Stomach Flashcards
What is the stomach?
highly involuted sac-like pouch that is divided into three main sections
What are the three parts of the stomach?
- fundus
- body
- antrum
What is the fundus section of the stomach?
portion lying above gastroesophageal sphincter
What is the body section of the stomach?
middle portion
What is the antrum section of the stomach?
most distal region, characterized by thickening of muscularis externa
What GI processing differences are there between the different regions of the stomach?
differences in motility and secretions
What are the 3 primary roles of the stomach?
- mixing and mechanical breakdown of stomach contents
- storage of ingested food, and regulated delivery of processed stomach contents to duodenum
- secretion of HCl and enzymes involved in protein digestion
Gastric Motility
What are the 4 categories of motility associated with the stomach?
- filling
- storage
- mixing
- emptying
Gastric Motility
Filling – What is receptive relaxation?
stomach reflexively relaxed upon food entry
Gastric Motility
What is filling mediated by?
via short loop ENS inhibition of muscularis externa
Gastric Motility
What happens to stomach structure during filling?
stomach can expand from volume of ~50 ml to ~1000 ml with little change in tension in gastric wall
Gastric Motility
Where is food stored?
in stomach body
Gastric Motility
What movement occurs during storage?
periodic (3/minute) weak peristaltic contractions occur, propagating from fundus toward antrum
these contractions result from fundal pacemaker cell activity – do little to move stomach body contents
Gastric Motility
What occurs during mixing?
- rhythmic slow wave propagation and associated muscularis externa contractions generate powerful antral peristaltic waves
- force antral contents toward pyloric sphincter – these are responsible for gastric mixing
- pyloric sphincter is only open enough to let small amounts of liquid through with each peristaltic contraction
- this wave of contraction also contracts sphincter so that solid luminal contents are diverted backward
Gastric Motility
Emptying – What does the volume of chyme that enters the duodenum dependent on? (2)
- strength of contraction of pyloric sphincter
- number of peristaltic waves per unit time
Gastric Motility
Emptying – What are the two apparent problems that the body must overcome?
- stomach receives variable input in terms of frequency, quantity and consistency of food, whereas duodenum requires stable environment
- gastric contents are highly acidic, while duodenum requires basic environment
Gastric Motility
Emptying – How does gastric emptying solve the two apparent problems (see previous question)?
gastric emptying is under tight regulation – emptying is influenced by both gastric and duodenal factors to control:
- strength of contraction of pyloric sphincter
- rate of gastric peristaltic contractions
Gastric Motility
What factors regulate gastric emptying?
- gastric factors – minor
- duodenal factors – major
Gastric Motility
What are the gastric factors that regulate gastric emptying? (2)
- amount of chyme in stomach
- degree of fluidity of gastric contents
Gastric Motility
What are duodenal factors that regulate gastric emptying? (4)
- acid
- fat
- peptides and amino acids
- osmotic pressure
Gastric Motility
What is the neuronal response to duodenal factors that regulate gastric emptying?
enterogastric reflexes
- short-loop reflexes (ENS)
- long-loop reflexes (ANS)
Gastric Motility
What is the hormonal response to duodenal factors that regulate gastric emptying?
enterogastrones
include cholecystokinin (CCK), gastrin, and secretin
Gastric Motility
How does acid regulate gastric emptying?
(pH < 3.5)
- neuronal reflex
- release of hormone secretin
Gastric Motility
How does fat regulate gastric emptying?
(monoglycerides or fatty acids)
- release of hormone CCK
- neuronal reflex
Gastric Motility
How do peptides and amino acids regulate gastric emptying?
release of hormone gastrin
Gastric Motility
How does osmotic pressure regulate gastric emptying?
(hypertonic chyme)
- unidentified hormone
- neuronal response
Gastric Secretion
What are the specialized infoldings of the gastric mucosa functionally divided into?
- oxyntic mucosa: lines fundus and body
- pyloric gland area (PGA): lines antrum
Gastric Secretion
What are the exocrine secretions? (3)
- mucus
- HCl and intrinsic factor
- pepsinogen
Gastric Secretion
What are the endocrine and paracrine secretions? (3)
- histamine
- gastrin
- somatostatin
Gastric Secretion
Where is mucus secreted from?
mucous cells in gastric pits and on epithelial surface of stomach lining
Gastric Secretion
Where is HCl and intrinsic factor secreted from?
parietal cells in gastric glands of oxyntic mucosa
Gastric Secretion
Where is pepsinogen secreted from?
chief cells in gastric glands of oxyntic mucosa
Gastric Secretion
Where is histamine secreted from?
enterochromaffin-like (ECL) cells in gastric glands of oxyntic mucosa
Gastric Secretion
Where is gastrin secreted from?
G cells in gastric glands of PGA
Gastric Secretion
Where is somatostatin secreted from?
D cells in gastric glands of PGA
Gastric Secretion
How is HCl secreted?
