GI 3 Small Intestine and Colon: Motility, Digestion and Absorption Flashcards
what is the valve that separates the stomach and the duodenum
pyloric valve
what is the valve that separates the ileum and the colon
ileocecal valve
what is the valve that separates the ileum and the colon
ileocecal valve
what are the segments of the small intestine in order
duodenum -> jejunum -> ileum
where does most secretion occur in the small intestine
duodenum
where does mot digestion occur in the small intestine
duodenum
where does most absorption occur in the small intestine
duodenum
what does the duodenum secrete
CCK
secretin
- GIP
HCO3-
what does the ileum secrete
PYY and HCO3-
where does the smallest amount of digestion and absorption occur
ileum
what does the dudoenum absorb
Fe
what does the ileum absorb
bile acids and vitamin B12
what are the motility patterns that occur in the duodenum
MMC
segmentation
peristalsis
describe the fasting pattern of motility in the small intestin
MMC
- uses motilin to sweep intestines of undigested material
- one every 90 minutes
describe the feeding pattern of motility in the small intestine and the primary control
-BER slow waves
- interstitial cells of cajal
- 3-12 waves per minute
- primary control is ENS
what are the stimuli of the feeding pattern of motility in the small intestine
-distension of duodenum
-nutrient content of chyme
- gastroenteric reflex
- hormones
what is the gastroenteric reflex
short feedback loop from stomach to small intestine
what hormones stimulate the feeding pattern motility in small intestine? inhibit?
-stimulate: CCK, gastrin, insulin, serotonin
- inhibit: secretin, glucagon
what are the 2 patterns of motility in the feeding pattern of the small intestine
segmentation and peristalsis
what happens in segmentation
- mixing of chyme with digestive enzymes to emuslify fats, adjust pH, and expose mucosa to chyme
describe peristalsis in the small intestine
-propels chyme through small intestine at a rate of 1cm/min
- spreads chyme across mucosal surfaces as it enters from stomach
-can begin anywhere in small intestine
-normally weak and dies out after 3-5 cm
how long does it take to pass food from pyloric valve to the ileocecal valve
3-5 horus
what does the duration of feeding pattern depend on
-caloric content of the meal
- nutrient composition of the meal
- proteins > fats > carbs
what is the net rate of movement of any substance across the intestinal epithelium influenced by
-surfacea rea
-motility
what is the east-west vector and what is it influenced by
- absorption
- influenced by SA
what is the north south vector and what is it influenced by
- how rapidly food moves from mouth to anus
- influenced by motility
what are the substances presented for digestion and/or absorption
- macronutrients
- electrolyes: Na+, K+, Ca2+, Mg2+, Fe2+, Cl-, PO4-
- water
- bile salts
-vitamins
-drugs
what are the factors that influence digestion
- motility
- surface area
- pH
- hydrolytic enzymes for carbs, protein and fat
- emulsifying factors for fat
what are the factors that influence absorption
- SA
- specialized cells
-specific transport mechanisms - carriers, pumps, pores - energy
- blood or lymph flow
what percentage of substances are completely digested and absorbed in the proximal small intestine and what is the exception
25% except dietary fat
what are the consequences of increased intake
increased absorption may produce increase storage and obesity
what is the gastric bypass surgery
diversion of chyme to distal small intestine
what pH do luminal enzymes in the small intestine require to function
neutral pH
what contributes to the pH of the small intestine
- H+ from the stomach
- bile HCO3-
-pancreatic HCO3-
what concentration of HCO3- does maximal stimulation of pancreas produce
145 mEq/L
where does most bicarbonate come from
the pancreas
what are the two sites for digestion of protein and carbohydrates
-intraluminal (stage I - pancreatic hydrolases)
- mucosal surfaces (Stage II - brush border hydrolases)
what does intraluminal digestion yield
di- and tripeptides, amino acids, maltose, maltotriose, alpha limit dextrins, glucose
where is fat digestion completed
in the lumen
what are the end products of mucosal surface digestion
amino acids, di- and tripeptides, glucose, galactose and frutose
what is the typical intake of carbohydrates in american diet and what percentage of the daily caloric intake does it make up
250-300 g/day
~50% daily caloric intake
what do polysachharides and disaccharides need to be broken down into before it can be absorbed
glucose fructose and galacose
what happens with fiber digeestion
fiber cannot be digested and is lost in feces
where does carbohydrate digestion begin
