gastrointestinal_week_1_20190518174122 Flashcards
what is the role of mouth and oropharynx
chops and lubricates food, starts carbohydrate digestion, propels food to oesophagi
what is role of oesophagus
muscular tube, propels food to stomach
what is role of stomach
stores/churns food, continues carbohydrate and initiates protein digestion, regulates delivery of chyme to duodenum
what are the parts of small intestine
duodenum, jejunum, ileum
what is role of small intestine
principle site of digestion and absorption of nutrients
what is the parts of large intestine
caecum, appendix and colon
what is role of large intestine
colon reabsorbs fluid and electrolytes, stores faecal matter before delivery to rectum
what is role of rectum and anus
storage and regulated expulsion of faeces
what is the accessory structures of the GI tract
salivary glandspancreasliver and gall bladder (hepatobillary system)
what is motility
mechanical activity mostly involving smooth muscle
how does secretion take place
secretion into the lumen of digestive tract occurs from itself and accessory structures in response to presence of food, hormonal and neural signals
what is secretion required for
digestion, protection and lubrication
what is digestion
chemical breakdown by enzymatic hydrolysis of complex foodstuff to smaller, absorbable units
what is absorption
transfer of absorbable products of digestion (with water, electrolytes and vitamins) from digestive tract to blood or lymph
what is components of mucosa (digestive tract wall)
epithelial cellsexocrine cellsendocrine gland cells lammina propia (capillaries, enteric neurones, immune cells)muscularis mucosae
what is components of submucosa (digestive tract wall)
connective tissue larger blood and lymph vesselsglandsnerve network (submucos plexus)
what is components of muscularis externa (digestive tract wall)
circular muscle layer nerve network (myenteric plexus)longitudinal muscle layer
what is components of serosa (digestive tract wall)
connective tissue
what happens during circular muscle contraction
lumen becomes narrower and longer
what happens during longitudinal muscle contraction
intestine becomes shorter and fatter
what happens during muscularis mucosae contraction
change in absorptive and secretory areas of mucosa (folding)mixing activity
what drives the slow wave electrical activity of digestive tract
interstitial cells of Cajal (ICCs)
what is the only condition which allows contraction to occur
if slow wave amplitude is sufficient to trigger smooth muscle cell action potentials
where are ICCs located and what types of junctions do they form
between longitudinal and circular muscle layers and in submucosa gap junctions with SMC
whether sloe wave amplitude reaches threshold depends on what
neuronal stimuli hormonal stimuli mechanical stimuli
how many waves per minute in stomach
3
how many waves per minute in small intestine
12 and 8 in duodenum and terminal ileum
how many waves per minute in large intestine
8 and 16 in proximal and distal (sigmoid) colon
what is the excitatory influences of parasympathetic system on digestive tract (synapse with ganglion cells in ENS)
increase gastric, pancreatic and SI secretionincrease blood flow and smooth muscle contraction
what is inhibitory influences of parasympathetic system
relaxation of some sphincters, receptive relaxation of stomach
what is excitatory influence of sympathetic system (synapse in pre vertebral ganglia)
increased sphincter tone
what is inhibitory influence of sympathetic system
decreased motility, secretion and blood flow
what is peristalsis (local reflex)
a wave of relaxation, followed by contraction that proceeds along gut in aboral directiontriggered by distension of gut wall
what is the short reflex of digestive tract
intestine-intestinal inhibitory reflex (causes inhibition of muscle activity in adjacent areas)
what is the long reflex of digestive tract
gastroileal reflex (increase in gastric activity causes increased propulsive activity in terminal ileum)
what is segmentation
rhythmic contractions of circular muscle layer that mix and divide luminal contents occurs in SI (in fed state) and LI (where its called haustration)
what is colonic mass movement
powerful sweeping contraction that forced faeces into rectum
what is migrating motor complex (MMC)
