Large intestine Flashcards
What are the functions of the large intestine?
- Extract Na+ & water from luminal contents
- Make & store faeces
- Move faeces towards rectum
What does the luminal epithelium secrete + what is its role?
Goblet cells secrete alkaline mucus for -
- protection from acid, abrasion + bacterial activity
- provide adherent medium for holding faecal matter together
What causes mixing in the large intestine?
Haustral contractions - combined contractions of the taeniae coli + circular muscle of the colon forms haustrations.
Segmented contractions causes mixing.
What is propulsion in the large intestine?
Mass movement that involves propelling luminal contents from T. colon to S. colon via series of peristaltic events.
What reflexes are involved in defaecation?
- Gastrocolic reflex
- Defaecation reflex
What is the gastrocolic reflex?
Initiates mass movement in colon, pushing contents into rectum
(Mediated by gastrin & extrinsic autonomic nerves.)
What happens in the defaecation reflex?
- Contraction of rectum in response to distention of rectum
- Relaxation of internal anal sphincter
- Initial contraction of external anal sphincter
- Increased peristaltic activity in sigmoid colon
- Relaxation of external anal sphincter
- Expulsion of faeces
What is the Valsalva manoeuvre?
- Full inspiration followed by forced expiration against closed glottis to cause diaphragm to move downwards
- Abdominal & thoracic muscles contract
- Increased abdominal pressure forces faecal contents into rectum
- Defaecation reflex initiated
What happens if defaecation is delayed?
Reverse peristalsis to move faeces back into colon.
Further water is absorbed so faeces harden.
How is Na+ + water reabsorbed in the jejunum?
- Na+ moves into epithelium from lumen via cotransporter SGLT1 with glucose.
- Na+K+ATPase moves Na+ into blood
- Water follows
How is Na+ and water reabsorbed in the colon?
- Electrogenic Na+ channel allows Na+ into cells
- Na+ leaves cell via Na+K+ATPase, changing electrochemical gradient
- Drives Cl- into cell via coutertransport with HCO3-
- Water is drawn through cell
What is the composition of faeces?
3/4 water, 1/4 solid matter -
- 30% dead bacteria
- 30% undigested material
- 10-20% fat
- 10-20% inorganic matter
What are the functions of the anaerobic bacteria in the gut?
Immunity against common bacteria. Covert bilirubin to urobilinogen. Form secondary bile acids. Degrade digestive enzymes. Digest mucus. Synthesis of some vitamins. Metabolise polysaccharides
What are the 4 types of diarrhoea?
MOISt -
Motility - increased GI motility (e.g. diabetic nephropathy)
Osmotic - loss of water (e.g. coeliac disease)
Inflammatory - damage to mucosal lining or brush border causes loss of protein-rich fluids, which are not absrobed - causde by infections/autoimmune conditions (e.g. IBD)
Secretory - increased active secretion/inhibition of absorption. (e.g. cholera)
What are the treatments for diarrhoea?
- Oral rehydration therapy - isotonic/hypotonic solution of glucose + NaCl. Increases Na+ absorption –> increases water absorption
- Anti-infective agents - e.g. erythromycin or ciprofloxacin for Campylobacter
- Anti-motility agents - opioids such as coedine or loperamide
