Gastrointestinal System Flashcards
What are the 3 main functions of the GI tract?
digestion
absorption
protection
define digestion
the process by which foodstuffs are degraded to produce smaller molecules that can be absorbed
define absorption
the process by which nutrient molecules are absorbed by cells that line the GI tract and enter the blood stream
define protection in terms of the GI tract
the process by which the lining of the GI tract is protected from damage during digestion of foodstuffs
what does the GI tract require protection from?
harsh environment created to break down food, e.g. acids and enzymes
name all the main parts of the GI tract
mouth, pharynx, oesophagus, stomach, duodenum, jejunum, ileum, colon, rectum, anus
is the GI tract structured the same from mouth to anus?
has a common design but there is difference between regions depending on function
what are the accessory organs of the GI tract?
liver, gall bladder, pancreas, salivary glands
what are the layers of the GI tract wall from lumen to basement layer?
mucosa, submucosa, muscularis externa, serosa
how does the mucosa structure in the stomach aid its function?
gastric pits with glands which secrete acid
how does the mucosa structure in the small intestine aid its function?
villi over apical surface to increase surface area for absorption
how does the mucosa structure in the large intestine aid its function?
has crypts full of mucous secreting cells to protect epithelium
what are the 3 layers of the mucosa?
epithelium, lamina propina, muscularis mucosae
within the lumen of the small and large intestine what are villi separated by?
crypts
what does the epithelium of the mucosa form?
the interface between external environment and the body
what is the epithelium made up of?
single layer of specialised cells
what is the lamina propina made from?
loose connective tissue
which of the 3 layers of mucosa is the largest?
lamina propina
what is the name of the most outer layer of mucosa?
muscularis mucosae
what is the muscularis mucosae formed of?
thin layer of muscle
what is the function of the muscularis mucosae?
local gut movement e.g. glands
what is the major site of gut motility that makes the GI tract a muscular tube?
muscularis externa
what is the next layer superficially to the mucosa?
sub mucosa
what is the submucosa formed from?
loose connective tissue with blood vessels and nerves
what the next gut layer located superficially to the sub mucosa?
muscularis externa
what 2 muscle layers is the muscularis externa formed of?
circular muscle (wraps around) longitudinal muscle (travels along length of tube)
which muscle layer of the muscularis externa is more deep to the lumen of the GI tract?
circular muscle with longitudinal superficial to it
what is the most outer layer of the GI tract?
serosa
what is the serosa made of?
connective tissue
what is the main function of the serosa?
holds tube together
where are lymph nodes found in the GI tract?
within mucosa
why are lymph nodes needed in the GI tract?
ensure immune response isn’t mounted incorrectly and also ensures quick immune response to pathogens when needed
where in the GI tract are glands found in the submucosa?
oesophagus and small intestine
where are myenteric plexi found in the GI tract?
between the 2 layers of muscularis externa all the way along GI tract from top of oesophagus to rectum
where are submucosal plexi found?
between muscularis externa and submucosa only in small and large intestine
what are the 3 levels of amplification of the surface area of the GI tract?
circular or spiral folds
villi
microvilli
where are microvilli located?
on apical membrane of epithelial cells (facing into the lumen)
what are glycocalyx?
network of proteins on the outside of microvilli which create an undisturbed area for final products of nutrient breakdown to sit in and then be absorbed
what are the 2 membranes found on absorptive cells?
apical and basolateral
what are the 2 pathways of movement through epithelium in the GI tract?
transcellular
paracellular
where do substances travel through the paracellular pathway?
gap between two epithelial cells
where do substances travel through the transcellular pathway?
through the epithelial cell from apical to basolateral membrane
what is the function of tight junctions?
act to separate proteins through the apical and basolateral membranes
give an example of a transcellular pathway
glucose and sodium pathway
what are parietal cells for?
vital for GI tract function to secrete acids
what does the presence of 2 membranes mean for the cell?
it is polar
what are the 5 main sites of secretion in the GI tract?
salivary glands, gastric glands, exocrine pancreas, liver-billiary system and small intestine
what is total saliva secretion throughout the day?
6-7 litres a day
what are the 3 types of gland?
in epithelium
in sub mucosa
gland outside GI tract
what does saliva contain?
enzymes, ions, water and mucus
what is the function of saliva?
breakdown of large compounds, regulate pH, dilute and protect
what is the name of the system that supplies the GI tract with blood?
splanchnic circulation
where does all blood from the GI tract drain before meeting the IVC?
into the hepatic portal vein and into the liver before exiting the liver into hepatic vein and the IVC
how is a villus supplied with blood?
central arteriole branching into capillaries and then exiting in a venule
what is absorbed into the blood stream through the villi?
simple sugars and amino acids
how do fats/fatty acids enter the blood stream?
drain into lacteals where they join the intestinal lymphatic system before entering circulation at the thoracic duct
where are fats reassembled?
in the epithelium of the villi
what are 3 types of regulation of blood flow to GI tract?
nerves
hormones
vasodilatory metabolites released by epithelial cells as they work to absorb nutrients
how does blood supply increase in the GI tract when moving from fasted to fed state?
sequentially as the different areas require blood
what are the 3 regulatory mechanisms that control GI function
endocrine
paracrine
neuronal
how does endocrine regulation of GI function work?
release of a transmitter (e.g. peptide) into the blood for delivery to distant target cells
how does paracrine regulation of GI function work?
release of a transmitter from a sensor cell that will have a local effect on adjacent cells with no travel in the blood stream
how does neuronal regulation of GI function work?
electrical signalling via neurons
where are endocrine cells located in the GI tract?
mucosa of the stomach, intestine and pancreas
what cells produce gastrin?
G cells
where are gastrin producing G cells located?
stomach
what is the role of gastrin?
stimulates acid secretion
what cells produce cholecystokinin (CKK)?
I cells
where are cholecystokinin (CKK) producing I cells located?
wall of small intestine
what cells is secretin produced by?
S cells
where are secretin producing S cells located?
small intestine
what cells is glucose dependent insulinotropic peptide (GIP) produced by?
K cells
where are the glucose dependent insulinotropic peptide (GIP) producing K cells located?
small intestine
what is the main role of glucose dependent insulinotropic peptide (GIP)?
regulates insulin production
what cells is somatostatin produced by?