powerful sweeping contraction from stomach to terminal ileum
what is the role of upper oesophageal sphincter (UOS)
relaxes to allow swallowing, closes during inspiration
what is the role of lower oesophageal sphincter (LOS)
relaxes to permit entry of food to stomach and closes to prevent reflux of gastric contents to oesophagus
what is the role of pyloric sphincter
regulates gastric emptying and usually prevents duodenal gastric reflux
what is role of ileocaecal valve
regulates flow from ileum to caecum - distension of ileum opens, distension of proximal colon closes
what regulates internal (smooth muscle) and external (skeletal muscle) anal sphincters
defection reflex
how to calculate BMI
weight (kg) / square of height (m)
what are the signals in satiation (feeling full)
CCK, PYY3-36, GLP-1, OXM and obestatin
what is chemical which signal for hunger
Ghrelin
what is the definition of satiety
period of time between termination of one meal and initiation of nxt
what is definition of adiposity
state of being obese
what two hormones are central appetite controllers
leptin (made and released from fat cells) and insulin (made and released from pancreatic cells)
what is the effect of diet induced obesity on leptin
leptin resistance
what happens to the insulin receptor which results in obesity
neuron specific deletion
what drug is licensed to treat obesity
Orlistat (Xenical or Alli)
how does this drug work
inhibits pancreatic lipase decreasing triglyceride absorption reduces efficiency of fat absorption
what is side effects of this drug
cramping and diarrhoea
what surgery is available for obesity
gastric bypass
what drug is in the pipeline for treating obesity
Contrave (mysimba) - combination of bupropion (dopamine reuptake inhibitor) and naltrexone (opioid antagonist)
what is the 5 parts of stomach
cardia (contains cardiac sphincter)fundus (below diaphragm)body (main part - where food breaks down)antrum (hold broken down food)pylorus (connects to SI)
what digests proteins in stomach
pepsin and HCl
how much gastric juice does stomach secrete
2 litres per day
how is food mixed
churning action of gastric smooth muscle against closed pyloric sphincter
when does the pylorus open intermittently
to allow movement of chyme into duodenum
what determines the escape of chyme through pyloric sphincter
strength of astral wave
what are the gastric factors which govern strength of astral wave
rate of emptying proportional to volume of chyme and the consistency of chyme
the duodenum also controls strength of astral wave since it must be ready to recieve chyme - how does it delay emptying
neuronal responses (enterogastric reflex decreases antral activity)hormonal response (release of enterogastrones eg secretin and CKK inhibits stomach contraction)
which stimuli within duodenum drives neuronal and hormone response
fat (high fat - delay)acid (time taken for neutralisation by bicarbonate from pancreas)hypertonicitydistension
what gastric secretions are released from oxyntic mucosa (fundus and corpus)
HCl, pepsinogen, intrinsic factor and gastroferrin, histamine and mucus
what is function of HCl
activates pepsinogen to pepsin (digestion of proteins), denatures proteins and kills most micro-organisms ingested with food
what is function of pepsinogen
inactive precursor of pepsin (once pepsin formed it activates pepsinogen)
what is function of intrinsic factor and gastroferrin
binds vitamin B12 and Fe2+ respectively, facilitating subsequent absorption
what is function of histamine
stimulates HCl secretion
what is function of mucus
protective
what gastric secretion are secreted by pyloric gland area (pylorus and antrum) and their function
gastric (stimulates HCl)somatostatin (inhibits HCl)mucus (protective)
what are three important secretoagogues that induce acid secretion from parietal cell
ACh, gastrin and histamine acting by direct and indirect mechanisms Acting on M2 and H2 receptors
which pathways do stimuli for secretion of H+ act by
PLC - IP3 (gastrin, ACh) andcAMP - PKA (histamine) signalling pathways
which pathways do stimuli for inhibition of secretion of H+ act by
cAMP - PKA (somatostatin, prostaglandins) signalling pathways
the rate of gastric secretion is controlled by what
stimulatory and inhibitory mechanisms that occur in three overlapping phases
what is the first phase
