gastric juices Flashcards
what is chyme
fluid consisting of gastric juices and digested food
define motility
movement of the gut wall - propulaive, retentive or mixing
what is transit time
time it tales for material to travel from one portion of the GI tract to another.
- increase propulasion will decrease transit time.
- increased retention will increase transit time
what is zymogen/proenzyme
- an inactive precursor of an enzyme that undergoes structural change before becoming active.
what is autocatalyse
catalysis of a reaction by on of it products
what is paracrine
a hormone that only has effects in the local vicinity
what are the 4 main functions of the stomach
- temporary storage of food
- mechanical degradation of food (principally protein) using muscles
- secretion of gastric juices for biochemical degradation of food
- destroy foreign bacteria (HCl)
what are the purposes of gastrointestinal movements
- propel food
- retain ingesta for digestion, absorption and storage
- circularte food around the gut interior to come into contact with absorptive surfaces
- in the stomach: physically break food up and mix with secretions for biochemical degradation
what enzyme digests proteins
pepsin
pepsin is a protease made from:
pepsinogen
what is the major role of HCl in the stomach
destruction of bacteria
- although HCl denatures proteins and helps enzymes to reach internal peptide bonds, this is not its primary function
explain how the stomach avoids being digested by its own secretions
- mucosal mucous cells secrete mucus containing HCO3 creating a protective layer protecting against pepsin and HCl
- negative feedback loop prevents constant release of gastric juices
how is pepsin prevented from breaking down Chief cells
- if pepsin broke down proteins of the stomach it would also digest chief cells so
- chief cells make and store pepsin as an inactive proenzyme called pepsinogen
- release pepsinogen into the stomach lumen where it is then converted to pepsin
what cells create pepsin
cheif cells. make pepsinogen which is then converted into pepsin once released into lumen
how is hydrochloric prevented from dissolving parietal cells
- if parietal cells made HCl, it would dissolve them so it is not produced by them directly
- parietal cells make H+ and Cl- ions which they release into the stomach lumen where HCl forms
- however, HCl would attack any unprotected surfaces in the lumen (peptic ulcer) which is why mucosal mucus cells secrete HCO3 containing mucus
histamine is released by what cells in the gastric mucosa
enterochromaffin like cells
which cells release gastrin
G cells
which cells release somatostatin and what does it do
D cells in the surface mucosa
inhibits G cells
what do the yellow and blue arrows indicate
yellow: parietal cells
blue: chief cells
what are the 3 phases of gastric secretion
- cephalic
- gastric
- intestinal
what is involved in the cephalic phase of gastric secretion. outline the steps
- short phase
- taste/smell of food and tactile sensations in the mouth stimulate medulla oblongata
- nerve impulses travel in vagus nerve to stomach. these are autonomic nerves to smooth muscles of stomach
- the nerve fibres stimulate postganglionic neurons in the enteric plexus of stomach which in turn stimulates secretion of H and Cl by parietal cells, pepsiogen by chief cells and gastrin byt endocrine G cells
- gastrin enters circulation to further stimulate H and Cl and pepsiogen secretion
outline the steps of the second phase of gastric secretion: gastric phase
- stretching (distension due to food in stomach) of stomach activates a parasympathetic reflex via chemo and stretch receptors. vagus nerve carries impulses to medulla oblongata
- medulla oblongata sends impulses in vagus nerve to further stimulate stomach secretions including mucus and to increase stomach muscle contraction (mixing waves)
- distension of stomach also activates local reflexes to increase secretions
largest volume of secretions occurs in this phase.
phase is initiated by presence of food in stomach
outline the steps of the third phase of gastric secretion: intestinal phase)
- as chyme enters duodenum, stomach becomes emptier so stomach distensions reduces
- if chyme is pH < 2 (or contains lipid digestion products) gastric secretions inhibited by 3 mechanisms:
- vagus sends sesory impulses to medulla oblongata to inhibit motor impulses arising there to stomach
- acidic solutions in duodenum inititate local entero-gastric reflexes in stomach and distension of duodenal wall also activate entero-gastric reflexes
- secretin hormone, gastric inhibitoru polypeptide and cholecystokinin produced by duodenum inhibit gastric secretions of the stomach
how is pepsinogen activated into pepsin in the stomach
- pepsinogen is released by chief cells packaged in zymogen granules
- HCl creates an environment where pepsinogen unfolds and becomes activated as pepsin
- pepsin can then continue to unfold other pepsinogens (autocatalysis)
discuss the release of gastrin (what stimulates its release and what stimulates stopping of secretion
- gastrin is released from G cells into the blood in response to peptides in the stomach and vagal stimulation
- secretion stops when pH < 2
discuss histamine secretion in the GIL for promotion of HCL formation
- secreted from enterochromaffin-like cells in response to gastrin stimulation and locally stimulates parietal cells to release H and Cl
gastric secretions are controlled by what branch of the nervous system? what nerves are involved?
- parasympathetic nervous system
- vagal stimulation = proximal stomach suppression of muscle contractions causing proximal stomach to relax allowing to dilate and store food, while at distal stomach results in intense peristalsis
vagal acetylcholine from PS impulses stimulates which cells to make which secretions (3)
- parietal cells to make and releas H and Cl
- chief cells to make and release zymogen pepsinogen
- G cells to make and release gastrin
what agent promotes secretion by chief cells and what do they produce
vagus nerve and histamine stimulates production/storage/release of pepsinogen from chief cells
what agents promotes secretion from G cells and what do they produce
vagal stimulation, peptides and amino acids in the stomach stimulates G cells to release gastrin into the blood
what agent stimulates ECL and what does it produce
gastrin promotes release of histamine by ECL
what agent promotes secretion by parietal cells and what do they produce
vagal nerves, wall stretch and peptides stimulate parietal cell response to release H and Cl ions
list negative feedbakc systems that prevent the stomach from continually secreting
- a drop in pH below 4.5 stimulates release of secretin by duodenum eneroendocrine cells
- secretin inhibits parietal and chief cells
- less stretch of the stomach reduces local stimuli
- if the stomach pH falls below 2 gastrin release is inhibited
- secretin stimulates buffer release from the pancreas
- D cells produce somatostatin to inhibit gastrin production
- enterogastric reflex reduces gastrin secretion and stomach contractions
- lipids and carbohydrates in the duodenum secrete CCK and gastric inhibitory peptide which reduce secretions and slow contractions
what does secretin do
- peptide hormone which infuences secretions withinn the GI tract
what are D cells
somatostatin producing cells
what is somatostatin
hormone that regulates the secretions of many other hormones (insulin for example)
what is CCK
cholecystokinin stimulates fat and protein digestion
