Large Intestine Flashcards
When can ingestion be considered a function of the large intestine?
When suppositories are used as a way of administering drugs
What are the main functions of the large intestine?
- mechanical processing - especially through reabsorption of fluid
- digestion - bacteria perform this
- secretion
- absorption - mainly of water
- excretion
Why does the large bowel act as more of a ‘storage space’?
The peristaltic waves in the large bowel are much slower
What part of the small intestine will open into the large intestine?
The terminal ileum is the last part of the small bowel
This opens into the cecum
What controls the flow of chyme from the small to large intestine?
Ileocecal valve
This is the opening between the ileum and the cecum (first part of large bowel)
Between which structures does the ascending colon run?
The ascending colon runs from the cecum to the hepatic flexure
This is a sharp bend between the ascending and transverse colons
Between which structures does the transverse colon run?
Between the hepatic flexure and the splenic flexure
What happens to the blood supply of the transverse colon as it moves towards the splenic flexure?
why is this important?
2/3 of the way along, the blood supply changes from the superior mesenteric artery to the inferior mesenteric artery
The distal 1/3 of the transverse colon is a hindgut structure
Where does the descending colon run to?
It runs from the splenic flexure into the sigmoid colon
The large bowel becomes the sigmoid colon when it enters the pelvis
what does the sigmoid colon lead to?
The rectum, and then eventually to the anus
What is the difference in the muscle of the small and large bowels?
In the SI there is a continuous sheet of smooth muscle
In the LI there are taenia coli - strips of longitudinal muscle
Where are the three taenia coli of the large bowel found?
What are they called?
- Tenia libera (free coli) - found on the interior
- Tenia mesocolica (mesocolic coli) - found on the posterior aspect of the large bowel
- Tenia omentalis (omental coli) - found on the superior aspect of the transverse colon
What results from tonic contractions of the teniae coli?
It bunches up the colon into a succession of pouches called haustra
What are the haustra?
Small pouches caused by sacculation, which give the colon a segmented appearance
What are epiploic appendages?
Small, fat-filled sacs of visceral peritoneum that are attached to the teniae coli
What is involved in a barium enema?
radioopaque barium is passed into the colon through the anus
this will show the very prominent haustra
What is shown in a barium enema about the nature of the haustra?
The dilated part of the haustrum is where the smooth muscle is relaxed
The narrowed part of the haustrum is where the smooth muscle is contracted
Why is the surface epithelium of the LI much flatter than the SI?
There are no circular folds and there are no villi
This is because no amplification of surface area is needed
In what other way does the nature of the LI differ to the SI?
There are very few enzyme-secreting cells located in the large intestine
Other than in the anal canal, what is the nature of the mucosa of the colon?
It is simple columnar epithelium
It is made mostly of enterocytes (absorptive cells) and goblet cells
What is the role of deep pit glands and enterocytes in the large bowel?
Deep pit glands contain goblet cells and enterocytes
Enterocytes absorb water, salts and vitamins produced by intestinal bacteria
what is the role of goblet cells in the large intestine?
They secrete mucus which eases movement of the faeces
what are the key roles of mucus in the large intestine?
- it protects the intestine from the effects of the acids and gases produced by enteric bacteria
- layers of mucus allow faecal material to be dried and compacted as it passes through the large bowel
Why does the large intestine contain lymphatic tissue?
As the colon contains a lot of bacteria
How may the ileocecal valve be described?
It is a one-way flutter valve that connects the terminal ileum to the cecum
What happens to the ileocecal valve when pressure is high in the colon?
The leaflets of the ileocecal valve are pushed together
This prevents chyme from returning to the terminal ileum after it has entered the cecum
What happens when the ileocecal valve relaxes?
During periodic relaxation of the valve, substances can flow into the caecum
When will the ileocecal valve be open and when will it be closed?
It is open during ileal distention
It is closed during caecal distention
What is significant about the relationship between the caecum and the appendix in most people?