in the mouth with salivary amylase but not significant
what is salivary amylase inactivated by
acid gastric juice
what percentage of carbohydrate digestion is done by the small intestine
95%
describe carbohydrate digestion in the small intestine
- starch and glycogen digestion by pancreatic amylase
- then brush border hydrolases produce the monsaccharides such as lactose, maltose and sucrose
describe the mechanism of carbohydrate digestion and absorption
- pancreatic amylase converts polysaccharides into maltose and then brush border enzymes convert into monosaccharides in the lumen
- frustose is transported passively across apical membrane and passively across BL membrane or converted into glucose
- glucose and galactose are transported actively across apical membrane with Na+
- glucose and galactose passively move across BL membrane into ISF
why is some fructose converted into glucose and galactose in the intestinal epithelial cell
glucose and galactose are absorbed faster than fructose
what is the typical protein intake daily in the american diet and what fraction of daily caloric intake does it make up
- 60-90 g/day
-1/6 average daily caloric intake
how much endogenous protein is added daily
about 50 g
protein is added to the lumen in the form of what
mucus and enzymes
what must polypeptides be digested into before absorption
small peptides or amino acids
where does protein digestion begin
in the stomach by pepsin
what is pepsin important for
collagen digestion
what is pepsin inactivated by
basic pH in small intestine
what percentage of total protein digestion does the stomach do
10-20%
how much protein is digested by the small intestine
80-90%
what are small peptides and amino acids in the small intestine produced by as a result of protein break down
trypsin, chymotrypsin, carboxypolypeptidases, elastase
- brush border peptidases produce amino acids
describe the mechanism of protein digestion across an intestinal epithelial cell
- pancreatic proteases and peptidases convert proteins into small peptides and amino acids
-amino acids move with Na+ across the apical membrane and facilitate diffusion across BL membrane - small peptides move across apical membrane with H+ and are converted into amino acids by peptidases
what transporter is responsible for movement of small peptidases across the apical membrane and what makes it special
peptide transporter 1 (PEPT1)
- no specificity
- can be used for drug delivery
what is the average fat consumption in the american diet daily and what percentage of daily calories does it account for
- 70-100g/day
- about 30%
what is the most abundant dietary fat and what are the other sources of dietary fat
-most abundant: triglycerides
- others: cholesterol, cholesterol esters, and phospholipids
what does fat emulsification for digestion
bile salts and lecithin
what do emulsifiers do
- reduce the interfacial surface tension of fat
- breaks down the fat globule down into smaller fragments, increasing the surface area for digestion
where does fat digestion begin, what enzyme, and what percentage of total fat digestion does it account for
- in the stomach by lingual lipase
- less than 10%
what enzyme does fat digestion in the small intestine
pancreatic lipase
how long does it take for pancreatic lipase to digest all TG it can reach
less than 1 minute
what does pancreatic lipase produce
fatty acid and monoglyceride
what are fat digestion products packaged into
micelles
what is the importance of micelles
- removes TG digestion products from fat globules so fat digestion can continue
- transports TG digestion products to brush border membrane
what are cholesterol esters digested by
cholesterol ester hydrolase
what are phospholipids digested by
pancreatic phospholipase A2
describe the mechanism of fat absorption across an intestinal cell
-FA and MG cross apical membrane by simple diffusion
- FA and MG move into smooth ER to keep [FA] and [MG] low in cell and maintain the gradient for their diffusion into the cell
- fat soluble vitamins. phospholipids and cholesterol esters absorbed by same mechanism
- TG and other hydrophobic substances are packaged into chylomicrons and secreted across BL membrane via exocytosis
- chylomicrons enter lymphatic vessels via lacteals and are transported to systemic circulation
what are the fat soluble vitamins
A,D,E, and K
what are surface cells are what do they do
- mature intestinal epithelial cells
-absorb Na+, Cl- and H2O
what are crypt cells and what do they do
- immature intestinal epithelial cells
-secrete Na+, Cl-, and H2O
what are crypt cells produced by
stem cells
where does fluid produced by crypt cells go and what does this fluid do
- flows to surface cells to be reabsorbed
