Stomach Flashcards
What are the primary functions of the stomach?
- STORE FOOD
- needs to be released at an appropriate rate for optimal digestion & absorption - SECRETE HCl & ENZYMES
- begins protein digestion - CREATE CHYME
-mixing food with secretions via movements through contractions of muscle layers.
Produce a liquid mixture (chyme)
What is gastric filling?
Eating triggers relaxation of stomach (folds)
[RECEPTIVE RELAXATION]
Process is mediated via vagus nerve
If >1L consumed, stomach becomes distended,
THUS, increasing intra-gastric P (= discomfort)
What is gastric mixing?
INTERSTITIAL CELLS OF CAJAL
-(in fundus) act as pacemaker cells!
Slow waves, entire length of stomach (~3/minute)
PERISTALSIS activated when slow wave potential reaches threshold, triggering an AP
Having food in stomach means the starting point of which SLOW WAVES OSCILLATES IS SET CLOSER TO THE THRESHOLD, & thus more likely to result in APs.
MUSCULATURE THIN IN FUNDUS (pocket of gas) + BODY (storage)
-THUS, less mixing
When peristaltic wave reaches the antrum,
- stronger contractions
- THUS, thicker musculature
- THUS increased mixing
What role does the pyloric sphincter play in terms of gastric mixing?
Peristaltic wave pushes chyme towards pyloric sphincter
Tonic contraction of the sphincter keeps it almost closed, but open enough for fluid to pass through
(amount of chyme that gets through is proportional to the strength of the antral contraction!)
Peristaltic waves reaches sphincher --> Sphincter contracts more forcefully --> Closes tightly --> Chyme hits sealed-off sphincter --> Chyme tumbles back into antrum --> More mixing!
What is gastric emptying (& motility)?
Digestion triggers increased gastric motility through a direct effect of stretch on the smooth m., as well as involvement of the intrinsic plexuses, vagus nerve and gastrin.
Chyme can only be released once it reaches appropriate degree of fluidity.
What factors increase gastric (antral) motility?
- Intensity of antral peristaltic contractions (motility) controls mixing and emptying.
- Factors change excitability of stomach by depolarising/hyperpolarising smooth m. cells
-Increase excitability –> basic electrical rhythm generates more frequent APs
What factors inhibit gastric (antral) motility?
NEURAL
(Intrinsic & Extrinsic nerves)
-Emotion and Pain
ENTEROGASTRONES
(Secretin, CCK)
DUODENUM::
-FAT: slowly digested, needs time to process fat
- ACID: chyme must be neutralised or inactivates enzymes & irritates duodenal mucosa
- HYPERTONICITY: a.a’s & glucose; draws H20 from plasma to reach istonicity
- DISTENSION: too much chyme, excess vol.
What is Emesis?
VOMITING
-coordinated by the vomiting centre in the medulla oblongata (in brainstem)!
Deep inspiration, glottis closed, uvula raised –>
Stomach, oesophagus & gastroesphageal sphincter RELAXED –>
Respiratory muscles (diaphragm & abdominal) are CONTRACTED. Stomach is squeezed between descending diaphragm & increasing intra-abdominal P
[Sensation of nausea, salivation, sweating, rapid heart rate - ANS]
Name the different types of gastric secretions?
SURFACE MUCOUS CELLS - secrete mucus
MUCOUS NECK CELLS - “ “
PARIETAL CELLS - secrete HCl & intrinsic factor
CHIEF CELLS - secrete pepsinogen & gastric lipase
G CELLS - secrete gastrin
What is gastric juice?
Collective exocrine secretions!
What type of mucosa is located in the fundus + body?
OXYNTIC MUCOSA
-predominately secretes acid
What type of mucosa is located in the antrum?
PYLORIC GLAND AREA
-predominately secretes mucus
Where are secretory cells located in the stomach?
IN MUCOSAL INFOLDINGS!
[may be exo-, para- or endocrine)
What is volume of chyme proportional to?
DISTENSION
stretch, intrinsic plexuses, vagus nerve, gastrin
Why is fluidity important in the stomach?
(CHYME) - MORE EASILY EMPTIED
How is HCl synthesised in Parietal cells?