cephalic phase - before food reaches stomach preparing it to receive foodvagal stimulation which results in ACh release which directly activates parietal cells
what is second phase
gastric phase - when food in stomachdistension of stomach activates reflex that cause acid secretionfood buffers, pH, D cell inhibition via ss of gastrin release is decreased
what is the third phase
intestinal phase - after food left stomach
how can gastric acid secretion be inhibited in cephalic phase
vagal nerve activity decreases upon cessation of eating and following stomach emptying increased pain, nausea and negative emotions also decrease vagal nerve activity
how can gastric acid secretion be inhibited in gastric phase
antral pH falls when food exits stomach - causes release of somatostatin from D cells to recommence, decreasing gastric secretionPGE2 continually secreted by gastric mucosa acts locally to reduce histamine and thus gastrin mediated HCl secretion
how can gastric acid secretion be inhibited in intestinal phase
factors that reduce gastric motility also reduces gastric secretion (neuronal reflexes, enterogastrones)
which drug class influence acid secretion
muscarinic receptor antagonists (pirenzepine) block ACh binding competitivelyproton pump inhibitors (omeprazole) block by covalent modification NSAIDs (aspirin) block cyclo-oxygenase irreversibly H2 histamine receptor antagonists (rantidine) block competitively
how does prostaglandins (PGE2 and PGI2) work to protect mucosa from attack by HCl and pepsin
it reduces acid secretion, increases mucus and bicarbonate secretion and increases mucosal blood flow
infection of the gastric antrum with which bacterium causes peptic ulcer
helicobacter pylori - secretes agents causing inflammation that weakens mucosal barrier leaving submucosa and deeper layers subject to attack by HCl and pepsin
drug treatment of peptic ulcer aims to promote ulcer healing by what
reducing acid secretion, increasing mucosal resistance and eradicating H. pylori
which drugs can trigger gastric ulceration and cause bleeding
non steroid anti inflammatory (NSAIDs) eg aspirin as they reduce prostaglandin formation (COX 1 inhibition)
gastric damage due to long term NSAID treatment can be prevented by what drug - what is the adverse effects of this drug
PGE1 analogue (ie misoprostol)inhibits basal and food stimulated gastric acid formation maintains (or increases) secretion of mucus and bicarbonate
what conditions are drugs that reduce acid secretion used in treatment of
peptic ulcer gastro-oesophageal reflux disease acid hypersecretion (eg zollinger-ellison syndrome or cushing’s ulcers)
how do proton pump inhibitors (PPIs) eg omeprazole work
activated by pH of stomach. React with sulphydryl groups in H+/K+ATPase responsible for transporting H+ out of parietal cells. The enzyme is irreversibly blocked so acid secretion only resumes after synthesis of new enzyme
when are PPIs used
peptic ulcer, GORD and Zollinger-Ellison
why is timing of dosing important
drug must be present in plasma at an effective concentration whilst proton pumps are active
how do H2 receptor antagonists eg cimetidine, ranitidine work
reduce acid secretion and increase healing of peptic ulcer by blocking action of histamine on parietal cells and reducing acid secretion
when are H2 receptor antagonists used
peptic ulcer and reflux oesophagitis
what two drugs are mucosal strengtheners
sucralfate - binds to ulcer base and forms complex gels with mucus - provides mucosal barrier to acid and pepsinbismuth chealate - mucosal strengthening, also toxic towards H.pylori so used in combo with antibiotics and H2 antagonists
what is component of temporomandibular joint
two cavities divided by articular disc (superior cavity for translation, inferior cavity for rotation)
what four muscles control mastication (chewing)
temporalis majormassetermedial pterygoidlateral pterygoid (3 close and 1 open respectively)
all muscles of mastication are controlled by which nerve
mandibular division of trigeminal nerve CNV3
what is the course of CNV3
from pons, through foramen oval to muscles of mastication and sensory area
what is the course of the facial nerve
from pontomedullary junction, travels through tempura bone via internal acoustic meatus then stylomastoid foramen this supplies anterior 2/3rds of tongue, muscles of facial expression and glands in floor in mouth
