B5.010 - Small and Large Intestine Flashcards
what is sucrose digested to
glucose and fructose
how does fructose enter the cell
GLUT 5 transporter
what is lactose digested into
glucose and galactose
uptake of glucose and galactose is depended on what
SGLT1 transporter in the apical membrane
what is the energy source for the SGLT1 transporter
its secondary active transport
gets energy from electrochemical gradient produced from Na/K ATPase in the basolateral membrane
how do monosaccharides leave the cell
through basolateral membrane via specific protein channels (GLUT2), sodium independent facilitated diffusion
describe Na absorption
Na traverses the apical membrane of the enterocytes through nutrient coupled Na transparent, Na/H exchange, Na channels
what generates the electrochemical gradient that provides the driving force for Na absorption
Na/K ATPase
what possible additional mechanisms are involved with Na absorption in small and large intestine
chloride/bicarb exchanger DRA or putative anion transporter 1 PAT1
what does DRA do
exchanges Cl/Bicarb
what dose KCC1 do
pottasium/2Cl cotransporter
what is ENaC
apical epithelial sodium channels allows sodium to enter epithelial cells
how does sodium exit the eipthelial cell
Na/K ATPase
describe water absorption in the stomach
Na pumps increase Na concentration in the intercellular space which increases the osmotic pressure, water flows across the brush border and out the sides of the intestinal epithelial cell to the paracellular space, this increase in hydrostatic pressure pushes fluid into capillaries
how much water is ingested/day
2 L
how much gastric secretions are released each day
7 L
where in the GI is water added to chyme
duodenum
where is the major site for water absorption
jejunum
how much water / day is absorbed in the colon
1.4 L
how much water is lost in feces
.1 L
describe chyme
hypertonic
osmolarity increases as digestion begins
what does the hypertonicity of chyme do
draws water into the intestine
what does Cl- do in the lumen of the intestine
draws Na and water into the lumen
describe bacterias effect leading to diarrhea
bacterial toxins can activate adenylate cyclase resulting in prolonged state of open CFTR leading to diarrhea
describe the electrolyte secretion by epithelial cells of intestinal crypts
Na is pumped out of cell
Cl-, Na+ and K+ are cotransported into cell by Na/K/2Cl cotransporter
K+ leaves by KCNQ1 potassium channels
what do activated mast cells do in intestine
release histamine, which either directly acts on epithelial cells or acts on submucosal neurons to stimulate release of ACh which then acts on epithelial cells
Ca absorption in small intestine
calcium enters intestinal epithelial cells via a calcium channel driven by ec gradient.
describe the movement of Ca within the cell
calcium bound to calbindin; calcium within the membrane bound vesicles. A Ca-ATPase moves caclum across basolateral membraen. A Na-Ca antiporter also moves calcium
in what state does iron have to be to be transported into the cell
ferrous (2+)
what is DMT1
transports Ferrous iron into the enterocyte
what is Dcytb
on enterocyte, converts Fe3+ into Fe2+
what happens with haem in the small intestine
the haem transporter takes up haem and then haem oxygenase converts it to Fe2+ for intracellular iron pool (ferritin)
what is hephaestin
converts Fe2+ to Fe3+
what is ferroportin
the channel that allows Fe2+ to exit the cell
where do you find vitamin B12
animal products like meat, eggs, milk, fish, bacteria in gut also make it
what is R protein
made by salivary gland protects vitamin B12 from acid degradation
what is IF
intrinsic factor made by parietal cells int eh dtoma hc but is outcompeted for vitamin B12 by R proteins
what happens to R proteins in the small intestine
get digested by pancreatic enzymes and IF and Vit. B12 associate and are resistant to degradation
where in the SI are there receptors for IF-Vit B12
distal ileum
what is transcobalmin 2
moves B12 into the blood
where is water absorbed
jejunum
where is Na absorbed
can be absorbed on any part of intestine, net rate highest in jejunum
where is HCO3+ absorbed/secreted
jejunum - absorbed
secreted - everywhere else
where is Cl- absorbed
all of SI
where is K+ absorbed
jejunum
ileum
where is Ca2+ absorbed
all segments of SI especially in duodenum and jejunum
where is iron absorbed
duodenum
where is Mg2+ absorbed
along entire length of intestine
most in ileum
where is phosphate absorbed
along entire length of intestine
duodenum > jejunum > ileum
where is copper absorbed
jejunum
what are the 3 .phases of interdigestive motility
1 - motor quiescence of stomach and duodenum
2 - sporatic peristaltic waves, segmenting contractions and peristaltic waves
3 - contraction of resivoir, forceful peristaltic waves
what do phase 3 peristaltic waves do
clean the intestinal segment from chyme and bacteria which accumulates aborally. Because of the successive waves tart and end further aborally het phase 3 slowly migrates distally
what is the migrating motor complex
starts 2-3 hr after digestion of a meal
triggered by hormone motilin
aborally clears small intestine of undigested debris
consists of 3 motility phases
when does a new MMC begin
when a MMC reaches the ileum
where does MMC start
antrum
when is the time between MMC cycles longer
during the day
when does the MMC terminate
when food enters the SI
will IV nutrition terminate MMC?