H20 dissociates into H+ and OH-
- H+ transported out of cell into stomach lumen via H+K+ATP pump
- -[H+ can’t pass back into mucosa due to the epithelial barrier]
-OH- converted to HCO3- via Carbonic Anhydrase (with CO2)
- HCO3- transported into interstitial fluid via HCO3- Cl- antiport
- -[THUS, Cl- enters cell against its electrochemical gradient] - it then diffuses into stomach lumen
The (-) potential set up by Cl- secretion enables H+ secretion against a huge concentration gradient
What are the properties of HCl?
- Not a digestive enzyme (but AIDS digestion)
- Activates PEPSINOGEN TO the active enzyme PEPSIN
- Breaks down CT & muscle fibres into smaller particles
- Denatures proteins (uncoiling of proteins means peptide bonds exposed for enzymatic attack!)
- Antibacterial (kills most organisms)
What are the properties on Intrinsic Factor (IF)?
Parietal cells secrete IF –>
IF binds with Vitamin B12 –>
This complex binds with a receptor in the terminal ileum, and the entire complex is ENDOCYTOSED –>
Vit. B12 is essential for the normal function of RBCs
(otherwise anaemia)
What are the properties of Pepsinogen?
- Major digestive component
- Synthesised and packaged by ER & Golgi
- EXOCYTOSED from storage vesicles (zymogen granules)
-Pepsinogen –(HCl) –> Pepsin
[autocatalytic activation]
-Initiates protein digestion by cleaving peptide bonds
What are the properties of Mucous cells and Surface epithelium?
SECRETE PROTECTIVE MUCUS
- Lubricating
(no mechanical injury) - Pepsin is inhibited
(no self-digestion) - Alkaline secretion
(no acid injury)
What are the function(s) of G cells, and where are they located?
- SECRETE GASTRIN
- STIMULATE acid secretion by acting on ECL & parietal cells
LOCATED in Pyloric Gland Area
What are the function(s) of ECL cells, and where are they located?
- SECRETE HISTAMINE
- STIMULATE acid secretion by acting on Parietal cells
LOCATED in Oxyntic Area
What cells are primarily present in ENDOCRINE secretion?
G cells
ECL cells
D cells
What are the function(s) of D cells, and where are they located?
- SECRETE SOMATOSTATIN
- INHIBIT gastrin & acid secretion
LOCATED in Pyloric Gland Area
What are the 3 phases of gastric secretion?
- Cephalic Phase
- Gastric Phase
- Intestinal Phase
What is the CEPHALIC stage of gastric secretion?
Increased secretion of HCl and pepsinogen in response to THINKING/SEEING/SMELLING/TASTING food
–>Cerebral cortex stimulates parasympathetic nervous system
Vagus nerve stim. gastric secretion & motility by increasing muscle and gland activity
-[Vagal stim. of the intrinsic plexuses]
What is the GASTRIC stage of gastric secretion?
Food REACHES STOMACH & proteins increase gastric secretion via BOTH:
- vagal stim.
- direct stim. of the intrinsic nerve plexuses (ENS)
Stretch & chemoRs provide info
(including distention, semi-digested protein, caffeine)
Activates short (myenteric) & long (vagal) reflexes, G cells
What is the INTESTINAL stage of gastric secretion?
Encompasses inhibiting factors originating FROM THE SMALL INTESTINE
This stops the flow of gastric juices )& gastric motility) as chyme is processed in S.I.
Gastric secretion decreases as:
- protein in stomach is removed
- pH falls very low in stomach
- food enters duodenum
What is the function of the gastric mucosal barrier?
- Epith. cells almost impenetrable to HCl
- Tight junctions prevent acid diffusing between cells
- Mucus stops acid from physically penetrating
- Bicarb. secreted into mucus neutralises acid & inactivates pepsin
Why is alcohol absorption slowed when consumed with a meal?
MORE SLOWLY ABSORBED WHEN GASTRIC EMPTYING IS DELAYED!
(keeping alcohol in the stomach for longer)
THUS, consuming a high fat meal
[(the most potent duodenal-derived gastric inhibitory factor)]
while drinking prevents alcohol from producing its effects as rapidly