D cells
where in the body are somatostatin producing D cells located?
stomach, pancreatic islets, small intestine
what is a key role of somatostatin?
inhibits acid secretion
how can the location of D cells affect somatostatin?
some are paracrine factors and others are hormones depending on location
what 2 hormones regulate secretion and motility in the GI tract?
gastrin and CCK
What is a key role of CCK in secretion and motility?
contraction of gall bladder to release bile into duodenum for fat digestion
pancreatic secretion of enzymes
what hormone regulates blood flow in the GI tract?
CCK - stimulates blood flow
what hormone regulates secretions of endocrine cells in the GI tract?
GIP - insulin release
what is the nervous system of the GI tract?
enteric nervous system
describe the nervous pathway in the enteric nervous system for local reflexes
sensory neurons stimulated by chemo/mechanoreceptors
signal transmitted to submucosal/myentric plexus (inter neurons)
response transferred via motor neurons to effectors (e.g. smooth muscle, secretory cell, blood vessel)
is there any intervention from the brain within the enteric nervous system?
no - local reflex control in a self-contained network
what is the name of the branch of the nervous system that relays information from the enteric nervous system to the brain?
extrinsic nervous system
what branches of the nervous system have effect on the GI tract?
sympathetic and parasympathetic nerves
what is the effect of sympathetic nerves on the GI tract?
direct blood away from GI tract
what is the vagovagal reflex?
direction of sensory information from the gut to the CNS
what are the 3 phases of GI control?
cephalic phase
gastric phase
intestinal phase
what happens during the cephalic phase of GI control?
before entry of food into stomach involves sight, smell and taste
what happens during the gastric phase of GI control?
distention and change in acidity of stomach as food enters
what happens during the intestinal phase of GI control?
distention, osmolarity and acidity of intestines
what are epithelial cells lining the GI tract responsible for?
nutrient absorption
What is the function of the GI tract tightly regulated by?
nerves, hormones and paracrine factors
what do epithelial cells lining the GI tract control?
quality and composition of secretions in the GI tract
what are the 3 main functions of motor activity of the GI tract?
non-propulsive movements (segmentation)
peristaltic movements
reservoir function
what is involved in non-propulsive movements in the GI tract?
churning and mixing of digestive products to ensure adequate exposure to acids and enzymes and aid absorption through the gut wall
what are peristaltic movements?
relaxation ahead of a bolus and contraction behind in order to move contents of gut through GI tract
what happens during reservoir function of the GI tract?
contraction of specialised circular muscles (sphincters) which causes isolation of a specific region (e.g. stomach)
what is a sphincter formed of?
specialised muscularis externa
what are the two types of muscle contraction seen in the GI tract?
phasic
tonic
how long do phasic contractions last in the GI tract?
seconds
what functions of the GI tract utilise phasic contractions?
non-propulsive movements (segmentation)
peristaltic movements
how long do tonic contractions last in the GI tract?
minutes to hours
what functions of the GI tract utilise tonic contractions?
reservoir functions for long term contraction of sphincters
is GI tract muscle single or multi unit?
single
how do innervated smooth muscle cells communicate with those that are not innervated?
through gap junctions
how is the membrane potential of GI smooth muscle cells described?
slow waves - oscillations of membrane voltage/potential
how does the resting potential of smooth muscle cells compare to nerves/skeletal muscle?
RMP of smooth muscle cells is more depolarised
what is smooth muscle activity and the slow waves of RMP regulated by?
hormones and nerves
what is the frequency of contractions in GI smooth muscle dictated by?
frequency of action potential firing which is determined by the frequency of slow waves
how do sphincters mediate reservoir function?
through long lasting tonic contractions
what are sphincters?
specialised circular muscles that separate two adjacent compartments of the GI tract
what does separation of different areas of the GI tract encourage?
regional specificity
what are the 6 sphincters found in the GI tract?
upper oesophageal sphincter lower oesophageal sphincter pyloric sphincter sphincter of Oddi ileocecal sphincter internal and external anal sphincters
which sphincter has the highest resting pressure of all sphincters in the tract?
upper oesophageal sphincter
what is the key role of the lower oesophageal sphincter?
prevents acid reflux into oesophagus from the stomach
which sphincters isolate the stomach?
lower oesophageal and pyloric
what sphincters isolate the small intestine?
pyloric and ileocecal
what sphincters isolate the large intestine?
ileocecal and internal and external anal sphincters
what muscle is the external anal sphincter formed of?
skeletal
what is the role of the sphincter of Oddi?
smooth muscle around bile duct and pancreatic duct which regulates enzyme and bile delivery to the small intestine
histologically how does the pyloric sphincter appear?
large bulge of circular muscle (from muscularis externa) into lumen
what is the difference between physiological and anatomical sphincters?
physiological sphincters do not have evident bulges of muscle into the lumen forming the sphincter like anatomical ones do
histologically how can the junction between oesophagus and stomach be identified?
change from stratified squamous epithelium and sub mucosal glands to gastric pits
what is the role of stratified squamous epithelium in the oesophagus?
protects from damage by abrasion/acid
what sort of resting pressure is mostly found in sphincters due to tonic contractions?
positive resting pressure
what types of movement do sphincters regulate?
forward/antegrade (mostly) and backwards/retrograde
what is sphincter regulation coordinated with?
smooth muscle contractions of adjacent compartments
what is the key role of sphincters?
mediate reservoir function through tonic contractions
is swallowing initiated voluntarily or involuntarily?
voluntarily
once swallowing is initiated what control is it under?
reflex
describe the process of the swallowing reflex?
food bolus reaches back of the mouth as directed by voluntary control of the tongue
touch receptors activated
stimulate the medulla and lower pons
vagal nerves stimulate the oesophagus and cranial nerves stimulate the pharynx and upper oesophagus
where are most of the touch receptors located that have a key role in swallowing?
those near the opening of the pharynx
what are the 3 phases of swallowing?
oral/voluntary phase
pharyngeal phase
oesophageal phase
what happens during the oral/voluntary phase of swallowing?
the tongue presses the food against the hard palette and the bolus is forced into the pharynx stimulating touch receptors
what happens during the pharyngeal phase of swallowing?
soft palette elevates, epiglottis close the trachea and upper oesophageal sphincter relaxes
what is the main aim of the pharyngeal phase of swallowing?
protection of the respiratory system
what does the elevation of the soft palette and closure of the trachea by the epiglottis prevent?
food entering the nasal cavity and trachea respectively
what happens during the oesophageal phase of swallowing?
