this was a presentation i did in my second year medical school. though about sharing it it

1. GIT PHYSIOLOGY
THE LARGE IN TESTINE
BY
• munyar
• Dread
• Gaffer

2. OBJECTIVES
 Describe the mechanisms, localization and regulation of colonic sodium
absorption.
 Describe the mechanisms mediating colonic bicarbonate and potassium
transport.
 Describe the role of dietary fiber in promoting colonic motility.
 Describe the factors contributing to intestinal and colonic gas composition and
the consequences of an altered colonic microflora.
 Describe the role of short chain fatty acids in colonic sodium absorption and in
both colonic and body energy metabolism.
 Describe the related roles of fluid malabsorption in the small intestine versus
colon on the potential to cause diarrheal disease.
 Describe the normal regeneration of the colonic epithelium from stem cells,
and how this process is changed if a stem cell becomes cancerous or in the
presence of inflammation.

3. Large Intestine:
 Extends from ileocecal junction to anus
 Consists of cecum, colon, rectum, anal canal
 Movements sluggish (18-24 hours)

4. Large Intestine
 Cecum
 Blind sac, vermiform appendix attached
 Colon
 Ascending, transverse, descending, sigmoid
 Rectum
 Straight muscular tube
 Anal canal
 Internal anal sphincter (smooth muscle)
 External anal sphincter (skeletal muscle)
 Hemorrhoids: Vein enlargement or inflammation

5. histology
• Simple columnar
• Absence of villi
• Presence of goblet cells
• Deep intestinal glands
• crypts

6. Functions of the Large Intestine
 Reabsorb water and compact material into feces
 ELECTROLYTE ABSORPTION
 Absorb vitamins produced by bacteria
 Store fecal matter prior to defecation
 NB: Most of the absorption occurs in the proximal half of the colon
(absorbing colon), the distal colon (storage colon) functions to store
faeces until time of excretion

7. Na+ absorption
 Mucosa of the large intestine has a large capability for Na absorption
(active)
 Electrical dy/dx created by this absorption causes Cl absorption as well
 Na ions are far more absorbed here than in the small intestine
 Aldosterone enhancement
 increasing expression of the basolateral NaK ATPase and luminal Na+ ion
channels.
 This Na and Cl absorption creates an osmotic gradient which in turn
causes absorption of water

8. Electrogenic Na+ absoption
 Is by epithelial sodium channels (ENaC)
 LUMEN SIDE
 Inhibited by amilorides and diuretics
 Na under an electrochemical gradient
 Na absoption is accompanied by Cl as well via the CFTR
 Basolateral Na/K pump
 Aldosterone increases these channels

10. Electroneutral transport
 Bulk transport of Na by luminal Na/H & Cl/HCO3 exchanger
 Na transport in ascending colon is mainly by electroneutral transport
while in the desending colon its electrogenic (via which channels and
under the enhancement of what hormone?)
 Parrallel Na/H and Cl/HCO3 exchanger on the colonic epithelial cells
 Transport of Na is coupled via changes in intracellular pH and[CL]
 Basolateral Na/K ATPase generates the driving force for the luminal Na
by lowering intracellular Na
 Regulated by Na depletion

11. Electroneutral mechanism

12. Regulation of Na Absorption
1. feedback regulation
 Ocuurs on ENaCs
 Na has a negative impact on the activity of the channels
 This occurs due to changes I intracellular Na
 Increase in [Na] in the colon cells downregulates and eventually endocytosis of ENaC
2 Steroid-Dependent Regulation of Ion TransporT
 upregulated by glucocorticoid and mineralocorticoid hormones.
 This leads to enhanced Na1 absorption and K1 secretion
 aldosterone induces only electrogenic absorption
 upregulation of the activity of other transport proteins like the basolateral
Na1-K1-ATPase by aldosterone is another mechanism by which aldosterone regulates Na1
absorption.

13. Fluid malabsorption disorders
Cholera
 Cholera bacillus produces a toxin that enters the cells
 toxin binds to the alpha unit of the Gs inhibiting the GTPase activity
 Prolonged stimulation of adenylyL cyclase increases cAMP
 Increased Cl secretion
 Decreased absorption of Na and Cl
 Water and electrolytes

14. Absorption of short chain fatty acids
a)In parallel with the absorption of NaCl, SCFA
are absorbed by the colonic epithelium usually by non ionic diffusion
 SCFA are produced during fermentation of dietary fibers by colonic bacteria.
b)an additional SCFA/HCO3 exchange mechanism
 Cl absorption is stimulated by increased HCO3 production during SCFA metabolism and
stimulation of the apical Cl2/HCO3 2 exchanger.
 Absorption of SCFA not only serves as an additional energy supply for colonic epithelial
cells, but has also a significant impact on NaCl absorption
 acidification of colonocytes and activation of apical Na1/H1 exchangers


