digestive secretions Flashcards
name the 4 secretions of digestive tract
saliva
gastric juice
pancreatic juice
bile
Digestive secretions contain large volumes of
fluids and electrolytes
animals must continually _____ the fluids and electrolytes to maintain hydration
absorb
what lubricates and moistens food
saliva
what is the antibacterial activity of saliva
- Antibodies
- Lysozyme
- Keep bacterial population under control
what begins digestion
salivary enzymes
what enzyme within saliva begins digestion
- Salivary amylase (omnivores)
- Lingual lipase (neonates)
how far do the salivary enzymes work, when do they stop working and why
The salivary enzymes are able to continue their digestive processes in the proximal stomach, where food is stored. They are deactivated by the lower pH of the
distal stomach.
what role does saliva have on thermoregulation
panting in sheep and dogs, as well as the application of saliva on the coat, such as we
see in some rodents
salivary acini secrete what 4 things
water
electrolytes
enzymes
mucous
where is saliva modified
salivary ducts
when saliva is modified in salivary ducts, what happens to it
electrolytes (especially sodium and chloride ions) being reabsorbed. This usually results in a hypotonic solution
what are afferent stimuli for salivation
chewing
stimulation of taste buds
anticipatory
can a salivary response be conditioned
yes ex classical conditioning, dog and bell
The salivary glands are largely under _____ control via what nerves
these nerves have acetylcholine (ACh) as they neurotransmitter and so they are _____
parasympathetic
the facial (VII) and
glossopharyngeal (IX) cranial nerves
cholinergic
Salivary glands also contain βadrenergic receptors, which are activated by
sympathetic nerves or by circulating catecholamines.
what explains why we see increased saliva production in dogs about to attack
Salivary glands also contain βadrenergic receptors, which are activated by sympathetic nerves or by circulating
catecholamines.
Unlike other digestive secretions, there is no ____ regulatory
component for saliva.
endocrine
is there endocrine regulation for saliva
no
what is atropine
anti-cholinergic drug used as anesthetic
since atropine is anti-cholinergic what does it do to saliva production
reduces saliva production; decreases risk of secretions into airways
is ruminant saliva ____tonic
isotonic
compared to other species ruminate saliva what a higher amount of what 2 ions
- bicarbonate (HCO3-)
- phosphate (PO4 2-)
does rumen saliva have high or low pH
high: neutralizes acids created during fermentation; maintains normal rumen microbiota
how much saliva do rumens produce per day
100-200L
gastric mucosa is ____ in most species
glandular
which species have non-glandular regions of gastric mucosa
horse and rat
cardiac gland secrete mainly
alkaline mucous
fundic glands aka proper gastric glands secrete mainly
mucous, HCl and pepsinogen
pyloric glands secrete mainly
mucous, but also gastrin
is gastric region surface mucosa produces
thick mucous to protect surface
parietal cells of stomach produce what
hydrochloric acid
parietal cells are what shape
pyramid; narrow apex and wide base
surface area of parietal cells increased by
deep infoldings in the apical surface
secretory part of parietal cell
canaliculus, subdivided into lots of little canaliculi
how is hydrochloric acid produced by parietal cells
long answer this is just a summary slide
It secretes both hydrogen and chloride ions.
water (H2O) and carbon dioxide (CO2) combine within the cytoplasm of the parietal cell to produce carbonic acid (H2CO3).
This reaction is catalysed by carbonic anhydrase, which is found in high concentrations in the gastric mucosa.
The carbonic acid then
spontaneously dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3–)
The hydrogen ion is then transported into the stomach lumen via a H+– K+ ATPase ion
pump aka proton pump. This uses ATP as an
energy source, to exchange potassium ions from the lumen into the parietal cells of the stomach, with H+ ions being transported in the opposite direction, (from the parietal cell into the lumen.) So 1 K+ is pumped into the cell for each H+ ion that’s secreted into the lumen. In this exchange process, one molecule of ATP is hydrolysed to ADP releasing the energy required.
The K+ ions that accumulate within the cell are released back into the lumen with Cl ions. This allows the potassium ions to be recycled, with little net movement of K+
ions.
results in the net secretion of both hydrogen and chloride ions, being present in the stomach lumen. Their opposing charges leads to them associating with each other, to form hydrochloric acid (HCl).