upper oesophageal sphincter is closed and peristalsis starts
define peristalsis
sequential contraction of rings of muscle behind a bolus
what are the two regions of the stomach distinguished by motility called?
orad and caudad
what is the function of the orad area of the stomach?
accommodation of food
what is the function of the orad and caudad areas of the stomach?
gastric emptying once small intestine is ready
what happens to the orad region during swallowing?
relaxes in coordination with peristalsis and opening of lower oesophageal sphincter
what is the relaxation of the orad following swallowing known as?
receptive relaxation
describe the process involved in vomiting
reverse peristalsis to remove toxic material from SI
pyloric sphincter and the stomach relax to receive contents
forced inspiration occurs against the closed epiglottis
sharply elevated intra-abdominal pressure causes forceful contractions of abdominal muscles
reflex relaxation of the upper oesophageal sphincter which drives vomit out
what are the 3 results of forced inspiration against the closed epiglottis?
lowered diaphragm
decrease in intrathoracic pressure
elevated intra-abdominal pressure
where do peristalsis contractions begin in the stomach?
corpus
where do contractions travel towards when they have been initiated in the corpus?
travel towards pylorus
what is the travel towards pylorus by peristalsis from the corpus known as?
propulsion
what happens to peristalsis contractions as they travel towards gastroduodenal junction (pyloric sphincter)?
increase in force and velocity
where does most grinding/mixing occur in the stomach?
mainly in the antrum
what is retropulsion?
movement of gastric contents back towards corpus from the antrum
what is retropulsion very effective at?
mixing and breaking down gastric contents
what does the tonic contraction of the pyloric sphincter control?
the speed of stomach emptying
in the fed state what size must gastric contents be to pass through pyloric sphincter?
<2mm in diameter
what is the most frequent type of movement in the small intestine?
non-propulsive movements
what are non-propulsive movements in the small intestine caused by?
rhythmic contraction and relaxation of the muscularis externa
what is the purpose of non-propulsive movements in the small intestine?
effectively mixes chyme and brings digested nutrients into contact with the mucosal surface (unstirred area to be absorbed into villi)
what is low frequency peristalsis in the small intestine caused by?
contraction of successive sections of the muscularis externa
what is the purpose of peristalsis in the small intestine?
propels chyme for a short distance, allowing time for digestion and absorption
how does gut motility appear in the fed state?
continuous contractile activity (segmentation and peristalsis)
how does gut motility appear in the fasting state?
presence of the migrating motor complex in the stomach and small intestine
what is the function of the migrating motor complex?
cleans stomach and small intestine of undigestable items
what happens to the pyloric sphincter during the migrating motor complex?
opens wider than 2mm to allow non-digested items to pass through
what hormone controls mobility in the fasting state?
motalin
what are the 4 phases of migrating motor complex?
no activity (silent) build up of mechanical activity short lived maximal activity drop back down to no activity
how does gut muscle contraction vary between fed state and fasting state?
during fasting state the MMC contractions are larger than normal contractions seen in fed state. However fed state contractions are ongoing whereas MMC is sporadic with silent phases
what are the 3 main functions of motility in the large intestine?
mixing of chyme to improve absorption of water and salts from the colon (mainly ascending and transverse)
kneading of semi solid contents
movement of contents towards anus
how quickly do non propulsive movements move products towards anus?
5-10cm per hour
what is mass movement/mass peristalsis?
specialised movement that happens 1-3 times a day and moves colonic contents towards the anus.
up to 20cm in one go
what system is motility in the large intestine controlled by?
enteric nervous system
what happens during the gastrocolic reflex?
distention of the wall of the stomach initiates mass peristalsis
what is the rectosphincteric reflex?
distention of the rectum causes signalling from pelvic nerves to the sacral spinal cord
internal anal sphincter relaxes in response to stimulus from spinal motor nerves
describe the route to defecation
distention of rectum
rectosphincteric reflex
relaxation of internal anal sphincter
relaxation of external anal sphincter
contraction of abdominal muscles and relaxation of pelvic wall muscles
flexure of hips and descent of pelvic floor
defaecation
what happens if it is not acceptable to defecate?
distention of rectum
rectosphincteric reflex
relaxation of internal anal sphincter
contraction of external anal sphincter
how much saliva is produced per day?
1.5l
is saliva hyposmotic, hyperosmotic of isosmotic compared to plasma?
hyposmotic
what is the pH of saliva?
~7
what is found within saliva?
mucin glycoproteins, lysozyme and alpha amylase
what is the only hyposmotic solution in the body?
saliva
what are the 3 main functions of saliva?
lubricate food to aid swallowing
clean and protect the cavity of the mouth
reduce starch to oligosaccarides and triglycerides to fatty acids
what elements of saliva are involved in lubrication of food to aid swallowing?
mucin glycoproteins and water
what elements of saliva are involved in cleaning and protecting the cavity of the mouth?
lysozyme
what elements of saliva are involved in digestion of starch and triglycerides?
alpha amylase and lipase respectively
what sort of saliva is released by the parotid gland?
serous (watery) secretion rich in alpha amylase
what sort of saliva is released by the submandibular and sublingual glands?
seromucous (water and mucous) secretion
what are minor salivary glands?
small glands found throughout the mouth
what sort of secretion is released by minor salivary glands?
mucous secretion rich in mucin glycoproteins
what is the main duct of a salivary gland called?
excretory duct
what are excretory ducts of salivary glands fed by?
intercalated duct
what are intercalated ducts of salivary glands fed by?
acinus
what are acinus formed of?
150-200 acinar cells
what is the functional unit of the salivary gland?
acinus
what intracellular structure is found in large quantities in acinar cells?
endoplasmic reticulum due to high levels of protein production
what are zymogen granules?
pockets of enzymes on the apical membrane of acinar cells which perform exocytosis to release enzymes into the intercalated ducts
what are acini lined by?
epithelial cells
what are acinar cells responsible for?
producing elements of saliva that are washed through the ducts
what type of epithelia do acinar cells have?
‘leaky’
what sort of transport do ‘leaky’ epithelial cells allow?
transcellular movement and paracellular movement
what sort of transport across the epithelium occurs in the duct lining cells of salivary glands?
transcellular movement only as the epithelia are tightly bonded
what is in the primary secretion?
isotonic sodium chloride and water
what is the role of Na/K ATPase in NaCl secretion by the salivary gland?
uses energy from the hydrolysis of ATP to transport sodium (out) and potassium (in) across the membrane creating a steep concentration gradient of sodium ions that is used by other transport channels
what is the role of the sodium/potassium/2chloride cotransporter in NaCl secretion by the salivary gland?
through secondary active transport using the gradient created by Na/K ATPase sodium ions move into the cell down electrochemical gradient and bring potassium and chloride ions with them. This sets up a chloride concentration within the cell
how much potassium and chloride is brought into the cell alongside 1 Na+?
