Gastrointestinal Physiology Flashcards
What are the 2 functions of the GI tract?
digestions and absorption of nutrients
In order to serve these 2 functions, what are the 4 major activities of the GI tract?
(1) Motility propels ingested food from the mouth toward the rectum and mixes and reduces the size of the food
(2) Secretions from the salivary glands, pancreas, and liver add fluid, electrolytes, enzymes, and mucus to the lumen of the gastrointestinal tract
(3) Ingested foods are digested into absorbable molecules
(4) Nutrients, electrolytes, and water are absorbed from the intestinal lumen into the bloodstream
Describe the linear arrangement of the GI tract
Mouth –> Esophagus –> Stomach –> Small Intestine (duodenum, jejunum, ileum) –> Large intestine –> Anus
What are the 2 surfaces of the GI wall?
- mucosal
- serosal
The mucosal surface faces the _____ and the serosal surface faces the _____.
lumen
blood
What 3 things does the mucosal layer consist of?
- a layer of epithelial cells
- a lamina propria
- a muscularis mucosae
What does contraction of the muscularis mucosae do?
Changes the shape and surface area of the epithelial cell layer
What lies beneath the mucosal layer?
a submucosal layer
What is found between the submucosal layer and the serosal layer?
2 layers of smooth tissue: circular and longitudinal muscle
What 2 plexuses contain the nervous system of the GI tract?
- submucosal plexus
- myenteric plexus
The GI tract is regulated by the _____ nervous system
autonomic
What are the 2 components of the autonomic nervous system?
- intrinsic component
- extrinsic component
What is the intrinsic component called and what does it do?
Enteric system, which is wholly contained within the submucosal and myenteric plexuses. It communicates with the extrinsic component
The extrinsic component is the ________ and _______ innervation of the GI tract
parasympathetic
sympathetic
Postganglionic neurons of the parasympathetic nervous system are classified as either _____ or _______.
cholinergic
peptidergic
What neurotransmitter do cholinergic neurons release?
ACh
Parasympathetic innervation results in what 5 things?
- contraction of smooth muscle in the GI tract wall
- relaxation of the sphincters
- increased salivary secretion
- increased gastric secretion
- increased pancreatic secretion
Postganglionic neurons of the sympathetic nervous system are classified as _____.
adrenergic
What neurotransmitter do adrenergic neurons release?
norepinephrine
Sympathetic innervation results in what 5 things?
- relaxation of smooth muscle in the GI tract wall
- contraction of the sphincters
- increased salivary secretion
GI peptides are classified as what 3 things?
- hormones
- neurocrines
- paracrines
What are hormones?
Peptides released from endocrine cells of the GI tract
Describe the path hormones take to reach their target cell
They are excreted into portal circulation, pass through the liver, and enter the systemic circulation.
The systemic circulation then delivers the hormone to target cells with receptors for that hormone.
What are paracrines?
Peptides released from endocrine cells of the GI tract
How do paracrines differ from hormones?
They act locally within the same tissue that secretes them.
Paracrines reach their target cells by way of _____.
diffusion
What are neurocrines?
Peptides that are synthesized in neurons of the GI tract and released following an action potential
After neurocrines are released following an AP they ______ across the synapse to act on its target cell.
diffuse
What does motility refer to?
contraction and relaxation of the walls and sphincters of the gastrointestinal tract
What 4 things does motility do to ingested food to prepare it for digestion and absorption?
- grinds
- mixes
- fragments
- propels
All contractile tissue of the GI tract is smooth muscle, except for what 3 structures? What kind of muscle are these?
- the pharynx
- upper 1/3 of the esophagus
- external anal sphincter
They are all striated muscle
The smooth muscle cells of the GI tract are coupled via what?
gap junctions
Contraction of the circular smooth muscle of the GI tract results in what?
shortening of a ring of smooth muscle, which decreases the diameter of that segment
Contraction of the longitudinal smooth muscle of the GI tract results in what?
shortening in the longitudinal direction, which decreases the length of that segment
Contractions of the GI smooth muscle can be either _____ or _____.
phasic
tonic
Describe phasic contraction of smooth muscle
Periodic contractions followed by relaxation
Where are phasic contractions found?