- H2O flow allows for absorbing intestinal digestates
what is the disorder not enough Cl- secretion
cystic fibrosis
what is the disorder of too much Cl- secretion
infectious diarrhea
describe the surface cells during the prandial state
-electrogenic
-glucose and Na+ move together across the apical membrane into the cell
- glucose diffuses passively across the BL membrane
- Na+ is moved into the ISF across the BL membrane by the Na+ K+ ATPase
- Na+ causes water to move across BL membrane into ISF and then Cl- to follow
describe the surface cells during the post prandial state
-electroneutral
- Cl- moves into the cell while HCO3- moves out into the lumen across the apical membrane
- Na+ moves into the cell while H+ moves out into the lumen across the apical membrane
- Cl- moves into the ISF via the K+ Cl- symporter
- Na+ moves into the ISF via the Na+ K+ ATPase
- water follows these ions into the ISF
how much sodium is secreted into the lumen per day? how much is ingested? how much is absorbed
-secreted: 30g/day
- 5-8g/day ingested
- more than 95% absorbed
describe the Na+, Cl-, and H2O secretion by crypt cells
- Cl- enters the cell in the apical membrane with K+ and Na+ and moves across the cell to be pumped out across the BL membrane by the CTFR channel
- Na+ and water will follow
- Na+ and K+ are pumped back out across apical membrane
- electrogenic
what increases Cl- secretion
anything that increases cAMP
how do cholera and E coli infections affect Cl- secretion
-they produce enterotoxins that increase cAMP
- cAMP activated Cl- secretion into the lumen causing massive diarrhea
- up to 20L produced a day
describe vitamin B12 absorption in the stomach, duodenum, ileum, and inside cells
- stomach: binds to R binding protein which stabilizes B12 in the acidic environment. IF secreted by parietal cells cannot interact with B12 at low pH
- in duodenum: proteases digest R binding protein and B12 binds to IF
- in ileum: IF binds to IFCR and taken into cells via receptor mediated endocytosis
- in cells: IF degraded, B12 binds to TCII, complex cross BL via exocytosis and enters hepatic portal blood
what water soluble vitamins are transported? what are they cotransported with? and where is this completed
- thiamin, riboflavin, niacin, pyridoxine, panthothenate, biotin, and ascorbic acid
- cotransported with Na+
- completed in upper small intestine
what other substances are actively absorbed
- Ca2+
-PO4- - Mg2+
- Fe2+
- bile salts
what is the absorbing function of the colon and how much is absorbed
-absorbing function in the proximal 1/2
-absorption of water and electrolytes from chyme to form solid feces
- about 1.4 L/day
what is the storage function of the colon and how long can it be stored
-storage function in the distal 1/2
- storage of fecal matter until it can be expelled
how long does it take to move contents from ileocecal valve to anus
8-15 hours
what does poor motility in the colon cause
greater absorption and hard feces in transverse colon causing constipation
how does diverticulitis happen
forms when too much pressure is in colon and little sacs pop out and fill with fecal matter and get infected
describe mixing movements of the colon called haustrations
- fecal material slowly dug into and rolled over
- all fecal matter exposed to mucosal surface
- facilitates absorption of water and dissolved substances
-movement of material from cecum through ascending colon
describe propulsive movements called mass movements in the colon and how often they occur
- 1-3x per day
- transverse to sigmoid
-occurs throughout colon for 10 minutes
-forces feces into rectum
what stimulate mass movements
gastrocolic and duodenocolic reflexes and irritation
what does slow motility in the colon promote with bacteria
bacterial growth
what keeps the bacteria in the colon
the ileocecal valve
what do colon bacteria do
- ferment undigested carbohydrates to short chain fatty acids that are soluble and easily absorbed for nutrient salvage
- produce vitamin K
- increase resistance of the intestinal mucosa to colonization with pathogenic microorganism
- produce flatus
describe the defectation reflex
- afferent nerve fibers cause parasympathetic fibers to relax internal anal sphincter and the descending and sigmoid colon
-sympathetic nerve fibers cause contraction of external anal sphincter and cause defecation
the rectum is empty until ____
mass movement
what stimulates movement of luminal contents into colon
-pressure and chemical irritation relax sphincter and excite peristalsis
- fluidity of contents promotes emptying into the colon
what inhibits movement of luminal contents into the colon
pressure or chemical irritation in cecum inhibits peristalsis of ileum and excites sphincter to contract