1 K+ and 2Cl-
what is the sodium/potassium/2chloride cotransporter also known as?
NKCC
what is the role of basolateral K+ channels in the secretion of NaCl by the salivary glands?
recycling of potassium ions across the membrane creating a source of extracellular potassium that can be used by NKCC and Na+/K+ ATPase
hyperpolarises the basolateral membrane which increases drive for Cl- to exit the cell across the apical membrane
what is the basolateral K+ channel gate opened by?
increase in intracellular Ca2+ concentration
is exit of Cl- across the apical membrane an active or passive process?
passive
what is the apical Cl- channel gated by?
Ca2+ concentration within the cell
how is salt brought into the lumen of the duct?
cl- channel opens and Cl- moves out down electrochemical gradient. The negative charge created by Cl- movement into duct draws Na+ across the apical membrane paracellularly completing movement of NaCl into duct
what is the effect of having NaCl in the lumen of the duct?
exerts osmotic effect that pulls water into the lumen of the duct via paracellular pathway
what does transcellular Cl- movement lead to?
paracellular sodium and water movement
how are salivary glands regulated?
parasympathetic nervous system
what does signalling from the parasympathetic nervous system cause in the epithelial cell of the salivary gland?
increase in intracellular Ca2+ concentration which opens the basolateral K+ channel and apical chloride channel
summerise NaCl secretion by salivary gland acinar cells
Na+/K+ ATPase creates inwardly directed Na+ gradient across basolateral membrane
The NKCC accumulates Cl- ions intracellularly driven by the Na+ gradient
Ca2+ activated K+ channels recycle K+ ions across the basolateral membrane and maintain the driving force of Cl-exit across the apical membrane
Ca2+ activated Cl- channels in the apical membrane provide a pathway for Cl- to exit the cell into the duct
the movement of Cl- ions into the duct lumen draws Na+ and water through paracellular pathways to complete the process of isotonic NaCl secretion
how is the primary secretion of the acinar cells modified in the duct?
duct lining epithelial cells reabsorb Na+ and Cl- by transcellular pathway
potassium bicarbonate is excreted across the same cells on the same pathway
what causes the final ionic composition of hyposmotic saliva?
reabsorption of Na+ and Cl- by transcellular pathway and potassium bicarbonate excretion
what is composition of saliva determined by?
ion transport mechanisms of acinar and ductal epithelial cells
what happens to the ionic composition of saliva as flow rate increases?
K+ remains at constant rate
Na+ rises exponentially
HCO3- increases quickly and then remains at a steady level
Cl- increases linearly
does saliva flow rate affect its osmolality?
no - remains hyposmotic throughout, whatever happens to flow rate
how is saliva secretion controlled?
nerves
what branches of the ANS stimulate saliva secretion/
sympathetic and parasympathetic
what is the effect of acetylcholine on acinar cells?
stimulates production of 0.9% NaCl (fluid) and enzyme release by exocytosis
what is the effect of acetylcholine on duct cells?
indirect - due to increased saliva production less NaCl is absorbed in the duct
what is the effect of noradrenaline on acinar cells?
stimulates release of enzymes by exocytosis
what is the effect of noradrenaline on duct cells?
stimulates secretion of bicarbonate by raising th eICF concentration of cyclic AMP
What are the 3 main roles of the stomach in digestion?
reservoir to aid gastric motility
digests proteins through the action of pepsins
essential for the absorption of vitamin B12 through production of intrinsic factors
what sort of environment do pepsins require in order to perform their function properly?
highly acidic
what is the role of the intrinsic factor in absorption of vitamin B12?
prevents the destruction of vitamin B12 in the small intestine by proteolytic enzymes from the pancreas
what is the name of the majority of the stomach?
corpus
where is the cardia of the stomach located?
just below lower oesophageal sphincter
what is the mucosa of the cardia of the stomach specialised for?
mucous secretion in order to protect the oesophagus from acid
how much gastric secretion is produced in the stomach per day?
2 litres
what is the pH of gastric secretion in the stomach?
0.9-1.5
is gastric secretion in the stomach isosmotic, hyposmotic or hyperosmotic?
isosmotic
what is found within gastric secretions?
HCl, pepsins, intrinsic factors, mucous and HCO3
what is the role of mucous and HCO3?
protection of GI mucosa from highly acidic gastric secretions
what is the gastric mucosa of the stomach made up of?
gastric pits
what is the name of the cell that produces HCl and intrinsic factors?
parietal cell
what is the most dominant cell in gastric glands?
parietal cells
what do parietal cells secrete?
HCl and intrinsic factors
where are gastric glands located?
beneath gastric pits
what is produced by chief cells?
pepsinogen (inactivated pepsin)
what are the 2 types of endocrine cell in the gastric mucosa?
ECL - (entro chromafin like)
D cells
what is produced by ECL endocrine cells in the gastric mucosa?
histamine
what is produced by D endocrine cells in the gastric mucosa?
somatostatin
what type of factor is histamine in the stomach?
paracrine
what type of factor is somatostatin in the stomach?
depending on location is either paracrine or peptide hormone
what is the role of histamine in the stomach?
stimulates acid secretion
what is the role of somatostatin in the stomach?
inhibits acid secretion
which cells produce HCO3 in the gastric mucosa?
surface epithelial cell
which cells produce mucous in the gastric mucosa?
mucous neck cells
where are mucous neck cells located?
in the top of glands
what are the dominant epithelial cells in the gastric mucosa?
surface epithelial cells
what do G cells produce?
gastrin in the antrum
what cells are found close to the G cells in the antrum?
D cells producing somatostatin
what cell mechanism causes the release of pepsinogens from chief cells?
increase in intracellular Ca2+ concentration
how are pepsinogens released from the chief cell into the lumen of the gastric gland across the apical membrane?
exocytosis
what is the role of pepsins?
digestion of proteins
what is the optimal pH for pepsins?
less than 3
what is the spontaneous conversion of pepsinogens to pepsins stimulated by?
