Large intestine Flashcards
what is the function of proximal large intestine ?
Cecum . ascending colon , small portion of transverse colon
Absorption of water and electrolytes mainly
1000-2000 ml of isotonic chyme each day
90% fluid reabsorbed
200-250 ml of semisolid feces
what is the function of distal large intestine ?
Most of transverse colon , descending colon, sigmoid colon
storage of feces
what is the significance of ileocecal sphincter ?
prevent feces reflux into small intestine
in large intestine we have plentiful helpful bacteria which benefit from feces
In small intestine , it might be harmful and cause infection and disruption of normal intestinal biome
what are the movements of large intestine ?
Similar to small intestine :
Mixing movements : haustration
Propulsive mass movement
but its slow and sluggish cuz we dont need forceful or fast contraction to propel feces
describe the mixing movement ? Haustration
Large circular constriction occur by contraction of the circular muscle
at the same time the three longitudinal strips of muscle —> Tenia coli also contract
The unstimulated portion bulge out in form of bags like sacs —> Haustration
Haustration occur when both muscles contract
Peak intensity in about 30 seconds and then disappear during the next 60 seconds
After few minutes, new haustral contraction occur in other areas nearby , as the content is slowly dug into and roll over
Feces exposed to the mucosal surface –> absorption of fluid and dissolved substances
Minor forward propulsion of contents in colon
80 to 200 ml of feces are expelled each day
when matter is in the absorbing portion of small intestine its called chyme
when it reaches the feces storage its called fecal matter
describe propulsive mass movement ?
1 to 3 times per day , especially for 15 minutes in the first hour after breakfast
Slow but powerful contractions move contents over long distances
Chyme becomes fecal in quality
Semisolid instead of semifluid
A series of mass movements persist for 10 to 30 minutes –> then they cease –> RETURN HALF DAY LATER
Desire for defecations - once mass of feces is in rectum
what are the sequence of events in mass movement /propulsive ?
Modified peristalsis
much of propulsion in cecum and ascending colon is from haustral contraction ( haustral contraction are modified peristalsis cuz they are slower and more powerful )
Constrictive ring appear due to distension of colon usually in transverse colon
Colon distal to the constrictive ring LOSES haustration and contract as a unit
Fecal material propelled down the colon
for about 30 seconds the contraction develop more force
Relaxation in the next 2-3 minutes
Another mass movement occurs
Cecum + ascending colon –> Haustrations
Transverse colon —> mass movement from constrictive ring + unit contraction distally
what is the gastrocolic reflex?
When you eat food your stomach get distended
it makes large intestine propel food since more is coming to the stomach
VIA AUTONOMIC NERVOUS SYSTEM
what is duodenocolic reflex ?
similar to the gastrocolic reflex
when the duodenum gets distended it will make the colon propel more cuz more food coming
also via Autonomic nervous system
what are 2 hormones in distention ?
Gastrin –> released in response to food in the stomach ( gastric distention )
CCK –> coming from the duodenum tells the stomach to decrease gastric emptying but INCREASE COLON EMPTYING
what else stimulate the motility of colon ?
irritation of colon also initiate mass movement
which explains diarrhea in ulcerative colitis
what is peritoneointestinal reflex?
inhibition of excitatory enteric nerves —> intestinal paralysis
Due to irritation of peritoneum
like peritonitis
what is renointestinal and vesicointestinal reflex?
inhibition of intestinal activity as a result of kidney or bladder irritation
describe defecation ?
Mostly rectum is empty –> to avoid irritation of the rectum , once fecal matter reaches there, defecation reflexes are initiated , rectum doesnt have enough capacity to store fecal matter
existence of weak functional sphincter between sigmoid colon and rectum
Sharp angulation provide additional resistance to filling
Continuous dribble of fecal matter through the anus is PREVENTED by TONIC CONSTRICTION
INTERNAL ANAL SPHINCTER :
Thickening of circular smooth muscle
supplied by sympathetic and parasympathetic lies immediately inside the anus , Sympathetic is excitatory to internal anal sphincter m parasympathetic is inhibitory ( so para will stimulate defecations )
EXTERNAL ANAL SPHINCTER :
Striated voluntary muscle
controlled by PUDENDAL nerve ( voluntary control )
surrounds the internal anal sphincter and extends distal to it , kept continuously constricted ( unless voluntary relaxed )
Urge to defecate occurs at rectal pressure : 18 mmHG
Rectal pressure at 55 mmHG will result in reflex expulsion of fecal contents
describe defecation reflexes?
it is mediated by enteric nervous system in the rectal wall :
Stimulus : feces enter the rectum , distention of rectal wall
Receptors : stretch receptor in rectal wall
Afferent signals : sensory fibers
Center : Myenteric plexus
Efferent : motor signals to smooth muscles
Effector : Smooth muscle cells of descending colon , sigmoid, rectum
Response :
1- peristaltic wave forcing feces toward anus
2- Internal anal sphincter relaxed by inhibition from myenteric plexus ( due to distension of rectal wall )
3- If external anal sphincter also relaxed voluntarily defecation occurs
describe external parasympathetic defecations reflx?
