Anatomy and Physiology of the Large Intestine Flashcards
What are the three functions of the large intestine?
Extract Na+ and water from luminal contents
To make and store faeces
To move faeces towards the rectum
Which cells secrete alkaline mucus in the large intestine?
Where are these cells found?
Goblet cells
Luminal epithelium
What is an alkaline mucus in the large intestine for?
Protect the epithelium from acid, abrasion, and bacteria
Provide a medium to hold faecal matter together
How does sodium move into the luminal cell in the JEJUNUM?
Na+K+ ATPase pump moves sodium out of cell (active transport)
Na+ is reabsorbed in co-transport with glucose or galactose via SGLT1 (secondary active transport)
Which transporter absorbs fructose?
GLUT5
How does glucose, galactose and fructose move out of a luminal cell?
GLUT2
How does water move into the intestinal epithelium?
Down an osmotic gradient created by sodium.
Sometimes between cells, sometimes through (‘tight’ or ‘leaky’ junctions).
How is sodium moved into a COLONIC epithelial cell?
Na+ H+ exchanger
Electrogenic Na+ channel
Name 2 functions of the ileocaecal valve
Prevents back flow of faecal contents from the colon
Controls the rate at which chyme enters the colon.
What are the two categories of colonic movements?
Haustral contractions (mixing)
Mass movements (propulsive)
Name two functions of colonic motility
Facilitates absorption of water and salts.
Permits orderly evacuation of faeces.
The gastrocolic reflex:
Is mediated by gastrin and extrinsic autonomic nerves
Initiates mass movements in the colon
Pushes colonic contents to rectum
Name the two sphincters in the anal canal
Internal anal sphincter (smooth muscle)
External anal sphincter (skeletal muscle)
Why is the mechanism of sodium reabsorption different in the jejunum to the colon?
Because there is no more glucose to be reabsorbed to act as a co-transporter (SGLT1)
Na+ is absorbed in the colon in exchange for H+ or via an electrogenic Na+ channel
Is absorption of Na+ in the colon active or passive?
Active
What are the longitudinal muscles of the colon called?
Taeniae coli
Name the six steps of the defecation reflex
- 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
What are the 4 parts of the Valsalva manoeuvre
- Full inspiration then forced expiration against a closed glottis, diaphragm moves downwards
- Abdominal and thoracic muscles contract
- Intra-abdominal pressure increases, faeces forced into rectum
- Defecation reflex initiated
What causes faeces to be brown?
Derivatives of bilirubin
What causes the smell in faeces?
Products of bacterial action
Name some functions of intestinal bacteria
- Provides immunity against common bacteria
- Forms secondary bile acids
- Digests mucus
- Degrades digestive enzymes
- Synthesises vitamin K
- Metabolises undigested polysaccharides
What do purgatives do?
Accelerate transit through the intestine
Define constipation
Infrequent or difficult evacuation of faeces
What are 5 causes of constipation?
Dietary Hormonal Anatomical Medication side effect Illness or disorder
What are the four major classes of purgatives?
Bulk laxatives
Osmotic laxatives
Faecal softeners
Stimulant purgatives
How do bulk laxatives work?
They are polysaccharide polymers that are not broken down.
Retain water in GI lumen, softening and increasing faecal bulk.
Increased bulk stimulates mechanoreceptors
Give 2 examples of bulk laxatives
Methyl cellulose
Ispaghula husk
How do osmotic laxatives work?
They are poorly absorbed solutes that maintain an increased volume of fluid in the GI tract, resulting in a high volume of fluid entering the colon.
Distention leads to purgation.
Give 3 examples of osmotic laxatives
Magnesium sulphate, magnesium hydroxide
Macrogol
Lactulose
How does lactulose work?
Moves into the large colon and mimics the action of lactulose in the small intestine.
Bacteria breakdown lactulose. The fermentation produces gases, lactic acid which causes increased bloating and motility
When is lactulose particularly useful?
In treating constipation secondary to opioids
Give 2 faecal softeners
Docusate
Arachis oil enema
How do faecal softeners work?
They contain anionic surfactants that lower the surface tension at oil-water interface, allowing water or fat to enter the stool
Give 2 stimulant purgatives
Bisacodyl
Senna
How does bisacodyl work?
What class of laxative is it?
Stimulates the rectal mucosa, resulting in mass movements
Stimulant purgative
How does senna work?
What class of laxative is it?
Contains derivatives of athracene which is absorbed into the colon and have an effect on the myenteric plexus.
Stimulates the longitudinal muscle of the colon
Stimulant purgative
Give 4 causes of diarrhoea
Infectious agents
Toxins
Anxiety
Drugs
What are the 4 types of diarrhoea?
Motility-related
Osmotic
Inflammatory
Secretory
Describe secretory diarrhoea
Increased active secretion or inhibition of absorption
Cholera
Describe osmotic diarrhoea
Loss of water due to heavy osmotic load e.g. maldigestion
Coeliac diseases
Describe inflammatory diarrhoea
Damage to mucosal lining or brush border leads to passive loss of protein-rich fluids and decreased ability to absorb these fluids.
Bacterial infection
Viral infections
Autoimmune disease e.g. IBD
Describe motility-related diarrhoea
High GI motility, less time available for absorption of nutrients and water.
Diabetic neuropathy
Name the 3 treatments for diarrhoea
Maintenance of fluid/electrolyte balance (Na+/K+)
Anti-diarrhoeal agents
(Anti-infective)
What is in oral rehydration therapy?
Glucose
NaCl
Isotonic or hypotonic solution
How does oral rehydration therapy work?
Na+ gets reabsorbed with glucose via SGLT1, water follows
Name 2 opioid anti-motility agents
Codeine
Loperamide
How do opioid anti-motility agents work?
Act on mui-opioid receptors in myenteric plexus
Increases tone and rhythmic contractions of colon but diminishes propulsion