HCl
what happens to the speed of pepsinogen conversion to pepsin at pH above 5
very slow
what does increased pepsin presence stimulate?
more pepsinogen to convert to pepsin
what 2 things is low pH int he stomach required for?
pepsinogen activation and pepsin activity
what process does HCl speed up once pepsin is produced?
breakdown of proteins to peptides
what are the 3 functions of HCl?
promotes activation and activity of pepsins
kills or inhibits microorganisms
stimulates secretions in the small intestine and so process of digestion is completed
what are tubulovesicle?
vesicle contained within the apical membrane of resting parietal cells that holds ion transporters essential for secretion of acid
what happens to tubulovesicles when the parietal cell is activated to secrete HCl?
form canaliculi on the apical membrane
what are canaliculi?
finger like projections on the apical membrane of parietal cells that are formed of tubular vesicles inserting into the apical membrane
how doe canaliculi affect the surface area of the apical membrane of a parietal cell?
50-100x increase in surface area
how does an active parietal cell appear?
larger secretory membrane with more H+ (proton) pumps, K+ and Cl- channels
what is the role of the H+/K+ ATPase pump (proton pump) on the apical membrane in secretion of HCl?
uses energy from the hydrolysis of ATP to move hydrogen out of the parietal cell and potassium into the parietal cell
what is required for the H+/K+ ATPase pump to work?
extracellular K+ source
how is the extracellular K+ source maintained to ensure the H+/K+ ATPase pump works?
apical K+ channel allows K+ to exit the cell so that it can be used by the proton pump to move back into the cell
what is required within the parietal cell to maintain the movement of protons across the apical membrane?
source of protons
what does the parietal cell do with water and CO2?
converts them to H+ (protons) and HCO3 (bicarbonate)
how does CO2 enter the parietal cell for binding with water to create H+ and HCO3?
diffuses across basolateral membrane
how does water enter the parietal cell for binding with CO2 to create H+ and HCO3?
requires specific aquaporin channels to cross the cell membrane
what are water and CO2 converted to initially before forming H+ and HCO3?
carbonic acid
what enzyme is used to speed up the process of conversion of water and CO2 to H+ and HCO3?
carbonic anhydrase
what must happen to the bicarbonate produced during proton production?
cell must release it into the blood vessels of the mucosa
what transporter is used to transfer bicarbonate into the blood vessels of the mucosa from the cell?
basolateral membrane exchanger (antiporter)
what is exchanged at the basolateral membrane of the parietal cell so that HCO3 may enter the blood vessels of the mucosa?
Cl- enters the parietal cell
what is an alkaline tide?
when parietal cells secrete acid so blood cells are alkalised by influx of HCO3
under what conditions will an alkaline tide only occur?
only occurs in the fed state when parietal cells secrete acid
how does Cl- exit the apical membrane of the parietal cell?
through gated channel into the lumen of gastric gland
how is HCl produced in the lumen of the gastric gland?
Cl- combines with H+
what is the role of Na+/K+ ATPase in the parietal cell?
creates a steep concentration gradient for sodium across the basolateral membrane which is used by other channels (e.g. regulation of cell pH where Na+ is moved out and H+ in)
what is the role of basolateral K+ channels in the parietal cell?
provides a drive for Cl- and K+ to exit the cell via apical membrane channels
summerise the cellular mechanism of HCl secretion
H+/K+ ATPase uses energy from ATP hydrolysis to pump H+ ions out of the cell across the apical membrane into the lumen of the gastric gland and K+ions into the cell. Apical membrane K+ channels recycle K+ ions across the apical membrane
The secretion of H+ ions across the apical membrane causes intracellular pH to rise. This rise in pH triggers passive uptake of CO2and H2O by the cell across the basolateral membrane. The production of H+ and HCO3-from H2O and CO2 via H2CO3 is catalysed by carbonic anhydrase
HCO3- ions produced by carbonic anhydrase are removed from the cell across the basolateral membrane by the anion (Cl-/HCO3-) exchanger. In exchange, Cl-ions are taken up by the cell
HCO3- ions that exit the cell across the basolateral membrane cause the alkalinisation of local blood vessels termed “alkaline tide”
The Cl-ions that enter the cell across the basolateral membrane via the Cl-/HCO3-exchanger exit passively across the apical membrane via a Cl-channel to complete the process of acid (HCl) secretion
The Na+-K+-ATPase creates the inwardly directed Na+ gradient across the basolateral membrane used by other transporters
Basolateral membrane K+ channels maintain the driving force for Cl- exit across the apical membrane
what 2 factors regulate mucus and HCO3- secretion?
acetylcholine
prostaglandins
what is the role of acetylcholine in the regulation of mucus and HCO3- secretion?
nervous stimulation leading to secretion of mucus and HCO3-
how is HCO3- drawn into surface epithelial cells?
secondary active transport alongside Na+ using the Na+ gradient set up by Na+/K+ ATPase
what is the main role of prostaglandins?
protect the integrity of gastric mucosa
what 3 ways do prostaglandins protect the integrity of gastric mucosa?
inhibit acid secretion by parietal cells
stimulate mucous secretion
stimulate bicarbonate secretion
how is the gastric mucosa protected?
gastric mucosal barrier
what is the gastric mucosal barrier formed of?
mucus impregnated with bicarbonate
how do acid and bicarbonate leave the gland without contact with surface epithelial cells?
viscous fingering - muscularis mucosae squeezes gastric glands and shoots the acid out of the glands
what is the bicarbonate and proton concentration like close to the epithelial cells of the mucosa?
bicarbonate concentration is high
proton concentration is low
bicarbonate neutralises any protons
what is the bicarbonate and proton concentration like close to the lumen of the stomach?
bicarbonate concentration is low
proton concentration is high
what is the physiological protection of the gastric mucosa?
mucous and HCO3-
what forms the anatomical protection of the gastric mucosa?
apical surface of the surface epithelial cells is impermeable to protons
tight junctions do not allow protons to pass through the paracellular pathway
how many physiological agonists stimulate HCl secretion directly?
3
what are the 3 physiological agonists that stimulate HCl secretion directly?
histamine
gastrin
acetylcholine
how do ECL cells stimulate HCl secretion directly?
secrete histamine as a paracrine factor
how do G cells stimulate HCl secretion directly?
secrete the hormone gastrin
how do enteric neurons stimulate HCl secretion directly?
release ACh
how is HCl secretion stimulated indirectly?
nerves stimulate ECL cells to release histamine
gastrin targets ECL cells causing them to release histamine
what nerve is gastric secretion in the cephalic phase entirely dependent on?
vagus
how much does secretion in the cephalic phase contribute to the total volume of secretion?
