GI Tract Flashcards
Where does most GI secretion occur?
Mostly in the stomach and small intestine
Where does most GI absorption take place?
primarily in the small intestine
Why must macromolecules from food, specifically proteins, be degraded into monomers before they can be absorbed?
it allows for avoidfance of the immune susystem recognizing ingested proteins as foreign
What are the only two categories of GI muscles under voluntary control?
muscles of mastication and external anal sphincter
What type of transport facilitates lipid transport across GI membrane?
simple diffusion
Basolateral cell membrane glucose transport is an example of what type of transport across the GI tract membrane?
facilitated diffusion
Na+/glucose and Na+/amino acid contransport in the luminal membrane of the small intestine epithelium is an example of what type of transport
cotransport (secondary active)
Na+/H+ ATPase in gastric parietal cells is an example of what type of membrane transport
primary active
Na+/H+ exchange in intestinal epithelium is an example of what type of transport
countertransport
What are the 3 characteristics of the epithelial layer of the intestinal wall?
high turnover rate
single layer of columnar epithelial cells
specialized functions
how often are intestinal epithelial cells turned over?
every 2-3 days
What happens if microorganisms are able to penetrate the intestinal epithelial layer?
they are picked up by the lymphatics in the connective tissue layer
What are the two goals of regulation of GI function?
-obtain complete absorption of ingested organic nutrients
-restore homeostasis via negative feedback mechanisms
What are the two neural plexuses of the GI tract
submucosal and myenteric
The neurons of the enteric (intrinsic) nervous system can synapse in 3 locations, what are they?
-eachother
-smooth muscle and glandular cells
-sympathetic and parasympathetic nervous system
What are the two main nerves of PNS in the GI tract?
vagus (upper GI- to left colic flexure)
pelvic (lower GI- after left colic flexure)
What are the parasympathetic effects on the GI tract
increase secretion and mobility
What are the sympathetic effects on the GI tract
decrease secretion and mobility except for sphincters which contract
What are the two types of neural reflexes involved in GI regulation? what is the difference?
long and short reflexes. Long involves the CNS whil short does not need CNS input
What is the purpose of short reflexes in the GI tract
-adjust to local conditions, while being modulated by the ANS
What are the 5 main neurotransmitters of the GI tract?
- Ach
2.NE - VIP
- NO
- Gastrin releasing peptide (GRP)
What are the main actions of Ach on the GI tract?
-constriction of smooth muscle in wall
-relaxation of sphincters
-increase salivary, gastric, and pancreatic secretions
What are the main actions of NE on the GI tract?
-relaxation of smooth muscle in wall
-contraction of sphincters
-increase salivary secretion
What are the main actions of VIP on the GI tract?
-relaxation of smooth muscle
-increase intestinal and pancreatic secretion
What are the main actions of NO on the GI tract?
relaxation of smooth muscle
What are the main actions of GRP on the GI tract
increase gastrin secretion
What are the 4 classical hormones of the GI tract
-Gastrin
-Secretin
-CCK
-GIP
What are the major effects of gastrin
stimulates HCL production in the stomach and growth of mucosa in stomach and intestines
What are the major effects of secretin
stimulates production of HCO3- in pancreas
What are the major effects of CCK
stimulates secretion and production of pancreatic enzymes. Contracts gallbladder
What are the major effects of GIP
stimulates insulin production. Reduces HCL production and stomach emptying
True/False: salivary secretion is almost exclusively regulated by the PNS
true
What is the main regulatory mechanism for the pancreas?
Hormonal
What is the main regulatory mechanism fro the small intestine (and most of the large intestine)
local reflexes ( enteric )
What are the 3 regulatory phases of the digestive process?