The lumen of the cecum continues into the appendix
This means that material can be trapped in the blind-ended tube
This leads to infection and appendicitis
Why is much of the motility of the colon NOT designed to move the contents along?
material needs to transit the colon slowly to allow water to be reabsorbed
The content is also compacted slowly
How do peristaltic waves in the large bowel compare to the small bowel?
The peristaltic waves are much less frequent and move a much larger distance
What is meant by the process of haustration?
There are slow contractions of the circular muscle to squeeze the content to and fro, mix material and release water
What stimulates slow-moving haustral contraction?
presence of food residues in the colon
What is movement like during haustration?
it involves sluggish segmentation, primarily in the transverse and descending colons
When a haustrum is distended with chyme, its muscle will contract to push the residue into the next haustrum
These movements happen every 30 minutes
What is meant by ‘mass movement’ in the colon?
Strong peristaltic waves which occur three to four times per day
Where does a mass movement commence?
The wave starts midway along the transverse colon
This forces the contents towards the rectum
what happens to the haustra during a mass movement?
the haustra briefly disappear as the mass movement sweeps by
they will reform immediately afterwards
When do mass movements usually occur?
During eating or immediately afterwards
How is motility of the large intestine mostly controlled?
- most control is intrinsic via the enteric nervous system
- there is some parasympathetic control
- there are enteroendocrine and neurocrine influences
What are the enteroendocrine and neurocrine influences on the large bowel?
They come from cells releasing 5-HT and Peptide YY
How is the action of Peptide YY an example of feedback activity?
Local cells produce Peptide YY
This acts locally and enters the circulation to influence regions elsewhere
What reflexes can trigger mass movement?
The gastrocolic and the orthocolic reflexes
When does the gastrocolic reflex occur and what is its effect?
It occurs when the stomach becomes full
It triggers a mass movement in the colon to move the faeces along
When does the orthocolic reflex occur and what is its effect?
It occurs when someone moves from lying in a horizontal position to standing in an upright position
It triggers a mass movement in the colon
What triggers the “ileal brake”?
why is it necessary?
The presence of undigested lipid in the distal ileum and proximal colon
If there is fat in the colon, this means insufficient processing has occurred earlier on
What is the action of the ileal brake?
Undigested lipid triggers the release of Peptide YY
Peptide YY slows gastric emptying and small bowel peristalsis
What is the difference in digestion in the colon and the small intestine?
Digestion in the colon is performed by bacteria
Digestion in the SI is performed by human enzymes
What will the bacteria in the colon digest?
- fibre
- urea and amino acids
- bilirubin from haemoglobin
- cysteine and methionine
- conjugated bile acids
What does the digestion of fibre produce?
Short chain fatty acids such as butyric acid, hydrogen and methane
Butyric acid is used as an energy source by colon cells
what does the digestion of urea and amino acids produce?
ammonia
What does the digestion of bilirubin produce?
urobilinogen and stercobilinogen
What happens to the urobilinogen produced from the digestion of bilirubin?
Half of the urobilinogen is reabsorbed
It is taken to the liver by the portal vein
It then enters circulation to be secreted by the kidneys in the urine
What happens to the stercobilinogen produced from the digestion of bilirubin?
Stercobilinogen is known as faecal urobilinogen
It is oxidised to stercobilin which is responsible for pigmentation of faeces
What does digestion of cysteine and methionine produce?
hydrogen sulphide
What does the digestion of conjugated bile acids produce?
Why is this process important?
Hydroxyl groups are removed to produce unconjugated bile acids
It is important for bile salt recycling as making the bile salts uncharged means that they can pass into circulation
How much gas is released by fermentation each year?
1L of nitrogen, hydrogen and CO2 is released a year
What transporter is needed for short-chain fatty acid absorption in the colon?
What type of transport is this?
It is secondary active transport via a Na+-linked transporter
This is the sodium monocarboxylate transporter
How are short-chain fatty acids absorbed in the colon?
The SMCT1 transports fatty acids into the cell alongside Na+ ions
This is driven by the export of Na+ from the cell by the Na+/K+ ATPase
why is it important that short-chain fatty acids are absorbed in the colon?
they are an important source of energy for colonocytes
What channels allow for transcellular sodium transport into the colon cells?