The bicarbonate ion is transported out of the cell into the blood. It does this via a transporter protein called an anion exchanger. This anion exchanger transports the bicarbonate ion out the cell
in exchange for a chloride ion (Cl–). This makes additional chloride available for
secretion into the glandular lumen.
during the process of parietal cells making Hydrochloric acid, bicarb ions are released into blood. This secretion of
bicarb ions across the basolateral membranes of the parietal cells produces what is called an _____ in the bloodstream.
“alkaline tide”
when is an “alkaline tide” normally seen
after a meal
how is the bicarb being released into blood (during process of parietal cells making hydrochloric acid) neutralized?
Normally this slight increase in blood HCO3- is neutralised by a secretion of hydrogen ions by the pancreas, when bicarbonate ions are secreted. This happens
when the gastric acid reaches the duodenum
how could alkalosis occur when bicarb ions are released into blood
can occur if something (a blockage or vomiting) prevents gastric acid
from reaching the duodenum (gastric acid neutralizes the bicarb ions)
peptic cells aka chief cells secrete
zymogens (inactive proenzymes) which are a type of pepsinogens (protein-digesting enzymes)
pepsinogen
protein-digesting enzyme
When pepsinogens are exposed to acid in the stomach lumen, what happens
a small portion is cleaved off and the enzymes become active pepsin.
this pattern of deactivated to active zymogen formation (by chief cells/ aka peptic cells) prevents what
digestive enzymes from digesting the cells that are making them
Parietal cell membrane is dynamic meaning
number of canaliculi rise and fall according to secretory need
_____ aka (“tubulovesicles“) fuse with the parietal cell membrane to increase surface area
Canalicular precursors
what decreases the surface area of the parietal cell membrane
Endocytosis of canaliculi (reforming the tubulovesicles) to decrease surface area
describe how SA of parietal cell membrane is increased and decreased
Canalicular precursors (“tubulovesicles“) fuse with the membrane to increase surface area
Endocytosis of canaliculi (reforming the tubulovesicles) to decrease surface area
at rest, what happens to the number of proton pumps within parietal cell membrane
number of proton pumps in minimal
the others are sequestered
within the tubulovesicles in the parietal cell
what happens when the parietal cells is stimulated
the vesicles fuse with the cell membrane. This leads to increased numbers of proton pumps in the parietal cell membrane. With more proton pumps, this allows increased movement of hydrogen ions into the stomach thus increasing acid production.
name the 3 ways in which parietal cells can be stimulated
histamine, acetylcholine or gastrin
how acetylcholine stimulates parietal cells
cephalic phase
- released via stimulation of vagus nerve, during cephalic phase of digestion when food is seen or chewed (anticipatory)
- signal sent to CNS –> vagus nerve –> ENS –> released ACh
- stimulates G cells (gastrin) and parietal cells (HCl)
- gastrin (endocrine) stimulates parietal cells (HCl)
gastric phase
- food entering stomach enters stomach –> gastric distension
- stimulates stretch receptors
- stimulates sensory stimulation of ENS
- ENS produces ACh
- same as above
- stimulates G cells (gastrin) and parietal cells (HCl)
- gastrin (endocrine) stimulates parietal cells (HCl)
Gastrin is secreted by
what are these activated by
G cells in stomach
vagus nerve, gastrin related peptide and by peptides in stomach lumen produced via protein digestion
what does activation of G cells lead to
(ie how does gastrin stimulate parietal cells)
production of gastrin which is released into blood, reaches parietal cells, binds to CCK, elevates calcium levels, increases fusion tubulovesicles within parietal cell membrane, more proton pumps, more H ions into stomach, increases acid production
histamine is secreted by
when do they secrete histamine
mast cells and enterochromaffin like cells in stomach mucosa
they secrete histamine is response to presence of gastrin and ACh
how does histamine stimulate parietal cells
The histamine binds to H2 receptors on the parietal cells. This leads to increased fusion of
tubulovesicles with the parietal cell membrane. The mechanism of action is a bit different, rather than elevating calcium levels, it works via the secondary messenger cAMP.
stimulation of gastric secretion
full picture summary
What happens to gastric secretions when an animal eats food?
The food acts as a buffer, raising the stomach pH.
This removes the inhibiting effect of acid on G-cell secretion, increasing gastrin secretion.