-celiac
-gastric
-intestinal
Describe the celiac phase of digestive regulation
-occurs in anticipation of food
-occurs via long reflexes
Describe the gastric phase of digestive regulation
-initiated by stomach distension
-occurs via short and long reflexes
- gastrin secretion occurs during this phase
Describe the intestinal phase of digestive regulation
-initiated by intestinal volume and composition (mainly duodenum)
-secretion from pancreas and gall bladder occur during this phase
What is the effect of ghrelin on appitite?
pro food intake hormone
Which salivary gland is responsible for 50% of total salivary volume?
parotid
What type of fluid is produced by the sublingual and mandibular salivary glands?
mixed serous and mucous
Describe the function of acinar cells in saliva formation
secrete initial saliva, comprised of water, ions, enzymes, and mucus
Describe the function of ductal cells in saliva formation
modify the initial saliva by altering electrolyte concentrations
Describe the function of myoepithelial in saliva formation
contract with neural input to eject saliva
True/False: the initial saliva is hypotonic to plasma
false, it is isotonic
Why is saliva hypotonic in the final product?
because of the Na+/Cl- absorption modification by ductal cells
Which species’ saliva is always isosmotic with plasma?
ruminants
Why is ruminant saliva always isosmotic with plasma?
because of the increased HCO3- and phosphate secretion into saliva
Saliva composition is dependent on the ____ with ductal cells
contact time
more contact time with ductal cells result in what type of saliva?
more hypotonic due to the increase in solute reabsorption
less contact time with ductal cells results in what type of saliva
more isosmotic due to less modification time
What are the 7 main functions of saliva?
-lubrication
-antibacterial effect
-enzyme digestion
-pH regulation
-binding of tannins
-providing urea for protein synthesis in the forestomachs (ruminants)
-thermoregulation
Salivary secretion is entirely under ____ control
neural
What ANS stimulation dominates salivary sectretion during meals?
PNS
Describe the swallowing reflex
pressure against the pharynx stimulates the swallowing reflex
sensory arms- located in the walls of the pharynx
afferent arm- vagus and glossopharyngeal muscles
integration- medullary swallowing center
effect arm- motor nerves to the striated muscles of pharynx and upper esophagus
What is the purpose of the secondary peristaltic wave of the swallowing reflex?
-clears the esophagus of food left behind after primary wave
True/False: the secondary peristaltic wave of the swallow reflex involves both the ANS and enteric nervous system?
false, it only involves enteric
What are the five main functions of the stomach
- some enzymatic degredation of starch and protein
- temporary storage location for ingested food
3.mechanical processing of digesta - HCl secretion into stomach to kill bacteria ingested with food
5.parietal cells secrete intrinsic factor (IF)
What is the purpose of intrinsic factor (IF)?
a glycoprotein which is required for the absorption of Vit B12
What is chyme
ingested food that has mixed with stomach secretions and converted to a semi liquid mixture
What is the non glandular part of the stomach (-ruminants)? lined by what class of cells?
esophageal, lined by simple squamous epithelium
What is the glandular part of the stomach lined with?
simple columnar epithelium
In what species is the non glandular portion of the stomach important for storage
horses
What part of the pig stomach serves as storage?
the cardia
The stomach wall is comprised of 3 layers of smooth muscle, what are they?
-outer longitudinal
-middle circular
-inner oblique
What are the two main functions of mucin producing cells?
they produce mucus as well as function as stem cells
Parietal cells secrete what?
HCL and IF
Chief cells produce what secretory product
pepsinogen
What is the function of somatostatin?
inhibitory effect on HCl production (paracrine effect)
What are the effects of histamine producing ECL cells?
histamine binds to neighboring parietal cells in a paracrine fashion and stimulate HCl secretion
Define responsive relaxation
relaxation of the proximal portion of the stomach in response to food intake
In what reflex process does postganglionic PNS fibers release VIP and NO, not Ach?
receptive relaxation
True/False: GI pacemaker cell frequency is not influenced by neural or hormonal inputs?
true
What is essential for GI pacemaker cells to reach AP threshold?
neural input
What are pacemaker cells and where are they mainly found in the GI tract
modified smooth muscle cells and they are abundant in the myenteric plexus
The depolarization and plateau phase of GI pacemaker cells are due to what current
inward Ca2+
Repolarization of GI pacemaker cells are due to what current
outward K+
Gastric emptying is determined by stimulatory mechanisms originating in the ____ and inhibitory mechanisms originating in the ___
Stomach, duodenum
What cells secrete gastrin?
endocrine epithelial cells (G cells)
What factor results in release of CCK
high fat content in the duodenum
What is the strongest signal for hormonal inhibition of gastric emptying?
high fat content in the duodenum
What 4 other signals (outside of high fat content) inhibits gastric emptying?
low pH
High osmolarity
distension of duodenum
high concentration of peptides
What are the 4 major components of gastric juice?