Epithelial sodium channels - ENaC
During salt absorption in the colon, what does the entry of Na+ ions into the colonocyte cause?
A paracellular Cl- flux as the Cl- ions leave the cell
This then leads to water following the osmotic gradient
What is salt absorption in the colon stimulated by?
How can the colon respond?
the process is stimulated by aldosterone
the colon can respond by expressing more channels (ENaC) and absorbing more sodium
What creates the sodium gradient needed for salt absorption in the colon?
The Na+/K+ ATPase pumps Na+ out of the cell
The K+ that is pumped into the cell leaves via a K+ channel
What are the major secretagogues of chloride in the colon?
what is a secretagogue?
cAMP and Ca2+
These are substances which promote secretion of Cl-
How do toxins increase the amounts of the chloride secretagogues in the colon?
cholera and E. coli toxins increase the amount of cAMP by triggering the insertion of CFTR channels
C. difficile toxin increases the amount of Ca2+
What is the result of inserting more CFTR channels into epithelial cells in the colon?
There is increased chloride excretion into the lumen
This leads to salty water being secreted back into the faeces
This leads to severe dehydration
What is the effect of increasing cAMP or Ca2+?
Secretory diarrhoea
what is the movement of potassium like in the colon?
It can undergo secretion or absorption
There is usually a net secretion, and passive secretion exceeds active secretion
How is passive secretion of K+ achieved in the colon?
This involves K+ moving paracellularly
As Na+ is reabsorbed, this creates an electrochemical gradient
How is active secretion of K+ achieved in the colon?
This involves opening the BK channel
This allows K+ to go from a high intracellular concentration, into the lumen
What determines the balance of passive and active secretion of K+ in the colon?
It is determined by plasma potassium concentration, aldosterone and cAMP
What is the role of the H+/K+ ATPase in movement of K+ across the colon?
It is usually involved in the absorption of K+ and the secretion of H+ into the lumen
How is the transfer from sigmoid colon to rectum described?
What does the rectum act as?
The sigmoid colon enters the rectum at a very sharp angle
The rectum is a storage space
what are rectal valves and what is their function?
They are folds in the wall of the rectum
They help to support the faecal material and prevent it from pushing down too quickly
What are the 2 different anal sphincters?
- internal anal sphincter
2. external anal sphincter
What is the role of the internal anal sphincter?
What is it made of and what control is it under?
It is made from smooth muscle
It contracts to constrict the faecal material moving through the anus
It is under involuntary control
What is the external anal sphincter made from and what type of control is it under?
It is made from skeletal muscle
It is under voluntary control
what is the process of anal sampling?
As tension rises, the internal anal sphincter relaxes
This allows some faecal material to pass into the space between the sphincters
What happens after anal sampling?
The external anal sphincter contracts
Voluntary control allows it to relax to empty the bowels at the appropriate time
What is the Dentate (pectinate) line?
What does it signify?
It is the boundary between the normal gut lining and the anal canal
- the epithelium becomes stratified squamous
- it reflects the different innervations of the different areas
What happens to the internal anal sphincter when faeces enter the rectum?
When faeces enter the rectum, the pressure rises
The rise in pressure triggers reflex relaxation of the internal anal sphincter
This comes with the urge to defecate
What happens after the reflex relaxation of the internal anal sphincter?
A small amount of material enters the anus, allowing for anal sampling
The external anal sphincter contracts if defecation is not desired
Why is sitting or squatting recommended during defecation?
It increases the rectosigmoid angle
This facilitates the passage of faeces
What other action will increase the rectosigmoid angle during defecation?
Relaxation of the puborectalis muscle
How is the passage for faeces opened during defecation?
Relaxation of the external anal sphincter and pelvic floor muscles
What is the Valsalva manoeuvre?
This describes that the motive force behind defecation is triggered by rectal peristalsis plus raised intraabdominal pressure
