physiology of large intestine

week 3 ctb

large intestine

• caecum

• ascending, transverse, descending and sigmoid colon

• rectum

• anal canal

• ileocecal sphincter prevents reflux into ileum

colonic musculature

• circular muscle of colon contracts to divide colon into segments called haustra

• longitudinal muscles are 3 non overlapping bands known as taeniae coli

• coupling of circular and longitudinal muscles less efficient than in SI

structure of LI

• no villi

• lined by surface epithelial cells

• surface interspersed with colonic crypts

• surface epithelial cells primarily responsible for electrolyte absorption

• colonic gland cells responsible for ion secretion

• folds increase SA

functions of LI

• reabsorption of Na⁺, Cl^-, and water

  • production of semi solid or solid faeces

• absorb short chain fatty acids produced by colonic bacteria

• secrete K⁺ , HCO₃^- and mucus

• make and store faeces

• moves faeces towards rectum

faeces

• non digested/non absorbed dietary food products

• colonic bacteria and metabolic products

• dead epithelial cells

• biliary metabolites

• water

• excretory products

  • iron and copper

  • anions and cations

intestinal fluid and electrolyte transport

• fluid load to SI: 8-9L/day

• 6.5L absorbed by SI

• 1.9 L absorbed by colon

• 0.1L excreted in faeces

• max absorptive capacity is 4-5L/day

tight junctions

• fluid and electrolyte absorption take place by transcellular and paracellular routes

• permeability of tight junctions determines which route is used

• tight junctions in SI are leaky

• tight junctions in LI are tight

• majority of fluid movement in colon is via transcellular route

what drives water absorption in the colon?

• NaCl absorption

• short chain fatty acid absorption

• absorption of Na⁺

NaCl absorption

• coupled uptake of Na⁺ and Cl^-

• stimulated by various growth factors

• inhibited by various hormones

• water follows NaCl

absorption of short chain fatty acids

• short chain fatty acids absorbed from lumen by surface epithelial cells

• absorbed in a Na⁺ dependent manner

• symporters

• uptake driven by low intracellular Na⁺ conc established by basolateral Na⁺K⁺ ATPase 

• short chain fatty acids used as an energy source

absorption of Na⁺

• localised to distal part of colon

• driven by epithelial Na+ channel: ENaC (also found in kidney)

• channel opens in response to neurotransmitters/hormones

• chloride ions follow via paracellular route to maintain electrical neutrality 

• water absorbed via paracellular route as a result of transepithelial osmotic gradient

• last line of defence to prevent excessive water loss in stool

chloride absorption

• Cl- absorption occurs throughout SI and LI

• closely linked to Na+ absorption

• can be passive:

  • driven by electrochemical grad for Cl-

  • paracellular or transcellular

• driving force for Cl- absorption derives from eitherL

  • nutrient coupled transport

  • ENaC (epithelial sodium channel) channels in distal end of colon

chloride secretion

• simulated by secretogogues: ACh

involves 3 transport pathways on basolateral membrane: 

• Na⁺-K⁺ ATPase

• Na⁺/K⁺/Cl^- cotransporter

• K⁺ channels

• Cl- channel present on apical membrane

• Na⁺-K⁺ ATPase generates a low intracellular Na⁺ conc

• generates driving force for Cl- entry through Na/K/Cl cotransporter

• intracellular Cl- conc rises and electrochemical grad favours Cl- secretion acrtoss apical membrane

control of chloride secretion

• Cl- secretion requires stimulation by secretogogues:

  • bacterial exotoxins

  • hormones and neurotransmitters

  • products of cells of immune system

  • laxatives

• works to increase Cl- conductance of the apical membrane either by activating channels ro inserting new channels

diarrhoea

• induction of apical Cl- channels is important in pathophysiology of diarrhoea

  • both a symptom: increase in no. of bowel movements or decrease in consistency

  • sign: increase in stool volume

• osmotic diarrhoea: failure to absorb a dietary nutrient

• secretory diarrhoea: endogenous secretion of fluid and electrolytes from intestine

  • infections with E. coli and cholera

  • raise concs of secondary messengers

LI motility

• material not absorbed in SI enters LI

• faecal material moves from caecum through colon to rectum

• proximal colon:

  • non propulsive segmentation

  • mass peristalsis

control of LI motility

• proximal colon stimulated by vagal input + ENS

• remainder innervated by pelvic nerves

• voluntary input from spinal cord via branches of pudenal nerves

• regulates contraction of external anal sphincter

• regulates pelvic floor muscles

non-propulsive segmentation contractions

• produces circular muscle contractions

• churn colonic contents

• material retained in proximal colon

• allows continuation of fluid and electrolyte absorption

• occurs in caecum and proximal colon

• mixes contents of LI

• associated with sack like segments called haustra

mass movements

• function to move contents of LI over long distances (mass peristalsis)

• occurs 1-3 times/day

• water absorption occurs in distal colon

• faecal contents become semi solid and increasingly difficult to move

• final mass movement propels faecal contents into rectum for storage until defecation

distal colon

• responsible for drying of faeces

• contractile movements of sigmoid colon causes retention of colonic contents

• promotes drying of faeces and reduction in faecal volume

• retention can be 36h or longer

• promotes water retention

rectum

• colon terminates in the rectum

• rectum and colon joined at an acute angle: rectosigmoid junction

• rectum lacks circular muscle

• surrounded by longitudinal muscle fibres

• reservoir for storage of faeces

• rectum joints anal canal

internal sphincter

• has both circular and longitudinal muscle

• involuntary control

external sphincter

• encircles rectum

• contains only striated muscle

• controlled by both voluntary and involuntary mechanisms

• disention of rectum inititates rectosphincteric reflex

defecation

• entry of faecal matter into rectum causes contraction of smooth muscle in rectal wall

• internal anal sphincter relaxes in the rectosphincteric reflex

• external anal sphincter relaxes in the rectosphincteric reflex

• external anal sphincter is still tonically contracted

• once rectum fills to 25% capacity, there is an urge to defecate

• when appropriate external anal sphincter is voluntarily relaxed, smooth muscle of rectum contracts

• faeces forced out through anal canal

enteric microbial ecosystem

• vast assortment of bacteria and other microorganisms

• established shortly after birth

• matures as child grows

• fluctuates depending on diet and circadian rhythms

• drastic perturbations can be caused by ABs or aggressive pathogens

roles of colonic microbiota

• metabolic reactions (e.g undigested polysaccharides)

• formation of secondary bile acids

• conversion of bilirubin to uribilinogen

• salvage nutrients

• degrade digestive enzymes

• digest mucus

• synthesise certain vitamins (e.g vit K)

limiting growth of pathogenic microorganisms

• synthesis and secretion of microbicidal compounds

• functions as a physical barrier

• influences gene expression in epithelium