As gastric digestion and secretion proceed, and the parietal cells produce hydrochloric acid, the pH in the stomach decreases.
When the pH falls to about 2, gastrin secretion is suppressed, and at a pH of 1, it stops completely.
This removes the gastrin stimulus to the parietal cells, and acid secretion is reduced.
As the acid contents move from the stomach into the
duodenum and the pH of the duodenum is reduced, gastric acid production is
suppressed.
might involve the hormone secretin (which is produced in the duodenum) or reflexes acting through the enteric nervous system.
treating gastric ulcers is based on inhibiting ______ with target of increasing gastric pH to:
gastric acid secretion
above 4
what are 3 treatments of gastric ulcers
- omezaprole: proton pump inhibitor
- antihistamines (H2 receptor antagonists) sich as cimetidine and ranitidine
- line mucosa (like a band aid) with sucralfate
pancreatic exocrine secretions are essential for digestion of complex nutrients such as (3)
- proteins
- starches
- triglycerides
acinar cells secrete enzymes in the form of
zymogens
centroacinar cells and duct cells secrete
sodium bicarbonate solution
pancreatic cells have surface receptors which are stimulated by: (3)
- acetylcholine
- cholecystokinin
- secretin
______ pancreatic secretes hormones into bloodstream
endocrine
_____ pancreas secretes zymogens which helps digest proteins, starches and triglycerides in small intestine
exocrine
centroacinar cells and duct cells modify the
pancreatic juice
centroacinar cells have an exchange protein that transport bicarb ions ___ of cells in exchange for ____
does this exchange protein need ATP?
out
chloride ions
no, driven by high intracellular bicarb concentration
so centroacinar cells hsve proteins that push bicarb out in exchange for CL- in, and so the pancreatic juice is higher pH (more basic) due to the bicarb concentration, so alkaline pancreatic juice, what does it do?
alkaline pancreatic juice helps to neutralise the acidic contents of the
GI tract, coming from the stomach
where does the high bicarb concentration in acinar cells come from
1) electrolyte transport proteins on basolateral surface of cell which include Na+-Hco3- co-transporter; sodium and bicarb ions cotransported into cell
2) carbonic anhydrase present in cell which generates bicarb within cell
3) chloride-bicarb exchanger at luminal membrane
H+ ions produced from carbonic anhydrase reaction, these are removed from acinar cells at basolateral membrane by a sodium/ H+ exchanger, what does this do
acts to balance the “alkaline tide” that was generated by the secretion of gastric acid.
acinar, centroacinar and duct cells have 3 different type of receptors which bind to
acerylcholine, cholecystokinin and secretin
Acetylcholine is released from _____ near the cells, to stimulate pancreatic secretion
nerve endings
Acetylcholine is released from nerve endings near the cells, to stimulate pancreatic secretion
These nerve fibres have their cell bodies in the ______, and then travel outside the gut wall and into the pancreas.
enteric nervous system
what is the main hormonal stimulus for acinar cells
cholecystokinin (CCK)
what is the main hormonal stimulus for centroacinar and duct cells
secretin
acinar cells secrete macimallyt when
Ach, CCK and secretin are bound. Secretin is therefore said to potentiate – or increase – the action of CCK on acinar cells, and CCK potentiates the action of secretin on centroacinar and duct cells
pancreatic stimulation divided into 3 phases
1) cephalic phase: sight and smell of food
2) gastric phase: stomach distension –> vasovagal reflex
3) intetsinal phase: duodenal distension –> ENS stimulation
fats and peptides in duodenum –> CCK released
low pH in duodenum –> secretin released
what produces bile
hepatocytes
order of bile secretion
hepatocytes –> bile –> canaliculi –> bile ductules
bile ductule epithelium modifies bile by adding
H2O, electrolytes, HCO3-
cholesterol is ____ in H2O
insoluble
hepatocytes form bile acids from
cholesterol
are bile acids hydrophobic or hydrophilic
Bile acids have hydrophilic
(water-soluble) and hydrophobic (lipid-soluble) side
bile acids make lipids ____ in H2O
soluble
- Emulsify dietary lipids
- Makes products of fat digestion soluble
what do bile acids do to some cell membrane constituents (like cholesterol and phospholipids) as they secreted
dissolve away
bile contains what 3 components
bile acids, cholesterol and phospholipids
also has other things such as bile pigments and drugs and toxins
what are the components of bile: bile acids, cholesterol and phospholipids, all important for