Normally myenteric defecations reflex is weak
Parasympathetic defecation reflex involving SACRAL SEGMENTS of spinal cord strengthens it
Nerve ending in the rectum stimulated
Signals sent to spinal cord
Reflexly back to the descending colon, colon , sigmoid , rectum and anus through parasympathetic nerve fibers IN PELVIC NERVES
Peristalsis intensified and internal anal sphincter relaxes
Intrinsic myenteric reflex strengthened
THIS IS FROM THE SPINAL CORD/sacral segments ( EXTERNAL ) NOT INSTRINSIC
what are other factors affecting defecation ?
Taking a deep breath
Closure of glottis
Contraction of abdominal wall muscle
Pelvic floor relaxes downward and pulled outward
These are produced from signals entering spinal cord
Newborn babies and people with transected spinal cords, defecation reflexes causes automatic emptying of lower bowel because of lack of conscious control of external anal sphincter
describe voluntary defecation ?
Initiated by straining :
At rest normally 90 degree angle is between the anus and rectum and the contraction of puborectalis muscle inhibits defecation
After straining
pelvic floor is lowered 1-3 cm puborectalis muscle relaxes and anorectal angle is reduced to 15 or less
then relaxtion of external anal sphincter —> defeaction
defecation is parasympathetic spinal reflex which can be voluntarily initiated or facilitated
It is not advided to voluntarily defecate cuz the natural defecation reflexes will see that theres no need for them and they become inhibited and lead to constipation
describe absorption in the large intestine ?
Tight junctions are tighter than those that in small intestine –> PREVENT BACK DIFFUSION, IMPERMEABLE EPITHLEIUM
Apical side : Na is absorbed and K secrete channels
Basolateral membrane : Na is secreted to the blood and K is taken inside via Na/K pump
Aldosterone :
increases Na channel synthesis
Increase Na absorption at the apical membrane
Increase Na pumping outside to the blood via basolateral membrane via NA/K pump
K pumped into the cell via Na/K pump and then its secreted at the apical side
Cl absorption follow Na absorption , electrical potential gradient
Water moves down the gradient, absorbed in colon and water permeability is less than small intestine
Maximum absorption capacity of large intestine : 5 to 8 Liters of fluid and electrolytes per day
Excess appears in the feces as in diarrhea
describe large intestines secretions?
Mucosa of large intestines has CRYPTS OF LIEBRKUHN but NO VILLI
epithelial cells secrete no digestive enzymes there are mucus cells which produce ONLY MUCUS
non mucus secreting epithelial cell secrete BICARBONATE IONS
Secretion contain mucus and moderate amounts of bicarbonate ions
Secretion regulated by TACTILE stimulation of epithelial cell lining, LOCAL NERVOUS REFLEXES and PARASYMPATHETIC NERVES
Mucus provide protection from abrasions . bacterial activity and alkainity of secretion protects from acid formed in the feces
Mucus also work as adherent material to hold fecal matter together
how are large intestinal secretion produced?
althought predominantly theres absorption , yet secretion also take place to maintain the fluidity of intestinal contents required for diffusion, mixing movement of the meal and residues , etc
CL enter epithelial cell via Na/K/2CL co transporter in the basolateral membrane
Cl secreted in lumen —-> VIA cystic fibrosis transmembrane conductance regulator
( CFTR ) as well as perhaps via other chloride channels
K channels are present on the luminal and basolateral membranes of ENTEROCYTES OF COLON
K is secreted into the lumen
describe colon bacilli ?
bacteria present normally in the absorbing colon
Capable of digesting small amounts OF CELLULOSE
Produce vitamin K and vitamin B12, Thiamine, Riboflavin and various genes –> Carbone dioxide, hydrogen and methane that contribute to flatus
what is the composition of feces?
3/4 water
1/4 solid matter :
30% dead bacteria
10-20% fat
10-20% inorganic matter
2-3% protein
30% undigested roughage
Brown color : Stercobilin and urobilin, derivates of bilirubin
Odor : indole, skatole, mercaptans , Hydrogen sulfide
describe aganglionic megacolon?
Hirschsprung disease genetically determined :
Condition of abnormal colonic movement
Clinical presentation :
Abdominal distention , anorexia, lassitude , bowl movement occur once every several days
Treatments :
Resection of aganglionic segment and anastomosis of the remaining segment with the rectum
if extensive segment of colon involved then this treatment is not possible, option is colectomy
Pathophysiology :
Congenital absence of ganglionic cells in myenteric plexus —> RESULT OF FAILURE OF NORMAL CRANIAL TO CAUDAL migration of neural crest cells during development
Impaired peristaltic motility and defecation reflex
Children with the disease may defecate as infrequently as once every 3 weeks
