Large Bowel Flashcards
What is the main function of the large bowel?
Absorption of electrolytes and water
Elimination of undigested food and waste
What is the large bowel comprised of?
Colon, caecum, appendix, rectum and anal canal
What is the caecum?
Right side of the abdomen
Blind pouch just distal to the ileocecal valve- larger in herbivores
What is the appendix?
Thin, finger-like extension of the caecum - not physiologically relevant in humans
What are the dimensions of the large bowel?
1.5m long, 6cm diameter
What is the 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).
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 lesser omentum (posterior side, mesocolon).
What is the descending colon?
Runs from the splenic flexure to the sigmoid colon.
What is the sigmoid colon?
s-shaped) colon runs from descending colon to the rectum
What supplies the proximal transverse colon with blood?
blood by the middle colic artery (branch of the superior mesenteric artery).
What majorly supplies the large bowel?
Branches of the superior and inferior mesenteric arteries
What is the distal their of the transverse colon perfused by?
Inferior mesenteric artery
What do the different blood supplies represent?
embryological division between the midgut and hindgut
What is the region between the midgut and handout susceptible to?
Ischemia
Because it supplied by very small vessels branching of the SMA and IMA
What is the role of 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 are the taeniae coli needed for?
Large intestine motility
What is the function of the appendices epiploica?
unknown- suggested to have a protective function against intra-abdominal infection
What are Peyer’s patches?
Nodules of lymphoid tissue are common in the walls of the distal small intestine
What are nodules found in the large intestine called?
Solitary nodules
What are the haustra?
Small pouches caused by sacculation which give the colon its segmented appearance
Where does the majority of absorption take place?
Proximal colon
How are electrolytes and water reabsorbed?
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.
Describe the histology of the rectum?
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 terminal portion of the rectum?
Anal canal
What is anal canal surrounded by?
internal (circular muscle) and external (striated muscle) anal sphincters
Describe the mucosal structure of the large bowel?
Mucosa
Submuscosa
Muscularis
Serosa
What comprises the mucosa?
Epithelium
Lamina propria
Muscularis mucosae
Where are the glands found?
Submucosal space
Site between mucosa and
muscularis
Also contains nerves
What are the components of the muscular?
Circular muscle
Longitudinal muscle
What do you find in the inner layer of the mucosa?
Enterocytes and goblet cells are abundant
Microvilli
Abundant crypts
Stem cells are found in the crypts
What is found above the microvilli?
Solution of mucus and water
Why does the mucosa appear smooth at the gross level?
Because it has no villi (smaller surface area than small bowel)
Only microvilli
What are the main features microvilli in the large bowel?
Enterocytes have short, irregular microvilli - primarily concerned with resorption of salts
Does the large bowel have more goblet cells than the small bowel?
Yes
Where are there the most goblet cells?
More prevalent in the crypts than along the surface, no. increases distally towards rectum
What is found in the apical ends of goblet cells?
Mucus-filled secretion granules awaiting release
What is the function of mucus?
Facilitates the passage of the increasingly solid colonic contents and covers bacteria & particulate matter
What is the function of acetylcholine in the large bowel?
(parasympathetic and enteric nervous system) stimulates Goblet Cell secretion
What cells are found more commonly in the small bowel?
Enteroendocrine cells
NO Paneth cells in the large bowel
What is the surface of the microvilli covered by?
Glycocalyx
Traps water and mucus
What is glycocalyx?
rich carbohydrate layer on apical membrane- protection from digestional lumen
traps a layer of water & mucous known as “unstirred layer”
regulates rate of absorption from intestinal lumen
What does muscular externa consist of?
inner circular and outer longitudinal layer
What is the appearance of circular muscles?
Segmentally thickened
How to the genial coli and circular layer interact?
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals
What can the haustra do?
Individual Ovid segments can contract individually
What is the motility of the large bowel?
Colonic contractions- kneading process- minimally propulsive- 5-10cm/hr at most
Why is movement so slow?
Promotes absorption of electrolytes and water
Longer time period
What occurs in the proximal movement? Why?
‘antipropulsive’ patterns dominate to retain chyme
Longer time for reabsorption
What occurs in the transverse and descending colon?
localised segmental contractions of circular muscle called Haustral contractions cause back and forth mixing
When do movements increase in frequency?
Following a meal
What happens every 30 mins?
Short propulsive movements?
What happens 1-3 times a day?
Mass movement
Resembles peristaltic wave
Can propel contents 1/3-3/4 of length of large intestine in few seconds
What does food containing fibre do?
promotes rapid transport through colon
How does the parasympathetic nervous system supply the large bowel?
ascending colon and most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves
How does the sympathetic nervous system supply the large bowel?
Lower thoracic and upper lumbar spinal cord
T8-L2
What is the external anal sphincter supplied by?
somatic motor fibres in the pudendal nerves
What do afferent sensory neurones do?
Detect presuure
Large amount of faeces
Stimulates bowel movement
Why is the generic nervous system important?
Interstitial cells of cajal
Makes up the internal pacemaker of the large bowel
Responsible for background motility
What is Hirschsprung’s disease?
Patients without enteric nervous system in bowel
Present in infancy with abdominal distension and constipation
Where is the myenteric plexus ganglia?
Concentrated below taenia coli?
How do hormone work on the large bowel?
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.
Describe the control of defecation
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, sensation subsides
What can the last few cm of the rectum do?
distinguish between solid, liquid and gas.
That perceptual ability is important in knowing what can be passed appropriately in what circumstance.
Describe faeces
150g/day adult.
Two thirds water.
Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+).
Bile pigments give colour.
Bacterial fermentation gives odour
Describe the flora of the large bowel
Diverse, highly metabolically active community
Microbiome in an average adult human comprises approximately 1.5 kg of live bacteria
What is the role of intestinal flora?
Synthesise and excrete vitamins e.g. Vitamin K-
Prevent colonisation by pathogens by competing for attachment sites or for essential nutrients.
Antagonise other bacteria through the production of substances which inhibit or kill non-indigenous species.
Stimulate the production of cross-reactive antibodies
Stimulate the development of certain tissues, including caecum and lymphatic tissues
What is the role of flora in the large bowel?
Fibre 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 is the most prevalent bacteria?
Bacteroides- Gram-negative, anaerobic, non-spore forming bacteria. Implicated in the initiation colitis and colon cancer
What is the second most prevalent bacteria?
Bifidobacteria are Gram-positive, non-sporeforming, lactic acid bacteria. Have been described as “friendly” bacteria. Thought to prevent colonization by potential pathogens
What is gut bacteria linked to?
Drug metabolism
Insulin resistance Bile acid metabolism
Lipid metabolism
Obesity