what supplies superior half of oral cavity (gingiva of oral cavity and palate)
general sensation CNV2
what supplies the inferior half of oral cavity (gingiva of oral cavity and floor of mouth)
general sensation CNV3
what sensory part of gag reflex is carried by nerve fibres within what
CNIX
the motor part of gag reflex is carried by nerve fibres within what
CNIX and CNX
what is the course of CNV2 (maxillary division of trigeminal nerve)
from pons, through foramen rotundum to sensory area (mid-face)
what is the course of CNIX (glossopharyngeal nerve)
from medulla, through jugular foramen to posterior wall of oropharynx (sensory), parotid gland (secretomotor) and post 1/2 tongue (sensation and taste)
what is the role of the parotid gland
parotid duct crosses face and secretes into mouth by upper 2nd molar
what is role of submandibular gland
submandibular duct enters floor of mouth and secretes via linguinal caruncle
what is role of sublingual gland
lays in floor of mouth, secretes via several ducts superiorly
what is the role of extrinsic muscles in tongue musculature
function to change position of tongue
what do the four pairs of skeletal intrinsic muscles in the tongue musculature do
located dorsally/posteriorly and modify shape of tongue during function
what is role of CNXII - hypoglossol nerves - in tongue
from medulla, through hypoglossal canal to extrinsic and intrinsic muscles of tongue (except palatoglossus)
what nerves innervate the pharynx
CNX and IX
what muscle and nerve work to close lips to prevent drooling
orbicularis oris and cranial nerve VII
what is the anatomical location of oesophagus
inferior continuation of laryngopharynx begins at inferior edge of circropharyngeas muscle (C6)
where does the lower oesophageal sphincter lie
immediately superior to gastro-oesophageal junction presence of hiatus hernia reduce effectiveness of LOS - can lead to relfux
where does the stomach lie
mainly in left hypochrondium, epigastric and umbilical regions when patient supine
what is components of small intestine, from proximal to distal
duodenum, jejunum and ileum
what is components of large intestine, proximal to distal
colon (caecum, appendix, ascending colon, transverse colon, descending colon and sigmoid colon), rectum, anal canal and anus
where is pain from foregut structures felt and where do they enter spinal cord
epigastric region T6-T9
what are foregut structures
oesophagus to mid duodenum, liver and gall bladder, spleen, 1/2 of pancreas
where is pain from midgut structures felt and where do they enter spinal cord
umbilical regionT8-T12
what are midgut structures
mid duodenum to proximal 2/3rds of transverse colon 1/2 of pancreas
where is pain from hindgut structures felt and where do they enter spinal cord
felt in pubic region T10-L2
what are the structures of the hindgut
distal 1/3rd of transverse colon and proximal 1/2 of anal canal
how is referred pain caused
pain from these organs tend to be perceived by patient in dermatomes of levels at which they enter spinal cord and there is little overlap
what direction is external oblique muscles
hands in pockets
what direction is internal oblique muscles in
hands on chest
what is guarding
abdomen becomes rigid muscles contract to guard abdominal organs when injury threatens
what is the peritoneal cavity
space between visceral and parietal layers - contains lubricating fluidperitonitis - blood pus or faeces in cavity - inflammation
what are intraperitoneal organs
almost completely covered in visceral peritoneum and minimally mobile
what are intraperitoneal organs with mesentery
covered in visceral peritoneum which wraps around organ to form double layer, making it very mobile
what are retroperitoneal organs
only has visceral peritoneum on its anterior surface
what is mesentery
usually connects organ to posterior body wall
what is omentum
double layer of perineum that passes from stomach to adjacent organs
what is the greater omentum
four layeredhangs like an apron attaches greater curvature of stomach to transverse colon
what is the lesser omentum
double layeredruns between lesser curvature of stomach and duodenum to liver has free edge
by which do the two omentum sacs communicate
omental foramen (foramen of winslow)
what is peritoneal ligaments
double layer of perineum that connects organs to one another or body wall