- parietal cells actively secrete HCl
- ionic pumps move H+ and Cl- ions against their concentration gradients
- biochemical neutrality maintained
Gastric Secretion
What are the 4 functions that HCl performs that assists GI activity?
- converts pepsinogen into active pepsin
- breakdown of connective tissue and muscle fibres
- breaks tertiary structure of proteins
- kills some ingested microorganisms
Gastric Secretion
What is pepsinogen?
inactive form of pepsin
Gastric Secretion
What is autocatalysis of pepsinogen?
- HCl activates
- pepsin cleaves peptide bonds between certain amino acids
- stored inactive in zymogen granules within chief cells
Gastric Secretion
What are the functions of mucus?
protective
- lubrication
- inhibits pepsin to protect against auto-digestion
- neutralizes gastric acid at epithelial surface
Gastric Secretion
How are ulcers formed?
- H. pylori secretes toxin that weakens mucosal barrier
- acid and pepsin penetrate mucosal barrier
- histamine release enhances gastric acid and pepsin production
- other contributing factors include chronic alcohol, NSAIDs and stress
Gastric Secretion
Is ulcer formation multifactorial?
yes
Gastric Secretion
What is intrinsic factor required for?
absorption of vitamin B12 red blood cell formation
Gastric Secretion
What happens if there is a failure to produce intrinsic factor?
pernicious anaemia
Gastric Secretion
What are the 4 chemical messengers that regulate secretion of gastric juices?
- acetylcholine
- gastrin
- histamine
- somatostatin
Gastric Secretion
What does acetylcholine stimulate? (4)
- parietal (H+) cell secretions
- chief (pepsinogen) cell secretions
- ECL (histamine) cell secretions
- G (gastrin) cell secretions
Gastric Secretion
What is gastrin secreted into the blood by? When?
secreted into blood by G cells in PGA gastric glands, in presence of protein products in stomach lumen
Gastric Secretion
What does gastrin stimulate? (3)
- parietal cell secretions
- chief cell secretions
- ECL cell secretions
Gastric Secretion
What is the primary factor responsible for increasing gastric secretions during the ingestion of a meal?
gastrin
Gastric Secretion
What does gastrin promote growth (trophic) of?
gastric and duodenal mucosa – maintains their functionality
Gastric Secretion
What is histamine?
paracrine substance released from ECL cells in oxyntic gastric glands in response to both gastrin and Ach
Gastric Secretion
What does histamine stimulate?
acts locally to stimulate parietal cell H+ production
Gastric Secretion
What does somatostatin do?
inhibits and acts in negative-feedback manner to turn off gastric H+, pepsinogen and histamine production
Gastric Secretion
What are the 3 phases of secretion of gastric juice during ingestion of a meal?
- cephalic phase
- gastric phase
- intestinal phase
Gastric Secretion
What occurs during the cephalic phase?
- secretion of pepsinogen and H+ in response to sight, smell or thought of food and process of swallowing
- initiated in hypothalamus and mediated by vagal efferents
- vagal input also stimulates G cell production of gastrin
Gastric Secretion
When does gastric phase begin?
when food actually enters stomach
Gastric Secretion
What occurs during the gastric phase?
- proteins and peptides within lumen are the most potent stimuli
- receptors in ENS initiate short reflexes resulting in gastrin release from G cells
- long reflex loop also activates H+, and gastrin secretion via vagal and ENS activity
- histamine release is also stimulated, augmenting H+ secretion
- distension/caffeine/alcohol can also stimulate gastric juice production
Gastric Secretion
When does the intestinal phase begin?
when chyme empties into duodenum
Gastric Secretion
What type of phase is the intestinal phase?
inhibitory
Gastric Secretion
What happens during the intestinal phase?
- as meal leaves stomach, protein is removed
- somatostatin is released from D cells in PGA gastric glands in response to drop in pH – inhibits parietal, chief, and HCl cell activity
- negative feedback influence on gastric secretions via enterogastric reflexes and enterogastrones CCK and secretin
- these factors also decrease gastric emptying
Gastric Digestion
What types of digestion occur?
- carbohydrate
- protein
- iron*
Gastric Digestion
What occurs during carbohydrate digestion?
- initiated in mouth from salivary amylase
- continues to occur in food bolus as it is stored in body of stomach
- no further amylase is secreted by stomach
Gastric Digestion
What causes protein digestion?
resulting from pepsinogen and acid exposure
Gastric Digestion
Where does protein digestion occur?
in antrum of stomach where ingested food is thoroughly mixed with gastric juice
Gastric Absorption
What does stomach absorb (2)?
- (ethyl) alcohol
- aspirin
Gastric Absorption
What is aspirin?
acetylsalicylic acid – weak acid that remains un-ionized in strongly acidic gastric juice
un-ionized → acids are lipid soluble and cross gastric epithelium, rapidly producing its effects