15. Cont….
 Absorbed SCFAs are metabolised and contribute to calories
 In addition, they exert a trophic effect on on colonic epithelial
cells
 They also combat inflammation
 They are absorbed in part by ab exchange mech for H+,
thereby helping to maintain acid-base equilibrium
 They also promote absorption of Na+

16. Role of dietary fibre
 Reduces risk of colon canser
 Increased fecal weight
 Shortened transit time
 Dilution of colonic contents
 Stimulation of colonic microbial growth
 lower your risk of developing haemorrhoids
 solidify the stool because it absorbs water and adds bulk to stool

17. Motility of the colon
 Movements include: 1. segmental contraction
2. peristaltic waves
 Segmental contraction mix up the contents of the colon,
exposing them to the mucose, thence facilitating absorption
 Peristaltic –propel towards the rectum

18. Movement in Large Intestine
 Mass movements
 Common after meals
 Local reflexes in enteric plexus
 Gastrocolic: Initiated by stomach
 Duodenocolic: Initiated by duodenum
 Defecation reflex
 Distension of the rectal wall by feces
 Defecation
 Usually accompanied by voluntary movements to expel
feces through abdominal cavity pressure caused by
inspiration

19. Reflexes in
Colon and
Rectum:

20. Absoprtion in the large intestine
 About 1500 ml of chyme pass thru the ileolcecal valve each day
 Most of the water and electrolytes in the chime are absorbed in the colon
 The large intestine can absorb up to 5-8 liters of fluid and electrolytes each
day

21. Defacation
 Distension of the rectum with faeces initiates contractions
 SNS-excitatory, PNS-inhibitory…in internal(involuntary) anal sphincter
 External anal sphincter-skeletal(which nerve)
 The urge to defacate occurs when the rectal pressure is about 18mmHg
 At >55mmHg , reflex defecation
 Normally there is a 90 degree anorectal jxn which inhibits defecation,in combination
with puborectalis
 With straining abdominal muscles contract puborectalis muscles relax; anorectal jxn is
reduced to 15 degrees

22. Defecation Reflex
1. Distension of the rectum.
2. Stimulation of the stretch
receptors in the rectum.
3. A. Short reflex: Stimulation of
myenteric plexus in sigmoid colon
and rectum.
B. long reflex: stimulation of
parasympathetic motor neurons
in sacral spinal cord.
C. stimulation of somatic motor
neurons.
4. Increased local peristalsis.
Relaxation of internal anal
sphincter and contraction of
external anal sphincter.

23. Gastrocolic reflex
 Distension of the stomach
 Contractions of the rectum, the desire to defacate
 It can be amplified by an action of gastrin on the colon
 Because of the response, defeaction after meals
commonly occurs in children
 Same as duodenocolic reflex

24. Composition of faeces
 about 75% of fecal weight is made up of water. The other 25% is
composed of solid matter which contains :
1.Undigested fiber and solidified components of digestive juices (30%)
2.Bacteria (30%)
3.Fat (10% to 20%)
4.Inorganic matter (10% to 20%)
5.Protein (2% to 3%)

25. Color and odour
 Brown…….largely due to bilirubin
 Which is converted to urobilin and stecobilin to give give stool its typical color
Odour
 differs among people
 Largely influenced by the foods eaten
 Due to bacterial action
 The actual odoriferous products includeindole, skatole, mercaptans, and hydrogen
sulfide

26. hahahahaha

27. Secretions of the Large Intestine
Mucus Secretion.
 The mucosa of the large intestine has many crypts of Lieberkühn.
 Absence of villi.
 The epithelial cells contain almost no enzymes.
 Presence of goblet cells that secrete mucus (provides an adherent
medium for holding fecal matter together).
 Stimulation of the pelvic nerves from the spinal cord can cause
marked increase in mucus secretion. This occurs along with increase
in peristaltic motility of the colon.
 During extreme parasympathetic stimulation, so much mucus can
be secreted into the large intestine that the person has a bowel
movement of ropy mucus as often as every 30 minutes; this mucus
often contains little or no fecal material.

28. Roles of the mucus
 protects the intestinal wall against excoriation,
 it provides an adherent medium for holding fecal matter
together.
 protects the intestinal wall from the great amount of bacterial
activity that takes place inside the feces, and,
 finally, the mucus plus the alkalinity of the secretion (pH of 8.0
caused by large amounts of sodium bicarbonate) provides a
barrier to keep acids formed in the feces from attacking the
intestinal wall.

29. constipation

30. constipation
 pathological decrease in bowel movements
 However, many normal humans defecate only once every 2–3 d, even though
others defecate once a day and some as often as three times a day
 the only symptoms caused by constipation are slight anorexia and
mild abdominal discomfort and distention.
 Some causes of constipation include:
1. Antacid medicines containing calcium or aluminum
2. Changes in your usual diet or activities
3. Colon cancer

31. references
 Ganong
 Guyton
 https://www.ncbi.nlm.nih.gov/pubmed/10089109
 Roles of dietary fibre
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1704926/?page=1

32. THANK YOU
Musayiiita iyoyo yekubhombana
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