30%
when does gastric secretion in the cephalic phase occur?
before food enters the stomach
what does the vagus nerve stimulate in the cephalic phase of gastric secretion?
ACh to stimulate parietal cell
ECL cell to release histamine to stimulate parietal cell
G cell to release gastrin to stimulate parietal cell and ECL cell
what cells does the the vagus nerve inhibit in the cephalic phase of gastric secretion?
D cells and so stops inhibition of acid secretion
what is the gastric phase of gastric secretion controlled by?
vagovagal reflexes, hormones and paracrine factors
how much does secretion in the gastric phase contribute to the total volume of secretion?
50%
what mechanical and chemical factors stimulate secretion of acid during the gastric phase?
distention of the stomach
digested proteins and amino acids
what receptors is distention of the stomach detected by?
mechanoreceptors
what receptors detect presence of digested proteins and amino acids?
chemoreceptors
what does distention of the stomach cause in the gastric phase of gastric secretion?
mechanoreceptors via vagovagal reflexes trigger parietal cells and G cells to release/trigger release of HCl
what does digested proteins and amino acids cause in the gastric phase of gastric secretion?
g cell stimulates secretory cell
how is secretion of HCl inhibited if levels become to high during gastric phase of gastric secretion?
HCl triggers chemoreceptors which trigger D cells that inhibit G cells by paracrine factors and parietal cells by endocrine hormones
what happens to gastric secretion during early gastric emptying?
gastric chyme pH is >3 so stimulation dominates
what happens to gastric secretion during late gastric emptying?
gastric chyme is <3 pH so inhibition dominates
what factors stimulate secretion of acid during the intestinal (gastric emptying) phase?
distention of duodenum (mechanoreceptors)
digested proteins and amino acids (chemoreceptors)
what does distention of the duodenum cause in the intestinal phase of gastric secretion?
parietal cell and G cell stimulated due to signalling from mechanoreceptors
what does digested proteins and amino acids cause in the intestinal phase of gastric secretion?
g cell stimulates parietal cell due to signalling from chemoreceptors
what factors inhibit secretion of acid during the intestinal (gastric emptying) phase?
HCl concentration in duodenum
products of digestion in duodenum
what does higher HCl concentration cause in the intestinal phase of gastric secretion?
D cells produce somatostatin which inhibits g cells, ECL cells and parietal cells
s cells produce secretin which shut down G cells in antrum and parietal cells
what does digestion products cause in the intestinal phase of gastric secretion?
cholysystekinine (CCK) released by I cells which directly inhibits parietal cells
what is gastric emptying?
delivery of chyme from stomach to duodenum
what are the fastest and slowest products to be digested and so leave the stomach?
carbs-proteins-fats-indigestable solids
what items from digestion do not exit the stomach during the fed state?
indigestible solids
due to control mechanisms in the intestinal phase what does gastric emptying not exceed?
the rate at which: acid can be neutralised, fat can be emulsified, the small intestine processes chyme
where in the GI tract are all foodstuffs digested completely?
the small intestine
how much pancreatic secretion is produced per day?
1.5 litres
what is the pH of pancreatic juice?
7.8-8.4
what is pancreatic juice formed of?
salts and enzymes
what are the 2 types of pancreatic secretions?
endocrine and exocrine
what is in endocrine pancreas secretions?
insulin and glucagon
what is in exocrine pancreatic secretions?
salts and water
enzymes
what are the main functions of the salts and water in pancreatic juice?
create the right environment for enzymes to work
what is the salts and water portion of pancreatic juice formed of?
HCO3-
NaCl
water
what is the main role of HCo3- in pancreatic juice?
neutralise acid and ensure correct mucous physical and chemical properties
what are the main functions of the enzymes in pancreatic juice?
important for the digestion of all the major classes of foodstuffs and therefore essential for life
what enzymes are found in pancreatic juice?
proteases
lipases
alpha-amylase
what do protease enzymes do?
digest proteins
what do lipase enzymes do?
digest fats
what do alpha-amylase enzymes do?
digest carbohydrates
what is the structure of pancreatic glands similar to?
salivary glands
what are the cells of the pancreatic glands?
acinar cells
what is the name of the secretory unit of the pancreatic/salivary glands?
lobule
what cells secrete enzymes in the pancreas?
acinar cells
which of the enzymes are released into the lumen of the gland of the pancreas already activated?
lipases and alpha-amylase
why can lipases and alpha-amylase be released from the gland as activated enzymes?
they won’t digest the cells of the pancreas on the way out of the duct
how do zymogen granules leave the cell?
exocytosis
what are zymogens?
inactivated versions of enzymes that must be transported to site of action before being activated
what enzymes are released as zymogens from the pancreatic acinar cells?
proteases:
trypsin
chrymotrypsin
carboxypeptidases
why must proteases be released from the gland as zymogens?
as they would digest pancreatic cells if activated in the pancreas
where are zymogens activated after leaving the pancreas?
small intestine
what is trypsin known as while inactive?
trypsinogen
what is chrymotrypsin known as while inactive?
chrymotrypsinogen
what are carboxypeptidases known as while inactive?
procarboxypeptidases
what is the main hormonal stimulator of secretion of enzymes from the pancreas?
CCK
what cells is CCK relesed by?
I cells
as well as CCk, what are the 2 other elements involved in regulation of enzyme secretion?
ACh and secretin
what is composition of pancreatic juice decided by?
acinar and ductal epithelial cells
what is the difference between duct lining cells of the salivary and pancreas glands?
salivary gland has tight epithelia so all transport of ions is transcellular whereas pancreas has leaky epithelia so paracellular transport is possible
what moves by paracellular transport in the leaky epithelia of pancreatic duct cells?
water and NaCl
what is the only difference in production of isotonic NaCl between salivary and pancreatic acinar cells?
regulation - pancreatic acinar cells are regulated by nerves and CCK
how is isotonic NaCl solution modified in the lumen of the duct lining epithelial cells?
transcellular - bicarbonate is added and chloride exits
paracellular - Na and H2O enter the lumen
what is the role of Na+/K+ATPase in the secretion of NaHCO3 from the pancreas?
creates a steep Na+ concentration gradient across the basolateral membrane which is used by other channels in secondary active transport.
how is bicarbonate produced in the ductal epithelial cell of the pancreas?
