Week 11- Large Bowel Flashcards
What are the 4 parts of the colon?
Ascending
Transverse
Descending
Sigmoid
How does the transverse colon receive blood?
Via middle colic artery (branch of superior mesenteric)
How does the distal 3rd of the transverse colon receive blood?
Inferior mesenteric artery
Where are fatty tags found?
The peritoneum
Describe the muscle coat of the peritoneum
Has 3 thick longitudinal bands
What is the function of the large bowel?
Reabsorption of electrolytes & water, and the
elimination of undigested food and waste
How much water can the large bowel absorb?
4.5L (usually 1.5 L)
How is the rectum different to the large bowel?
Distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa.
What muscles are found in the anal canal?
Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.
Describe the mucosal structure of the large bowel
Similar to small bowel, enterocytes and goblet cells are abundant, crypts present where stem cells are found
Describe the mucosal layers of the large bowel
Mucosa
Submucosa
Muscularis (2 layers- circular and longitudinal muscles)
Serosa
Where are Meissener’s plexus found in the mucosal layers?
Submucosal plexus
How does the surface of the large bowel differ from the small bowel?
Has fewer villi (smaller surface area)
Enterocytes have some irregular villi that are mainly for salt reabsorption
How are crypts of the large bowel different to the small bowel?
They are dominated by goblet cells
How is water absorbed in the large bowel?
Passively- it follows electrolytes
Describe the distribution of goblet cells in the large bowel
Mainly in crypts, dont increase in abundance along the bowel , apical end filled with mucous granules
What stimulates goblet cells secretion?
Acetylcholine (parasympathetic and enteric nervous system)
Do you find paneth or enteroendocrine cells in the large bowel?
No
How does the glycocalyx in the large bowel differ to the small bowel? What is its function?
No digestive enzymes but does trap a layer of water and mucous (unstirred layer), serves as protection from digestional lumen yet allows for absorption. Regulates rate of absorption from intestinal lumen
Describe the muscle layers 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
What is the tenia coli?
Longitudinal layer concentrated in 3 bands
Describe colonic contractions
Minimally propulsive, 5-10cm/hr
What activity is found in the proximal colon?
Antipropulsive patterns (dominate and retain chyme)
How often is mass movement found in the bowel?
1-3 times a day, can propel 1/3 to 3/4 length of large intestine in a few seconds
How does fibre affect motility in the bowel?
It promotes rapid transport through colon (is indigestibel)
Describe parasympathetic influence on the large bowel
Ascending colon and most of transverse colon innervated by vagusnerve. More distal innervated by pelvic nerves
Describe sympathetic influence on the large bowel
Lower thoracic and upper lumbar spinal cord.
What is the external anal sphincter controlled by?
Somatic motor fibres in the pudendal nerves
How is pressure detected in the large bowel?
By afferent sensory neurons
What is Hirschsprung’s disease?
Lack of enteric intramural ganglia resulting in no large bowel movement
Where is the myenteric plexus ganglia concentrated?
Below taenia coli
What stimulates sodium and water absorption?
Hormonal/paracrine control. e.g. aldosterone
Where is the rectum found?
Last few centimetres of the large bowel
What is the role of the rectum?
Can distinguish between solid, liquid and gas.
Important in knowing what can be passed appropriately
in what circumstance
Describe the faeces
Two thirds water. Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+). Bile pigments give colour. Bacterial fermentation gives odour.
How much faeces is produced daily?
150g
How does bacterial presence differ in the large bowel compared to the small bowel/stomach
Lots more present- few in stomach and small bowel
Describe the relationship the large bowel has with bacteria
All mammals have symbiotic relationships with their gut microbial community
What is the mass of the human microbiome?
1.5 kg (equivalent to an organ)
Describe the roles of intestinal flora?
Synthesise and excrete vitamins e.g. Vitamin K- germ-free animals can have clotting problems.
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. Antibodiesproduced 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 is most prevalent in the flora? Describe it
Bacteroides- Gram-negative, anaerobic, non-spore forming bacteria
What gram positive bacteria is commonly found in the flora? Describe their role
Bifidobacteria, thought to prevent colonization by potential pathogens.
What are the 5 parts of the large bowel?
Colon Caecum Appendix Rectum Anal canal
What is the caecum and where is it found?
A blind pouch just distal to the ileocecal valve- larger in herbivores
What is the appendix and where is it found?
A thin, finger-like extension of the caecum - not physiologically relevant in humans
Describe the movement of ions in the large bowel?
Na+ and Cl- absorbed via ion channels
Water moves by osmosis
K+ moves passively into lumen
How long is the large bowel and what is it diameter?
1.5m long, 6cm diameter
Describe the location of 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)
Describe the location of 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)
Describe the location of the descending colon
Runs from the splenic flexure to the
sigmoid colon
Describe the location of the sigmoid colon
Colon runs from descending colon to the rectum
How is lymphoid tissue present in the large bowel
As small nodules in the walls
Describe tenia coli and haustra
Taenia coli necessary for large intestine motility
Taenia Coli shorter than small intestine, cause the formation of pouched ovoid segments called haustra (singular haustrum)
Describe the structure and abundance of 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 haustra
Shorter than circular muscle layer, ovoid segments. Can contract individually
What are haustral contractions and where do they occour?
In transverse and descending colon, localised segmental
contractions of circular muscle that cause back and forth mixing.
How often do haustral contractions occour and how are they affected by meals/
Short propulsive movements every 30 mins.
Increase in frequency following a meal