- the esophasgus
- gastric antrum
- small intestine
Describe tonic contraction of smooth muscle
Sustained contraction at a constant level or tone without regular periods of relaxation
Where are tonic contractions found?
- upper region of the stomach
- lower esophageal sphincter
- iliocecal sphincter
- internal anal sphincter
What are the first steps in the processing of ingested food?
chewing and swallowing
What are the 3 functions of chewing?
- it mixes food with saliva, lubricating it to facilitate swallowing
- it reduces the size of food particles, which facilitates swallowing
- it mixes ingested carbohydrates with salivary amylase to begin carbohydrate digestion
What are the 2 types of chewing?
- voluntary
- involuntary
What does the involuntary component of chewing involve?
Reflexes initiated by food in the mouth
Swallowing is initiated ______ in the mouth, but thereafter it is under _____ control
voluntarily
reflex (involuntary)
The reflex portion of swallowing is control by what?
the swallowing center located in the medulla
What are the 3 phases of swallowing?
- oral phase
- pharyngeal phase
- esophageal phase
When is the oral phase initiated? What does it result in?
when the tongue forces a bolus of food back toward the pharynx, this activates the receptors which initiate the involuntary reflex
What is the purpose of the pharyngeal phase?
To propel the food bolus from the mouth through the pharynx to the esophagus
Describe the 4 steps of the pharyngeal phase
1) the soft palate is pulled upward
2) the epiglottis moves to cover the larynx
3) the upper esophageal sphincter relaxes, allowing food to pass into the esophagus
4) a peristaltic wave of contraction propels the food through the open sphincter
Breathing is _______ during the pharyngeal phase.
inhibited
The esophageal phase is controlled by what 2 things?
- the swallowing reflex
- enteric nervous system
What occurs during the esophageal phase?
Food is propelled through the esophagus to the stomach
Primary and secondary _____ waves travel down the esophagus propelling the food toward the stomach
peristaltic
Emptying of the gastric contents to the duodenum takes approximately _ hours.
3
In order to enter the duodenum, solids must be reduced to particles that are how small?
at least 1mm cubed
What are 2 factors that slow or inhibit gastric emptying (increase gastric emptying time)?
- the presence of fat
- the presence of H+ ions
The effect of fat in the duodenum is mediated by what?
CCK
How does CCK mediate the presence of fat?
It slows gastric emptying, ensuring the gastric contents are delivered slowly to the duodenum which provides adequate time for fat to be digested and absorbed
The effect of H+ in the duodenum is mediated by what?
reflexes in the enteric system
How do reflexes in the enteric system mediate the presence of H+?
H+ receptors in the duodenal mucosa detect low pH of the intestinal contents and relay this information to the gastric smooth muscle. This permits enough time for neutralization of H+ by pancreatic HCO3-
Small intestinal motility serves what 3 functions?
- to mix the chyme with digestive enzymes and pancreatic secretions
- expose the nutrients to the intestinal mucosa for absorption
- propel the unabsorbed chyme along the small intestine into the large intestine
What innervates the small intestine?
both sympathetic and parasympathetic nervous systems
Parasympathetic stimulation ______ contraction of intestinal smooth muscle, whereas sympathetic stimulation _______ it.
increases
decreases
What are contents of the large intestine called?
feces
Where do segmentation contractions occur?
in the cecum and proximal colon
In the large intestine, segmentation contractions are associated with characteristic saclike segments called what?
haustra
Where do mass movements occur?
In the colon
A final mass movement propels fecal contents into what?
Into the rectum, where they are stored until defecation occur
As the rectum fills with feces, the smooth muscle wall of the rectum contracts and the internal anal sphincter relaxes via what reflex?
the rectosphincteric reflex
Once the rectum fills to __% of its capacity, there is an urge to defecate
25
Distention of the stomach by food increases the motility of the colon and increases the frequency of mass movements in the large intestine via which reflex?
the gastrocolic reflex
What are the functions of saliva?