what are the four main peritoneal ligaments
hepatogastric ligament hepatoduodenal ligament gastrosplenic ligament splenorenal ligament
what pouch does peritoneum form in male by its inferior aspect draping over superior aspect of pelvic organs
rectovesical pouch
what 2 pouches does peritoneum form in female
vesico-uterine pouch and recto-uterine pouch
what is ascites
collection of fluid in peritoneal cavity commonly caused by liver disease - cirrhosis or portal hypertension
how is abdominocentesis perfomed
needle places lateral to rectus sheath - avoids inferior epigastric artery
what is the course of the abdominal sympathetic nerves
leave spinal cord between T5-L2enter sympathetic chains but do not synapseleave chain within abdominopelvic splanchnic nervessynapse at pre vertebral ganglia which are located anterior to aorta at exit points of major branches of abdominal aorta
how is the adrenal gland sympathetic nerve different
leaves at T10-L1. Do not synapse, carried with periarterial plexus to adrenal gland and synapse directly onto cells
what is the course of abdominal vagus nerve (CNX)
presynaptic fibres enter abdominal cavity on surface of oesophagustravel into periarterial plexus around abdominal aortacarried to walls of organs where they synapse in gangliasupply parasympathetic nerve fibres to GI tract plus abdominal organs up to distal end of transverse colon
what is course of pelvic splanchnic nerves (S2, 3 and 4)
presynpatic parasympathetic nerve fibressmooth muscle/glands of descending colon to anal canal
the somatic motor, sensory and sympathetic nerve fibres supplying structures of abdominal part of the body wall are conveyed within what
thoracoabdominal nerves (7-11 intercostal nerves)subcostal nerve (T12 anterior rami)Illiohpogastric nerve (half of L1 anterior ramus)Illioguinal nerve (other half of L1 anterior ramus)
where is appendix usually located
right iliac fossa
describe the pain of appendicitis
visceral afferents enter spinal cord at T10as it worsens, appendix will start to irritate parietal peritoneum in right iliac fossa, which lies anterior to it
what are 3 parts of small intestine
duodenum (30cm), jejunum (3.5m), ileum (2.5m)
what does the small intestine recieve
chyme from stomach (from pyloric sphincter), pancreatic juice from pancreas, bile from gall bladder (both via sphincter of oddi)
what is the other roles of small intestine
secretes intestinal juice (succus entericus) and moves remaining residues to the large intestine via the ileocaecal valve (opens in response to proximal pressure and in response to gastrin)
the small intestine secretes (into blood) various peptide hormones from endocrine cells within mucosa. What are these hormones
Gastrin - from G cells of gastric antrumCholecystokinin (CKK) - from I cells of duodenum and jejunumSecretin - from S cells of duodenumMotilin - from M cells of duodenum and jejunum GIP - incretin from K cells of duodenum and jejunumGLP-1 - an incretin from L cells of gut Ghrelin - from Gr cells of gastric antrum, SI and elsewhere (pancreas)
all of these secretions act on which receptors
G protein coupled receptors
what is the control mechanism by which succus entericus (gastric juice) is secreted
distension/irritation, gastrin, CKK, secretin, parasympathetic nerve activity (all enhance)sympathetic nerve activity (decreases)
what does this secretion (succus entericus) contain
mucus (protection/lubrication - from goblet cells) aqueous salt for enzymatic digestion (mostly from crypts of lieberkuhn)no digestive enzymes
what does the secretion of succus entericus involve
Na+/K+ATPaseNa+/K+/2Cl- co transporterchloride channel (CFTR)
how is segmentation (mixing) of chyme initiated
by SI pacemaker cells causing BER which is continuous. At threshold activates segmentation which in the duodenum is primarily due to distension by entering chyme
what triggers segmentation in empty ileum
gastrin from stomach (gastroileal reflex)
what is the difference in segmentation contractions between duodenum and ileum
duodenum - 12 per min ileum - 9 per min net movement if aboral
what takes place after segmentation
peristalsis
which two activities occur in interdigestive, or fasting, state
few localised contractionsmigrating motor complex (MMC) - occurs between meals, strong peristaltic contractions passing length of intestine (stomach - ileocaecal valve)