no
what hormones/chemicals terminate MMC
gastrin and CCK except in ileum
what happens to digestive activity in the small intestine during sleep
reduces phase 2 activity and increases duration of phase 1
what does ingestion of a meal do to interdigestive motility
suppresses it and induces fed motor patter
describe postprandial motility
characterized by a lower amplitude of the antral waves occurring at maximal frequency, rhythmic pyloric opening and closure and coordinated duodenal contractions occurring in sequence with the antral waves
what NTs and neurohormones alter GI motility
ACh Serotonin Gastrin CCK (gall bladder, SI) Enkephalin PP - colon Substance P motilin
what NTs and neurohormones decrease motility
NE Somatostatin Secretin CCK (stomach) Enteroglucagon GLP1 Adenosine NO VIP GIP PP
what is motilin
produced in duodenal mucosa by M cells in response to stimulation via vagus nerve
what does motilin do during fasting
may initiate phase 3 of MMC but the trigger is unknown
when is MMC terminated
when food enters the small intestine
what hormones can germinate the MMC
gastrin and CCK
describe regulation of chyme into the cecum
chemoreceptors and mechanoreceptors and mechanoreceptors in cecum feedback into ileum and ileocecal sphincter to regulate chyme entry
what does the cecum do to allow entry of chyme
undergoes receptive relaxation similar to proximal stomach
what prevents backflow to SI from cecum
ileocecal sphincter, releases with jejunum distended, contracts when colon distended
efficiency of large intestine in reabsorbing water
> 90%
what does the large intestine do
reabsorbs water
absorbs electrolytes
stores fecal matter
evacuate 200-300 ml solid stool/day
what does the ascending colon do
receives chyme from ileum
receptive relaxation
short transit time
resevoir for transverse colon
what does the transverse colon do
primary region for absorption
long transit time (like 24 hrs)
where is the splenic flexure
between the transverse and descending colon
what does the descending colon do
completion of absorption
long transit time
sigmoid colon highly distensible for collection of feces
what is segmented motility
similar to mixing patter of SI but less dynamic and slower moving
circular and longitudinal muscle contractions; net movement is aboral
what are haustra
unstimulated areas of the small intestine, disappear and reappear with contractions and reform at other loci
what are power propulsions
mass movement 1-3x/day
movement along large segment of colon occurs in transverse to sigmoid colon or rectum
what triggers power propulsinos
arrival of large volume of chyme into cecum and transverse colon, also in response to gastrocolic reflex, response to stretch in stomach or digestion products in small intestine
what happens to haustra in power propulsions
they disappear
describe the internal anal spincter
smooth muscle that is tonically contracted, principle responsible for maintaining continence during the resting state
why is the rectum so compliant
bc it needs to distend when theres a bunch of stool that you cant evacuate yet
what happens when the rectum becomes distended
triggers rectoanal reflex which results in relaxation of the internal anal sphincter so that sampling of contents can be made
what other than the sphincter helps contribute to the closure of the anus
mucosal folds and anal endovascular cushions
describe the external spincter
skeletal muscle
contracts during voluntary squeeze and is responsible for maintaining continence when feces enter the rectum
filling of the rectum with stool causes what
initial decrease in internal sphincter tone, which is counterbalanced by a reflex contraction of external anal sphincter
describe the holding phase of defecation
rectal distension
sensory perception of stool
contracted puborectalis
contracted external anal sphincter
describe the neuronal pathways for defecation reflex and fecal continence
when the rectum is distended neural pathways from chemo and mechanoreceptors send inhibitory motor signal to internal sphincter
mechano/chemo receptors in anal canal determine consitency of contents
what does the conscious cortex do in the context of fecal continence
makes appropriate decision to control external anal and puborectalis muscles
when does the puborectalis contract and relax
contracts to maintain continence, causesa nd acute rectoanal angle which helps maintain continence
relaxes to allow for defecation - allows widening of the rectoanal angle for defecation
describe the muscular actions for defecation
relaxation of external anal sphincter and puborectalis muscle