Co2 diffuses into the cell and water delivered through aquaporins. They are then converted to carbonic acid and then to protons and bicarbonate
what is the production of protons and bicarbonate catalysed by?
carbonic anhydrase
what is a secondary source of bicarbonate?
there are sodium bicarbonate channels on the basolateral membrane
how are the excess protons removed from the cell once bicarbonate has been produced?
Na+/H+ antiporter utilising the Na+ gradient produced by Na+/K+ ATPase
how does bicarbonate leave the cell?
exits through anion exchanger in exchange for Cl-
where does the supply of extracellular Cl- come from to supply the anion exchanger?
apical Cl- channel
what is the role of basolateral K+ channels in secretion of NaHCO3 from the pancreas?
hyperpolarises membrane and so drives Cl- and HCO3- to leave the cell across the apical membrane
how do Na+ and H2O move into the duct of the pancreatic gland?
via paracellular channels
how is secretion of NaHCO3 from the pancreas regulated?
secretin increases the intracellular concentration of cAMP which phosphorylates apical Cl- channels and opens them
summerise secretion of NaHCO3 by pancreatic duct cells
Na+, K+-ATPase creates the inwardly directed Na+gradient across the basolateral membrane
HCO3-accumulates intracellularly by two mechanisms: (i) direct entry across the basolateral membrane by the Na+-HCO3-cotransporter and (ii) intracellular generation from CO2 and H2O via carbonic anhydrase3
The Na+-H+exchanger removes H+ from the cell generated by HCO3-synthesis from CO2 and H2O4. The Cl-/HCO3-exchanger secretes HCO3-into the lumen of the duct in exchange for Cl-
cAMP-stimulated Cl-channels secrete the Cl-required by the Cl-/HCO3-exchanger
Basolateral membrane K+channels maintain the driving force for Cl-exit across the apical membrane
HCO3-secretion into the duct lumen draws Na+and H2O through the paracellular pathway to complete the process of isotonic NaHCO3-secretion
what enzyme activates pancreatic enzymes from zymogens?
enteropeptidase
where is enteropeptidase found?
brush boarder of the small intestine mostly although can be dislodged by bile and then found in the lumen
what substance can dislodge enteropeptidase from the brush boarder?
bile
what does enteropeptidase do in order to activates pancreatic enzymes from zymogens?
converts trypsinogen to trypsin which then converts chrymotrypsinogen and procarboxypeptidase to chrymotrypsin and carboxypeptidase therefore activating them
what is the role of a trypsin inhibitor?
prevents premature activation of trypsin in the pancreas
what cells release enteropeptidase?
epithelial cells of duodenum and jejunum
what cells release trypsin inhibitor?
acinar cells
what are the major stimulants of pancreatic secretion in the cephalic phase of digestion?
vagal impulses
what proportion of pancreatic secretion occurs in the cephalic phase?
25% of total volume
what does distention of the stomach in the gastric phase lead to with regards to pancreatic secretion?
distention stimulates mechanoreceptors and so sets off vagovagal and gastropancreatic reflexes and stimulates secretion
what proportion of pancreatic secretion occurs in the gastric phase?
10% of total secretions
is the enzyme concentration of the pancreatic secretion during the gastric phase high or low?
high
what system does the major stimulation of pancreatic secretion come from during the intestinal phase?
hormones
what is triggered by presence of acid with regard to pancreatic secretion?
chemoreceptors detect H+ and lead cells to release secretin which in turn causes ductal cells to release NaHCO3 leading to a large volume of secretion low in enzymes
what does the presence of fatty acids, monoglycerides and peptides in the duodenum stimulate with regard to pancreatic secretion?
CCK is released by I cells and stimulates acinar cells to release enzyme rich pancreatic juice
what does distention and osmolarity increase in the duodenum trigger with regard to pancreatic secretion?
mechanoreceptors with a vagovagal enteropancreatic reflex increase pancreatic secretion
where are all foodstuff completely digested?
the small intestine
how much bile is produced per day?
0.5l
what is the pH of bile?
7.4
what is bile formed of?
bile salts, cholesterol, phospholipids
what is bile essential for?
digestion of fats
what cells is bile secreted by?
hepatocytes
where is bile stored?
in the gall bladder
what makes up bile?
bile salts (65%)
cholesterol (4%)
phospholipids (20%)
bile pigments (0.3%)
what is the role of bile salts in digestion?
emulsify and so improve the digestion of lipids
why is cholesterol found in bile?
mechanism for cholesterol excretion
what do bile salts, cholesterol and phospholipids form?
mixed micelles
what is the role of mixed micelles found in bile?
aid delivery of fatty acids and monoglycerides to unstirred layer of intestine which helps absorption
what are bile pigments formed of?
breakdown products of haemoglobin (bilirubin)
why is bilirubin found in bile?
so it can be excreted via the GI tract
what happens to bile between meals?
concentrated and stored in the gallbladder
what happens to the gall bladder during the cephalic phase of digestion?
neuronal stimulation of secretion causing the contraction of the gallbladder and relaxation of the sphincter of Oddi
what happens to the gall bladder during the gastric phase of digestion
distention of stomach stimulates mechanoreceptors. Vagovagal reflex via the parasympathetic nervous system causes contraction of the gallbladder and relaxation of the sphincter of Oddi
how is the intestinal phase of digestion regulated?
hormones
what stimulates release of bile from the gall bladder during the intestinal phase?
presence of products of digestion (fatty acids, monoglycerides, peptides) in the duodenum stimulate release of CCK which causes contraction of gallbladder and relaxation of the sphincter of Oddi
what effect do sympathetic nerves have on the gallbladder?
stop contraction of the gallbladder and contract the sphincter of Oddi
why are bile acids reabsorbed?
bile synthesis is hugely metabolically expensive and so only a fraction of required bile is produced per day
where in the GI tract are bile acids reabsorbed?
ileum
what happens to reabsorbed bile acids once they leave the ileum?
returned to the liver and taken up by hepatocytes
what type of mechanism is recycling of bile acids?
homeostatic
what mechanism is used to reabsorb bile acids from the ileum?
Na+ exchanger, Na+ dependent mechanism
how many rounds of bile recycling can take place per meal?
up to 5 depending on richness of meal
how much secretion does the small intestine produce per day?
1l
what is the pH of small intestine secretion?
