Physiology of the Large Intestine Flashcards
What are the 3 major functions of the large intestine?
- Produce and store faeces
- Move faeces towards the rectum
- Extract Na+ and water from the luminal contents
What does the large intestine secrete?
Why?
Alkaline mucus is secreted from goblet cells to:
- Protect the epithelium from acid, abrasion and bacterial activity
- Provide an adherent material for holding faecal matter together
How does water move across the intestinal epithelium?
Depends on how tight the tight junctions are between epithelial cells:
- Leakiest in the duodenum, tightest in large intestine
Paracellular route:
- Between cells with leaky tight junctions (top of small intestine)
Transcellular route:
- When tight junctions are tighter, they take this route (large intestine)
How is water primarily reabsorbed in the small intestine?
Through paracellular route as tight junctions are leaky in this part of the intestine
How is water primarily reabsorbed in the large intestine?
- Transcellular route as tight junctions between epithelial cells are tightest in this part of the intestine.
- Follows Na+ by osmosis
How is sodium taken up by colonic epithelial cells?
How is it exported out of the cell into the blood?
Lumen to cell:
- Electrogenic Na+ channel
- H+/Na+ exchanger
Cell to blood:
- Na+/K+ATPase into the lateral space
What are the functions of the ileo-caecal valve?
- Allows chyme to enter the caecum from the terminal ileum
- One way valve guarded by a sphincter: prevents backflow of faecal contents into ileum
- Controls rate at which ileal chyme enters the caecum
What are the functions of the movements of the colon?
- Facilitates absorption of water and salts
- Facilitates evacuation of faeces
What are the 2 movements of the colon and their functions?
Haustral contractions:‘mixing’ of faecal matter to absorb water and salts
Propulsive: mass movements
What are taeniae colae and haustral contractions?
What are their functions?
Longitudinal smooth muscle arranged in 3 bands
Contractions of the taeniae colae and circular smooth muscles of the colon cause the colon to bulge into segments called haustrae.
Haustral contractions allow the mixing of faecal matter to absorb water and salts.
What are the predominant motor movements of the ascending colon?
First half of colon (ascending and first 1/2 of transverse)
- Haustral contractions predominate
- Very slow propulsive movements
What triggers haustral contractions?
When each haustrum is filled with fluid chyme this triggers its contraction to expel the contents.
What is the predominant motor activity of the transverse colon to the sigmoid colon?
Propulsive mass movements
What is the gastrocolic reflex?
What is it mediated by?
When food enters the stomach, gastrin secretion is increased and autonomic nerves are stimulated.
Gastrin and extrinsic autonomic nerves trigger the gastro colic reflex which initiates mass movements in the colon
Pushes colonic contents into the rectum. triggering the defaecation reflex.
Why is the rectum normally empty?
Due to the sphincer between the sigmoid colon and the rectum and the sharp angle at which the sigmoid colon becomes the rectum (provides resistance)
What are the 2 sphincters guarding the anal canal?
What are they comprised of?
Internal anal sphincter:
- Smooth muscle (involuntary)
External anal sphinter:
- Skeletal muscle (voluntary)
What is the defaecation reflex?
What triggers it?
- Triggered by sudden distension of the walls of the rectum due to faeces.
- Detected by mechanoreceptors
Consists of:
- Contraction of the rectum
- Relaxation of the internal anal sphincter
- Initial contraction of the external anal sphincter
- Increased peristaltic activity in the sigmoid colon
- Relaxation of the external anal sphincter
- Expulsion of faeces
How can the defaecation reflex be overriden by voluntary control?
What does voluntary defaecation involve?
The external anal sphincter is made of skeletal muscle and is under voluntary control therefore can be overriden to prevent involuntary defaecation.
Voluntary defaecation involves the valsalva manouvre
What occurs when the defaecation reflex is overriden?
If voluntary control prevents defaecation this can trigger reverse peristalsis, where faecal contents are moved back up the colon and more water is absorbed.
What is the valsalva manouvre?
Conscious process
Full inspiration followed by forced expiration (using thoracic and abdominal wall muscles) against a closed glottis:
- Causes depression of the diaphragm which combined with contraction of abdominal and thoracic wall muscles increases abdominal pressure.
- Increased pressure forces faecal contents into the rectum
- Defaecation reflex is initiated
Describe the composition of faeces
3/4 water
1/4 solid matter:
- 30% dead bacteria
- 10-20% fat
- 10-20% inorganic matter
- 2-3% protein
- 30% undigested material including bile pigment and sloughed epithelial cells
Colour= bilirubin derivatives (stercobilin, urobilin)
Odour= products of bacterial action (skatole, indole, mercaptans and hydrogen sulphide)
What are the functions of intestinal bacteria?
What can disrupt all of these functions?
(Anaerobic bacteria)
- Provide immunity against common pathological bacteria
- Convert bilirubin to urobilogens
- Form secondary bile acids
- Degrade digestive enzymes
- Digests mucus
- Synthesises certain vitamins (e.g. vit K)
- Metabolises undigested polysaccharides
All of these functions can be disrupted by oral antibiotics
What can cause normal variations in bowel habit?
- Age
- Diet
- Social and cultural influences (stress, lifestyle etc)
What changes in stool could indicate disease?
- Changes in frequency, consistency or volume
- Presence of blood, mucus, pus or fatty material