digestion and absorption of fats in diet
liver serves as excretory organ for _____ compounds
lipid soluble
bile pigments are breakdown products of
hemoglobin from RBCs
what gives bile is green colour and later gives poo brown colour
bilirubin
when no/little food in duodenum the ______ closes and bile is diverted to gall bladder
sphincter of Oddi
sphincter of Oddi
where common bile duct enters into the duodenum
the epithelium of gall bladder does what to bile
absorbs sodium, chloride and bicarb from bile; concentrates the bile and volume is reduced
what happens to bile in species with no gall bladder like horse and rat
sphincter of oddi is non-functional and bile secretion is continous
bile acids aid in digestion and absorption of fats in ____ but are not absorbed themselves until they reach ____
jejunum
ileum
when bile acids are absorbed in ileum, they travel back to liver in
hepatic portal vein, reabsorbed in liver, back into bile (enterohepatic circulation)
some drugs and toxins are recycled by _____
what does this do to them
enterohepatic circulation
extends duration of action
what is secreted in repsonse to fat in duodenum
CCK
CCK is secreted when fat is in duodenum,
what does CCK do to sphincter of oddi and gall bladder
relaxes sphincter of oddi and contracts gall bladder
bile acids returning in enterohepatic circulation stimulates
bile production
bile acids returning in enterohepatic circulation stimulates bile production
what kind of feedback is this
positive
what is secreted in response to acid in duodenum
secretin
secretin is secreted in response to acid in duodenum, what does this stimulate from bile ductules
stimulates H2O HCO3- secretion from bile ductules
ie bile also acts to help neutralize stomach acid
when fat is digested in SI what happens to CCK, and what is the result of this
- CCK no longer released
- Sphincter of oddi closed
- bile no longer secreted into SI
- no bile acids in enterohepatic circulation
- bile production decreases
In monogastric animals, saliva produced during periods of rapid secretion has a higher electrolyte concentration than saliva produced during periods of slow salivary secretion. From your understanding of salivary gland physiology, which of the following appears to be the MOST LIKELY explanation?
During rapid secretion, fluid produced by the acinar cells is exposed to the actions of the duct cells for a shorter time than during slow rates of secretion
Some nutritionists are researching a drug that increases the secretion of saliva in cattle. What effect do you think this would have on the pH of the rumen?
Increase rumen pH
If the enzyme carbonic anhydrase is inhibited, what effect is this likely to have on gastric pH?
Increase gastric pH
Which of the following is NOT a stimulus for gastric acid secretion?
A.
Norepinephrine secretion arising from stimulation of sympathetic nerves
B.
Vagal nerve activity associated with an animal seeing food
C.
The presence of undigested protein in the pyloric antrum
D.
Acetylcholine release stimulated by the gastric stretch receptors sensing the presence of food in the stomach, acting on the enteric nervous system
E.
Histamine release from cells in the gastric mucosa
A.
Norepinephrine secretion arising from stimulation of sympathetic nerves
Fundic glands contain 4 types of secretory cells: mucous neck cells, chief cells, parietal cells, and enteroendocrine cells. Pepsinogen and prorenin are zymogens, which are converted to the active enzymes pepsin and renin in the gastric lumen, where they help to break down protein. Pepsinogen and prorenin are secreted by which of the following types of cells?
Chief cells
There are 5 major gastrointestinal hormones that are part of the intrinsic endocrine system. Which of the following hormones is produced in the distal stomach and stimulates parietal cells to secrete hydrochloric acid?
Gastrin
Which of the following is NOT a natural ligand for pancreatic receptors?
A.
Cholecystokinin
B.
Acetylcholine
C.
Gastrin
D.
Secretin
C. Gastrin
During which phase of pancreatic secretion is secretin released?
Intestinal phase
Bile secretion is initiated by the presence of food in the __________ and stimulated by the return of __________ to the liver.
Duodenum; bile acids
When an animal with a gall bladder ingests food that contains some fat, the gallbladder is signalled to contract. The presence of fat in the proximal __________ stimulates __________ in the mucosa to produce __________, which signals the gall bladder to contract.
Duodenum; enteroendocrine cells; cholecystokinin