7.6
what is small intestine secretion formed of?
mucous, enteropeptidase, salt and water
what is enteropeptidase important for?
activation of trypsinogen
in what part of the intestine are villi found?
dominant in SI
what is the role of crypts in the small intestine?
secretion of NaCl and water
what is the role of crypts in the large intestine?
secretion of mucous to protect the surface of the colon
what is the length of the small and large intestine?
small - 6m
large - 2.4m
what is the surface area of the small and large intestine?
small - 300m2
large - 25m2
are there villi in the small and large intestine?
small - yes
large - no
are there microvilli in the small and large intestine?
yes
are there crypts in the small and large intestine?
small - yes (NaCl and water)
large - yes (mucous)
is there nutrient absorption in the small and large intestine?
small - yes
large - no
is there active Na+ absorption in the small and large intestine?
yes
is there active K+ secretion in the small and large intestine?
small - no
large - yes
is there secretion of HCO3- in the small and large intestine?
for protection
small - yes
large - yes
how much water is usually taken in through the mouth per day?
2l
what fluids are secreted by the GI tract?
salivary glands stomach bile duct pancreas small intestine
how much fluid does the small intestine absorb throughout the day?
6.5l
how much fluid does the small intestine absorb throughout the day?
1.9l
what is fluid balance in the intestine achieved by predominantly?
absorption in the small intestine
what process occurs on the villi of the small intestine?
absorption
what happens in the crypts of the small intestine?
secretion of salt and water
what are the 5 mechanisms of digestion and absorption in the small intestine?
none needed (e.g. for glucose)
brush boarder hydrolysis of oligomer to monomer
luminal hydrolysis of polymer to monomer
intracellular hydrolysis
luminal hydrolysis followed by intracellular resynthesis
what happens during brush boarder hydrolysis of oligomer to monomer in the small intestine?
final breakdown of products (e.g. sucrose to glucose and fructose) occurs in the brush boarder of apical membrane
what happens during luminal hydrolysis of polymer to monomer in the small intestine?
breakdown occurs in the lumen of small intestine (e.g. proteins to amino acids)
what happens during intracellular hydrolysis in the small intestine?
partial digestion occurs in the lumen of small intestine (e.g. protein to tri-peptide), this is then absorbed and broken down (e.g. to amino acids) in the cell
what happens during luminal hydrolysis followed by intracellular resynthesis in the small intestine?
triglyceride is broken down to monoglyceride and fatty acids in the lumen and absorbed across the apical membrane. These are then recombined into triglycerides in the cell and absorbed into lacteals
by what enzyme are carbohydrates originally broken down?
alpha amylase
what are the intermediate products of carbohydrate breakdown?
maltose, maltotriose and alpha-limit dextrins
what enzymes are used to split maltose, maltotriose and alpha-limit dextrins into glucose?
maltase, lactase and sucrase-isomaltase
what is maltose broken down into by maltase?
2 glucose molecules
what enzyme is used to break down maltose into glucose?
maltase
where are maltase, lactase and sucrase-isomaltase found in the small intestine?
brush boarder
what is maltotriose split into?
1 maltose and 1 glucose molecule
what are alpha-limit dextrins split into?
1 unit maltose and 1 unit maltotriose
what enzyme are alpha-limit dextrins split by?
sucrase-isomaltase
what is the difference between maltase and lactase and sucrase-isomaltase?
maltase and lactase sit in the apical membrane alone, sucrase-isomaltase is a complex of 2 enzymes
what is produced from the breakdown of sucrose?
fructose
what is produced from the breakdown of lactose?
galactose
what are the transport mechanisms found in the apical membrane of the small intestine?
SGLT1
GLUT5
what does SGLT1 stand for?
Na coupled glucose transporter 1
what does GLUT5 stand for?
glucose transporter 5
what does SGLT1 transport?
glucose and galactose
how does SGLT1 transport glucose and galactose?
secondary active transport using gradient created by Na+/K+ ATPase in the basolateral membrane
what does GLUT5 transport?
fructose
how does GLUT5 transport fructose?
facilitated diffusion
what is the transport mechanism for products of carbohydrate digestion across basolateral membrane into blood vessels called?
GLUT2
what does GLUT2 transport?
glucose, galactose and fructose
how does GLUT2 transport glucose, galactose and fructose across the basolateral membrane?
facilitated diffusion
where is sugar absorption greatest in the GI tract?
duodenum
what are the 3 methods of digestion and absorption of protein in the small intestine?
Luminal enzymes digest proteins to amino acids before absorption by villus epithelial cells.
Luminal enzymes digest proteins to peptides, enzymes in the apical membrane of villus epithelial cells digest the peptides to amino acids before absorption by villus Luminal enzymes digest proteins to peptides, which are absorbed by villus epithelial cells and then digested within the cell (intracellular enzymes) to amino acids
how many mechanisms exist to move amino acids across the apical membrane?
7
are all mechanisms to move amino acids across apical membrane sodium dependent?
no - some not all
what is transport of peptides across the membrane coupled to?
secondary active transport alongside H+ (cotransport)
how many mechanisms exist to move amino acids across the basolateral membrane?
3
where does most amino acid and peptide absorption occur?
duodenum
what are emulsification droplets formed of?
lipid droplets and bile salts
what is the function of an emulsion droplet?
increases the surface area on which enzymes can act
what enzyme is used to break down lipids into monoglycerides and fatty acids?
pancreatic lipase
what is the key preliminary step in reducing the size of lipid droplets and increasing oil water interface?
emulsion
when is pancreatic lipase effective?
only at the oil water interface of an emulsion droplet
what is formed with monoglycerides and fatty acids?
mixed micelles
what is found in mixed micelles?
phospholipid and cholesterol, monoglycerides and fatty acids
what is the role of mixed micelles?
diffuse into acid unstirred layer on apical membrane of epithelial cells
how do fatty acids and monoglyceride cross the apical membrane?
diffusion
what happens to fatty acids and monoglyceride in the smooth ER of the cell?
reassemble into triglyceride and form chylomicrons
what happens to chylomicrons once formed in the smooth ER?
exit villus epithelial cells across basolateral membrane via exocytosis and enter lacteals
what is key to emulsion droplet creation and movement?
the non- propulsive movement of the small intestine
what happens to the bile salt micelle once the fatty acids and monoglycerides have crossed the apical membrnae?
they are recycled and form more mixed micelles
In what part of the small intestine does most lipid absorption occur?
jejunum