- initial digestion of starches and lipids
- dilution and buffering of ingested foods
- lubrication of ingested food with mucus to aid its movement through the esophagus
What are the 3 anatomic divisions of the stomach?
- fundus
- body
- antrum
What are the 4 kinds of cells that are contained in the gastric mucosa?
- parietal cells
- chief cells
- G cells
- mucous cells
The body of the stomach contains _____ glands. What do they do?
oxyntic glands
What are the 2 cell types found in oxyntic glands?
- parietal cells
- chief cells
What do the parietal cells secrete?
HCl and intrinsic factor
Describe the process behind HCl secretion in the parietal cells
1) CO2 combines with H20 to form H2CO3 which is broken down into H+ and HCO3-
2) H+ is secreted
3) Cl- which is reabsorbed from the blood is secreted with H+
What are 3 substances that alter HCl secretion?
- ACh
- histamine
- gastrin
ACh binds to _____ receptors on the parietal cells.
M3
Histamine binds to _____ receptors on the parietal cells.
H2
Gastrin binds to _____ receptors on the parietal cells.
cholecystokinin B
CCK(B)
Both ACh and gastrin stimulate H+ secretion via which second messenger?
IP3/Ca2+
Histamine stimulates H+ secretion via which second messenger?
cAMP
What do the chief cells secrete?
pepsinogen
The antrum of the stomach contains ______ glands.
pyloric
What are the 2 cell types found in pyloric glands?
- G cells
- mucous neck cells
What do the G cells secrete?
gastrin into the circulation
What do the mucous neck cells secrete?
- mucus
- HCO3-
- pepsinogen
Pepsinogen is important in the digestion of what?
proteins
What is the only essential secretion of the stomach?
intrinsic factor
What is intrinsic factor required for?
absorption of vitamin B12 in the ileum
What occurs in the absence of intrinsic factor?
pernicious anemia
The pancreas secretes approximately how many liters of fluid per day? Where does it secrete it?
1 L into the lumen of the duodenum
Pancreatic secretions consist of what 2 components?
aqueous and enzymatic
The aqueous component is high in what? What is the significance behind this?
HCO3-
Which functions to neutralize the H+ delivered to the duodenum from the stomach
What is the function of the enzymatic component of pancreatic secretions?
to digest carbs, proteins, and lipids into absorbable molecules
Describe the structure of the pancreatic exocrine glands
It resembles a bunch of grapes, with each grape corresponding to a single acinus
What is an acinus? What lines it?
The blind end of a branching duct system that is lined with acinar cells
What do acinar cells do?
secrete the enzymatic portion of the pancreatic secretion
What are the ducts of the pancreas lined with?
ductal cells
These ductal cells extend into a special region of _____ cells in the acinus
centroacinar
What do the ductal and centroacinar cells do?
Secrete the aqueous HCO3- component of the pancreatic secretion
What innervates the exocrine pancreas?
both parasympathetic and sympathetic nervous systems
Parasympathetic activity ______ pancreatic secretion, and sympathetic activity _____ it.
stimulates
inhibits
What is bile necessary for?
the digestion and absorption of lipids in the small intestine
What are the components of the biliary system?
- liver
- gallbladder and bile duct
- ileum
- portal circulation
Describe the steps involved in bile production and secretion
1) hepatocytes of the liver synthesize and secrete bile
2) bile flows out of the liver through the bile duct and fills the gallbladder
3) CCK acts on the gall bladder allowing the bile to flow into the lumen of the duodenum
4) bile salts are recirculated to the liver via the enterohepatic circulation
5) bile salts are delivered back to the liver
What are the components of the bile salts?
- Bile salts (including bile acids)
- Bile pigments like bilirubin
- Cholesterol
- Phospholipids
- Electrolytes
- Water
