Ascending colon is on the right side of the abdomen, runs from the caecum to the hepatic flexure (the turn of the colon by the liver).
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).
Descending colon runs from the splenic flexure to the sigmoid colon.
Sigmoid colon (s-shaped) colon runs from descending colon to the rectum.
What is the blood supply like for the colon?
The proximal transverse colon is supplied with blood by the middle colic artery (branch of the superior mesenteric artery).
Distal third of transverse colon is perfused by the inferior mesenteric artery.
Reflects embryological division between the midgut and hindgut.
Region between the two is sensitive to ischemia.
What is special about the peritoneum?
The peritoneum carries fatty tags (appendices epiploicae), and the muscle coat has 3 thick longitudinal bands (taeniae coli); the gut wall is pouched in appearance (haustra)
What is the suggestive function of the appendix?
Structural or functional purpose of appendices epiploica unknown- suggested to have a protective function against intra-abdominal infections.
Why is the taenia coli necessary?
large intestine motility
Where are nodules of lymphoid tissue common?
Nodules of lymphoid tissue are common in the walls of the distal small intestine (Peyer’s patches) and large intestine (solitary nodules)
What are taenia coli (google)?
Teniae coli are three bands of longitudinal smooth muscle on the colon surface.
What are the haustra?
gut wall pouched in appearance
How would you compare the length of the taenia Coli and the small intestine?
it is shorter this causes the formation of pouced ovoid segments (the haustra)
Describe the (re)absorption function of the colon.
Colon absorbs electrolytes & water
- More in proximal colon
Na+ and Cl- absorbed by exchange mechanisms and ion channels.
Water follows by osmosis.
K+ moves passively into lumen.
How much water can the large intestine reabsorb?
Large intestine can reabsorb approx 4.5 litres water (usually 1.5 litres). Above this threshold diarrhoea.
What is the rectum?
Dilated distal portion of the alimentary canal
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 is the anal canal?
Terminal portion is anal canal. Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.
Diagram of the mucosal structure of the large bowel.
What does the myenteric plexus do?
innervate muscularis layer
Describe the large bowel mucosal structure?
Like the small intestine
Enterocytes and goblet cells are abundant.
Abundant crypts
Stem cells are found in the crypts.
What is the mucosal organisation of the large bowel?
Mucosa appears smooth at the gross level because it has no villi (smaller SA than small 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
Describe the goblet cells of the large bowel.
Higher no. of goblet cells than small bowel.
More prevalent in the crypts than along the surface, no.
increases distally 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 cell secretion?
Acetylcholine (parasympathetic and enteric nervous system) stimulates Goblet Cell secretion.
Describe the microvilli in the large bowel.
Microvilli (~0.5-1.5μm high) make up the “brush border”.
Several thousand microvilli per cell
Surface of microvilli covered with glycocalyx
Describe the mucosal organisation of the large bowel.
Villi are absent from large bowel
Enterocytes are still the dominant cells facing the gut lumen, but the mucus-secreting goblet cells (stained red) dominate the crypts.
As in the small bowel, new cells arise from crypt stem cells.
Crypts dominated by goblet cells
No Paneth cells & enteroendocrine cells are rarer than in small bowel
Glycocalyx does not contain digestive enzymes.
What is 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 muscle layer of 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.
What can contract individually in the muscle layer of the large bowel and describe muscle layers of rectum and canal?
Shorter than circular muscle layer, ovoid segments called haustra- can contract individually.
Apart from rectum and anal canal- substantial and continuous muscle layers
Movement of large bowel more complicated than small intestine
Describe the motility of the large bowel.
Colonic contractions- kneading process- minimally propulsive- 5-10cm/hr at most.
Promotes absorption of electrolytes and water.
In the proximal colon, ‘antipropulsive’ patterns dominate to retain chyme.
In transverse and descending colon, localised segmental contractions of circular muscle called Haustral contractions cause back and forth mixing.
Short propulsive movements every 30 mins.
Increase in frequency following a meal.
What is mass movement in the large bowel.
1-3 times daily- mass movement- resembles peristaltic wave.
Can propel contents 1/3-3/4 of length of large intestine in few seconds.
Food that contains fibre (indigestible material) promotes rapid transport through colon).
How is the large bowel controlled.
para and symp
somatic motor fibres (sphincter)
enteric nervous system
What is controlled by para and symp in the large bowel?
Parasympathetic: ascending colon and most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves.
Sympathetic-lower thoracic and upper lumbar spinal cord
what controls the external anal sphincter?
External anal sphincter controlled by somatic motor fibres in the pudendal nerves.
What detects pressure in the large bowel?
Afferent sensory neurons detect pressure
How is the enteric NS important for large bowel control?
Enteric nervous system also important Hirschsprung’s disease (no enteric intramural ganglia).
What can trigger mass movement?
Presence of food in stomach can stimulate mass movement-hormonal?
Neural?
Where is the myenteric plexus ganglia?
concentrated below taenia coli
What promotes sodium and water absorption?
Hormonal/ paracrine control .e.g., aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)
Describe defecation.
Rectum filled with faeces by mass movement in the sigmoid colon
Stores stool until convenient to void
Defecation reflex controlled primarily by the sacral spinal cord-both reflex and voluntary actions.
pt. 2
Reflex to sudden distension of walls of rectum
Pressure receptors send signals via myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum. Internal anal sphincter inhibited.
Weak intrinsic signal augmented by autonomic reflex
External anal sphincter under voluntary control
Urge resisted, sensations ubsides
What sphincter in rectum is under voluntary control?
external
What is the social part of the rectum?
Last few centimeters of the rectum
Can distinguish between solid, liquid and gas
That perceptual ability is important in knowing what can be passed appropriately in what circumstance.
How much faeces is made a day?
150g
What is faeces made of?
2/3 water
solids: cellulose, bacteria, cell debris, bile pigments, salts (K+)
What give colour and smell to faeces?
bile pigment give colour
bacterial fermentation gives odour
What is the large bowel flora?
All mammals have symbiotic relationships with their gut microbial community (microbiome)
Stomach and small bowel have few bacteria- protected.
Large bowel contains many
Essential to normal function
How would you describe the flora of the large bowel?
Diverse, highly metabolically active community
The microbiome in an average human adult comprises approximately 1.5kg of live bacteria, with the active biomass equivalent to a major human organ
What is the role 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 cross react 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.
Roles of intestinal flora diagram.
With recent research there are links between gut bacteria and what? (5)
1) Drug metabolism
2) Insulin resistance
3) Bile acid metabolism
4) Lipid metabolism
5) Obesity
What are the types of normal flora?
Most prevalent bacteria are the Bacteroides-Gram-negative, anaerobic, non-spore forming bacteria. Implicated in the initiation colitis and colon cancer.
Bifido bacteria are Gram-positive,non-spore forming, lactic acid bacteria. Have been described as “friendly” bacteria. Thought to prevent colonization by potential pathogens.