what is beneficial about this MMC
clears small intestine of debris, mucus and sloughed epithelial cells between meals - housekeeper function
what inhibits MMC
feeding and vagal activity
what triggers MMC
motilin (it is suppressed by gastrin and CCK)
what is endocrine secretions of pancreas (released directly into blood stream)
insulin and glucagon
what is exocrine secretions of pancreas (secreted to duodenum collectively as pancreatic juice)
digestive enzymes (acinar cells), aqueous NaHCO3- solution (duct cells)
what is the role of the HCO3- fluid released by pancreatic duct cells
neutralises acidic chyme entering duodenum - provides optimum pH for pancreatic enzyme function and protects mucosa from erosion by acid
what is special about pancreatic enzymes
can completely digest food in absence of all other enzymes eg trypsinogen directly converted to trypsin (proteases can go from inactive to active states)
what controls 20% of total pancreatic secretion
cephalic - mediated by vagal stimulation of mainly the acinar cells
what controls 5-10% of total pancreatic secretion
gastric distension evokes a vagovagal reflex resulting in parasympathetic stimulation of acinar and duct cells
what controls 70-80% of control of pancreatic secretion
intestinal phase - mechanism depends on wether its acid or fat and protein in duodenal lumen
what happens when there is acid in duodenal lumen
increased secretin release from S cells (carried by blood) to pancreatic duct cells - increased secretion of aqueous NaHCO3 solution into duodenal lumen which neutralises acid
what happens when there is fat and protein in duodenal lumen
increased CCK release from cells (carried by blood) to pancreatic acinar cells - increased secretion of digestive enzymes into duodenal lumen which digests fat and protein
what is main constituent of carbohydrates (approx 400g per day)
starch (greater than 50% total carbohydrate ingested)cellulose (indigestible in humans)glycogen disaccharides (sucrose, lactose)
what is main constituent of lipids (approx 25-160g per day)
triacylglycerols (approx 90% of total lipid ingested as fats and oils)phospholipidscholesterol and estersfree fatty acids lipid vitamins
what is main constituent of protein
70-100g per day ingested, plus 35-200g from endogenous sources (eg digestive enzymes and dead cells from GI tract)
what is definition of digestion
enzymatic conversion of complex dietary substances to a form that can be absorbed
most digestive processes occur in the small intestne as what
luminal digestion (mediated by pancreatic enzymes secreted into duodenum)membrane digestion (mediated by enzymes situated at brush border of epithelial cells)
what is absorption
process by which absorbable products of digestion are transferred across both apical and basolateral membranes of enterocytes (absorptive cells of intestinal epithelium)
what is the overall process of digestion and absorption called
assimilation
all dietary carbohydrates must be converted to which form to be absorbed
monosacchardies (glucose /fructose)
what accomplishes the sequence of carbohydrate digestion
a-amylase (salivary and pancreatic)
how is this achieved
starch converted to oligosaccharides (not absorbed) by intraluminal hydrolysis oligosaccharides converted to monosaccharides by membrane digestion (at brush border)
give example of olgiosacchardies
lactose, sucrose (can come from diet)
what is the role of a-amylase (endoenzyme)
breaks down linear internal a-1,4 linkages but not terminal a-1,4 linkages so no production of glucose products are thus linear glucose oligomers (maltotriose, maltose) and a-limit dextrins
what is an olgiosaccharide
internal membrane proteins with a catalytic domain that faces the lumen of GI tract
what do olgiosaccharides do
cleave terminal a-1,4 linkages of maltose, maltotriose and a-limit dextrins to yield glucose
how is lactase special
only has one substrate - breaks down lactose to glucose and galactose
what is specific role of maltase
degrade a-1,4 linkages in straight chain oligomers up to nine monomers in length
what is specific role of sucrase
specifically responsible for hydrolysing sucrose to glucose and fructose
what is specific role of isomaltase
unique in that it is the only enzyme that can split branching a-1,6 linkages of a-limit dextrins
what is lactose intolerance
