Large Bowel Flashcards
What does the large bowel consist of?
Large bowel consists of the 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.
what are the principal functions of the large bowel?
the reabsorption of electrolytes & water, and the elimination of undigested food and waste.
What are the four sections to the colon?
Ascending colon, Transverse colon, Descending colon and Sigmoid colon
Where does the ascending colon run from?
The caecum to the hepatic flexure
Where does the transverse colon run from?
The hepatic flexure to the splenic flexure
Where does the descending colon run from?
The splenic flexure to the sigmoid colon
Where does the sigmoid colon run from?
from descending colon to the rectum.
Which band of tissue attaches the transverse colon to the stomach?
Greater omentum
What is the blood supply of the proximal and distal transverse colon?
Proximal - blood from middle colic artery
Distal third - perfused with the inferior mesenteric artery
What is the region between the proximal part and the distal third of the colon sensitive to?
ischemia
What is the appendices epiplociae? and what is their suggestive purpose?
Fatty tags - protective function against intra-abdominal infections
What is the name given to describe the pouched appearance of the muscle coat of the colon?
haustra
What is the taeniae coli?
longitudinal muscle - 3 thick bands which are needed for large intestine motility
What immunological tissues are common in the walls of the distal small intestine and large intestines?
Small intestine - Peyers Patches
Large Intestine - Solitary nodules
How do haustra form?
The taenia coli is shorter than the small intestine which cause the formation of pouched ovoid segments called haustra
What does the colon reabsorb?
Electrolytes and water
In which part of the colon is more water and electrolytes absorbed?
More in the proximal colon
How does Na and Cl get absorbed?
Through exchange mechanisms and ion channels
How does K+ get reabsorbed?
Moves passively into the lumen
What happens when the large intestine absorbs more than 4.5L of water?
Diarrhoea
What histological feature distinguishes the rectum from the colon?
Transverse rectal folds in its submucosa and absence of taenia coli
Which two muscles surround the anal canal?
The internal sphincter and external sphincters
What type of muscle is the internal anal sphincter?
Circular muscle
What type of muscle is the external anal sphincter?
Striated muscle
What are the mucosal layers of the large bowel, starting with the inner most?
Mucosa
Submucosa
Muscularis
Serosa
What does the serosa harbour?
Blood vessels and nerve endings
What are found in crypts?
Stem cells
Which cells are abundant in the large bowel?
Crypts with stem cells
Goblet cells
Enterocytes
Why does the mucosa appear smooth at gross level?
No vili
What are enterocytes primarily concerned with?
The resorption of salts
What do enterocytes have on their surface?
Short, irregular microvilli
Does the large or small bowel contain more goblet cells?
Large bowel
Which cell dominates inside the crypts?
Goblet cells
What are the apical ends of the goblet cells packed with?
Mucus filled granules awaiting release
What is the role of mucus in the large bowel?
Mucus - facilitates the passage of the increasingly solid colonic contents and covers bacteria & particulate matter.
what neurotransmitter stimulates goblet cell secretion?
ACh
Are there paneth cells in the large bowel?
No
Does the glycocalyx of the large bowel contain digestive enzymes?
No
What makes up the brush border?
Microvilli
What is the surface of the microvilli covered in?
Glycocalyx
what does the glycocalyx do?
traps a layer of water & mucous known as “unstirred layer” - this regulates the rate of absorption from the intestinal lumen
What is the glycocaylx?
A rich carbohydrate layer on the apical membrane
What does the muscularis externa consist of?
an inner circular and outer longitudinal layer
How are the circular muscles thickened?
Segmentally thickened
Where are the longitudinal muscles of the large bowel concentrated?
In three bands - taenia coli
What is the thickness of the longitudinal layer between the taenia coli?
Thin
Where are the ovoid segments of the large bowel not seen?
Rectum and anal canal
What are haustra?
Ovoid segments, which are are shorter than circular muscle layer and can contract individually
What large bowel movements dominate in the chyme in order to retain chyme?
Anti-propulsive patterns
What causes back and forth mixing in the transverse and descending colon?
haustral contractions
What do colonic contractions promote?
Absorption of electrolytes and water
How does the motility of the large bowel change following a meal?
Increase in the frequency of colonic contractions following a meal in order to promote the reabsorption of electrolytes and water
What is mass movement?
High amplitude propagated contractions which resemble a peristaltic wave. Can propel the contants 1/3-3/4 of the length of the large intestine in a few seconds
What promotes the process of mass movement through the large intestine?
Food containing fibre (undigestible material)
What is the parasympathetic innervation of the ascending colon?
Vagus nerve
What is the parasympathetic innervation of most of the transverse colon?
Vagus Nerve
What is the parasympathetic innervation of the distal colon?
Pelvic nerves
What is the sympathetic innervation of the large bowel?
Lower thoracic and upper lumbar spinal nerves
What is the external anal sphincter controlled by?
Somatic motor fibres in the pudendal nerves
What is Hirschsprung’s disease?
Lack of enteric intramural ganglia
Where are myenteric plexus ganglia concentrated?
Below the taenia coli
What can stimulate mass movement?
Presence of food in the stomach
how does aldosterone promote water and na+ reabsorption?
synthesis of Na+ ion channel, Na+/K+ pump)
How does the rectum become filled with faeces?
Through mass movement in the sigmoid colon
What controls the defacation reflex?
Sacral spinal cord
Describe the defacation reflex?
- 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.
- This weak, intrinsic signal is then augmented by autonomic reflex
How is the sensation for defacation able to subside?
External anal sphincter under voluntary control, so if the urge is resisted then the sensation subsides
Which part of the rectum can distinguish between solids, liquids and gas?
The “social” part of the rectum - last few cm of the rectum
How many grams of faeces do adults pass a day?
150g
what proportion of faeces is water?
2/3rds
What are the solid components of faeces?
cellulose, bacteria, cell debris, bile pigments, salts (K+).
What gives faeces such a strong odor?
Bacterial fermentation
Describe the microbiome of an average adult?
Diverse, highly metabolically active community.
comprises approximately 1.5 kg of live bacteria, with the active biomass equivalent to a major human organ.
What are the 7 roles of the intestinal flora - summary?
- Synthesize and excrete vitamins
- Prevent colonisation by pathogens through competitive inhibition
- Produce substances which are toxic to non-indigenous species
- Stimulate prouction of cross-reactive antibodies
- Stimulate development of some tissues
- Break down fibre
- Produce short chain fatty acids
How do intestinal flora prevent colonisation by pathogens?
pathogens by competing for attachment sites or for essential nutrients.
How does the production of cross reactive antibodies by intestinal flora prevent infection?
Antibodies produced against components of the normal flora can crossreact with certain related pathogens,
Which tissues do intestinal flora stimulate the development of?
Caecum and lymphatic tissues
What is the purpose of intestinal flora producing short chain fatty acids?
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
Recent research has shown links between gut bacteria and..
1) Drug metabolism
2) Insulin resistance
3) Bile acid metabolism
4) Lipid metabolism
5) Obesity
What are the most prevalent types of bacteria in the gut?
Bacteroides - gram negative, anaerobic and non-spore forming bacteria
Describe Bifidobacteria?
Gram-positive, non-sporeforming, lactic acid bacteria. Have been described as “friendly” bacteria. Thought to prevent colonization by potential pathogens.