contraction of abdominal muslces and diaphragm
power propulsions in sigmoid colon and rectum
which muscles in the actions for defecation are under voluntary control
external anal sphincter and puborectalis
what is intestinal obstruction
partial or complete blockage of the bowel resulting in failure of contents to move through intestine
what causes intestinal obstruction
mechanical
bowel doesnt work correctly (ileus)
causes of mechanical intestinal obstruction
abnormal tissue grwoth adhesions or scar tissue foreign bodies gallstones hernias impacted feces itussusception volvulus
what is a pseudo obstruction
impairment to intestinal motility resembles mechanical obstruction but no blockage is present
underlying symptoms due to nerve, muslce, ICC issues
chronic pseudo obstruction causes
neuropathic - DM, dysautonomia, CNS, panneoplastic, chagas
myopathic - scleroderman, amyloid, polymysitis
secondary pseudo obstruction
chemical, electrolyte, mineral disturbances
complications of intra abdominal surgery
decreased blood supply
injury, infection, kidney disease
what is bloat
sensation is common in women
most often due to abdominal distension (not increased gas)
associated with diminished propulsion of small and large bowel
if gas the cause then due to incomplete digestion and absorption of carbs in SI and bacteria in colon
what is dyssynergic defecation
prolonged colonic transit time, discoordination of abdominal rectoanal and pelvic floor muscles, rectal hyposensitivity paradoxical increase in sphincter pressure, inadequate propulsion forces
what does dyssynergic defecation resultin
chronic constipation due to pelvic floor dysfunction i.e. muscles and nerves are not functioning properly
what is fecal incontinence
involuntary passage of fecal material
causes of fecal incontincence
weakness of anal sphincter loss of sensation of rectal fullness constipation stiff rectum due to scarring diarrhea
Dx of hirschsprungs disease
newborns fail to have their first bowel movement within 48 hrs of birth
rectal biopsy; absense of ganglion cells in submucosal and myenteric plexuses
failure of normal peristalsis and relaxation of internal anal sphincter
what is hirschprungs disease
loss of intrinsic nerves to sort segment HD (80% aganglionosis in rectosigmoid)
or long segment HD (15-20% up to splenic fixture and beyond)
neural crest cells stop growing along intestine towards anus
genetic contribution to hirshsprungs disease
a chromosome abnormality present in 12%
most common down syndrome
most common gene associated with HD
RET - proto oncogene, tyrosine kinase receptor
what does RET stimulate
survival, proliferation, migration, differentiation of neural crest cells
cephalic phase of digestion
stimuli include sight, taste, sound, or thought of food which are processed by brain
describe the gastric phase of digestion
stomach is notified by brain that it should prepare for arrival of food. Stomach leaves interdigestiive phase and starts a low level of motor and secretory activity. 30% before food arrives
what mediates the gastric phase
vagus to the DMN of vagus in the medulla, PARA efferents target direct and indirect pathways
what does the pancreas do
ACh from vagus stimulates pancreatic secretions
short lived dissipates rapidly
what does the gall baldder do
contracts (vagal efferents ACh)
Sphincter of Oddi (vagal efferents act on intrinsic neurons that release NO and VIP)
describe the esophageal phase
mechanical stimulation of pharynx and esophagus initiates reflex pathways via the brainstem or intrinsic pathways via the ENS
peristalsis
relaxation of UES
relaxation of proximal stomahc transfer of bolus
protection of distal esophageal mucosa
what is the vasovagal gastropancreatic reflex
detects presence of food in stomach and stimulates secretions from pancreas
how does the sphincter of Oddi relax
vagal efferents act on intrinsic neurons that release NO and VIP
what are enterogastrones
hormones that inhibit stomach processes like aid secretion released when intestinal duodenal phase starts
describe the role of I cells in duodenal phase
factors like lipids stimulate I cells to release CCK which activates vagus resulting in stimulation pancreatic acinar cell production of enzymes
what do S cells do in duodenal phase
lipids and other factors also stimulate release of secretin from S cells which increases bicarb and water secretion by ducts of pancreas and gallbladder
gastric acid better at stimulating HCO3- production than enzymes
what is the major site for digestion and absorption of nutrients in small intestine
jejunum
what is peptide YY
released by ileum to inhibit gastric functions