relative common digestive problem resulting from inability to adequately digest lactose - caused by lactase insufficiency
what causes lactase persistence (LP) - continuance of digestion of lactose in milk in adults
polymorphisms in MCM6 gene that regulates expression of lactase (LCT) gene
lactose intolerance can result from what
primary lactase deficiency (primary hypolactasia - due to lack of LP allele)secondary lactase deficiency (caused by damage to proximal SI)congenital lactase deficiency (rare autosomal recessive - no ability to digest lactose from birth)
what do the ileum colonic microflora produce if lactose is delivered to the colon
short-chain fatty acids (which can be absorbed), hydrogen (can be detected in breath of lactase deficient individuals) CO2 and methane
what is the by-product of these productions
bloating, abdominal pain and flatulence
what does undigested lactose cause
acidification of colon and an increased osmotic load - loose stools and diarrhoea
where does the absorption of the final product of carbohydrate digestion (glucose, galactose and fructose) take place
duodenum and jejunum - it is a two step process involving entry and exit from the enterocytes via the apical and basolateral membranes respectively
what absorbs these final products
glucose and galactose are absorbed by secondary active transport mediated by SGLT1fructose by facilitated diffusion mediated by GLUT5
exit for all monosaccharides is mediated by what
facilitated diffusion by GLUT2
which form must SGLT1 be in to absorb glucose
a hexose in the D-conformation an one that can form a pyranose ring
what is the mode of operation of SGLT1
2Na+ binds Affinity for glucose increases, glucose binds Na+ and glucose translocate from extracellular to intracellular2Na+ dissociate, affinity for glucose fallsGlucose dissociates Cycle repeated
proteins must be digested to oligopeptides and amino acids for efficient absorption within how long
6 months
how many different pathways exist and what is the basic pathway
4 protein - peptides - amino acids - amino acid in enterocyte - amino acid in bloodthird pathway (contains intracellular hydrolysis) is different: protein - peptides - peptide in enterocyte - amino acid in enterocyte - amino acid in blood
what different enzymes and transporters are used
luminal enzymes, (can be bush border enzymes - 2), apical membrane transporters, (can be intracellular hydrolysis - 3), basolateral membrane transporters
what is special about the fourth variant
when peptide transported out of enterocyte without intervening intracellular hydrolysis by proteases
how does digestion of proteins take place in stomach
HCl denatures proteins and pepsin cleaves proteins into peptides (is endopeptidase with preference for bonds between aromatic and larger neutral amino acids)
how does digestion of protein take place in duodenum
five pancreatic proteases are secreted as proenzymes from exocrine pancreas and converted to active form in duodenum - they act as endopeptidases or exopepitases
what are examples of endopeptidases that produce oligopeptides (2-6 amino acids)
trypsin, chymotrypsin, elastase
what are examples of exopeptidase that produces single amino acids
procaroxypeptidase A and procarboxypeptidase B
what completes protein digestion
additional proteases which are present at the brush border and within the cytoplasm of enterocyte
step 1 in summary of protein digestion and absorption
protein digested in lumen to amino acids or olgiopeptides by pepsin and pancreatic proteases
step 2 in summary of protein digestion and absorption
peptidases at the brush border further hydrolyse oligopeptides to amino acids
what is step 3 in summary of protein digestion and absorption
amino acids transported . across the apical membrane via variety of amino acid transporters, 5 of which are Na+ dependent (system B0AT1) and 2 are Na+ independent (system b0+AT)
what is step 4 in summary of protein digestion and absorption
olgiopepties transported across apical membrane by H+/oligopeptide co-transporter - PepT1
what is step 5 in summary of protein digestion
olgiopeptides within cytoplasm are hydrolysed to amino acids by peptidases within enterocyte
what is step 6 in summary of protein digestion
amino acids exit enterocyte across the basolateral membrane by several Na+ independent transporters