HCl
Pepsinogen
IF
Mucus
True/False: after a high protein meal, the blood and urine pH will be elevated due to alkaline tide
true
What is an alkaline tide?
increased secretion of HCl into the intestinal lumen results in more HCO3- being pumped into the capillary lumen and thus the blood to balance acid base
What three substances stimulate HCl secretion? what are they secreted by? what are their effector paths?
Histamine from ECL cells (paracrine)
Ach from vagus efferents (neurocrine)
Gastrin from G cells in the pyloris (endocrine)
gastrin, in addition to directly stimulating HCl secretion, can indirectly stimulate it by stimulating secretion of which substance
histamine
almost all of the HCl secretion into the stomach occurs during what two phases of the digestive process?
celiac and gastric
True/False: the same duodenal signals that reduce gastric contractility also reduce HCl secretion?
true
What is the role of prostoglandins on HCl secretion
they inhibit by blocking the effects of histamine, they also stimulate mucus and bicarb secretions
What are the sequence of events of small intestine peristaltic contractions?
- expansion of the intestine release of serotonin from ECL cells which bind to afferent enteric neurons
- excitatory NT (Ach) are released behind bolus to contract smooth muscle
- Inhibitory NT (VIP NO) are released in front of bolus to relax smooth muscle
What is the main regulatory pathway of peristalsis in the small intestine
Intrinsic (enteric) reflexes
What is the role of the ANS in small intestine peristalsis
-PNS activity via the vagus increases contractile strength
-SNS activity via postganglionic fibers from the cranial and caudal mesenteric plexus decrease contractile strength
In omnivores and carnivores: when digestive processes are happening what type of motility predominates?
mixing contractions, peristalsis is weak
what is the interdigestive state?
when most of the nutrients have been absorbed from the small intestine
What is migrating motility complex
pattern of electrical activity that clears the intestine of residual, often indigestible substances
where is bile acids and B12 absorption concentrated to?
the ileum
What are the 3 transcellular mechanisms of absorption?
-diffusion
-endocytosis/exocytosis
-transport mediated by carrier proteins
What are the two steps of carbohydrate digestion/degredation
-luminal phase: amylase breaks down starch and glycogen to poly and disaccharides
-membranous phase: maltose and other simple carbs broken down to monosaccharides by enzymes in the apical membrane of the epithelial cells
What form(s) of carbohydrates are able to be absorbed?
monosaccharides only
Describe the routes of absorption of monosaccharides on the apical membrane? basolateral?
glucose and galactose use SGLT1s and fructose utilizes facilitated diffusion on the apical surface, all monosaccharides use GLUT2 on the basolateral
What are the absorbable forms of protein
amino acids, di and tripeptides
Where are the two locations of protein digestion/degradation? by what enzymes
stomach-pepsin and small intestine-trypsin
What brush border enzyme activates trypsinogen?
enterokinase or autocatalyzed by activated trypsin
What are the routes of protein absorption on the apical surface of the cell?basolateral?
amino acids pair with Na+ for secondary active transport, and di/tripeptides pair with H+ on the apical surface, all use facilitated diffusion on the basolateral
What is the one example of intact protein absorption
passive transfer of maternal antibodies to neonates
How does maternal antibody absorption happen without degredation?
-membranes of intestinal epithelia are permeable to intact proteins for 24 hours after birth
-production of HCl is low for 1-2 post birth
-pancreatic secretions is not fully developed
-colostrum contains substances that block trypsin
True/False : carbohydrate, protein, and lipid digestion all require a luminal and membranous phase
false, lipid digestion does not have a membranous phase
What are the 4 main enzymes involved in lipid digestion?
-Colipase
-ligual/gastric/pancreatic lipases
-cholesterol ester hydrolase
-phospholipase A
What enzyme is essential in lipid digestion that is displaces bile salts to allow other enzymes access?
colipase
What comprises the outside of a chylomicron?
Apo proteins and phospholipids
