lecture 27 - the gut 1: saliva and stomach Flashcards
processing of food
- motility
- secretion
- digestion
- absorption
- excretion
motility
mechanical breakdown
controlled movement along gut
secretion
addition of enzymes and electrolytes
digestion
chemical breakdown
absorption
transport proteins
excretion
secretion of metabolic waste production and xenobiotics
elimination of faeces
structure of the gut tube
- plica
- mucosa
- submucosa
- circular mucosa
- longitudinal mucosa
- submucosal glands
- villi
regulation of gut function
- neural
- hormonal
- local
neural regulation of gut function
ANS
• vagus nerves
enteric nervous system - in the gut wall
hormonal regulation of gut function
endocrine
dispersed enters-endocrine cells
local regulation of gut function
paracrine
eg. histamine
3 key stages of regulation
cephalic
gastric
intestinal
functions of saliva
- lubrication - fluid and mucus
- digestion - amylase & R proteins
- solution
- moistness
- protection - fluid and bicarbonate keeps pH up so amylase will work, lysozyme and immunoglobulins
salivary glands
3 pairs of glands account for 90% of saliva
parotid glands
• serous (watery)
• amylase
sublingual & submandibular glands
• mixed secretion
10% from minor glands
• mucous
• mucins
total secretion from salivary glands
1.5 litres a day
salivary gland structure
acinus leads to duct
secrete K+ and HCO3-
salivary fluid secretion
primary fluid from acinar epithelial cells
• isotonic - plasma like
secondary ductal modification
• Na+ & K+ reabsorption
• K+ & HCO3- secretion
• ducts impermeable to water
hypotonic saliva - HCO3- rich
control of salivation
in cephalic phase
• sight, smell, taste and thought of food
parasympathetic - cholinergic
• cranial nerves VII and IX
• large volume
sympathetic - noradrenergic
• smaller volume
• rich in enzymes and mucus
gastric functions
- reservoir
- mixing food with gastric secretions
- digestion
- controlled gastric tempting - food to duodenum
- protection
2 types of digestion
mechanical
chemical
structure of stomach
starts at oesophagus and enters body of stomach
contains rugae - surface folding increases area
leaves via Antrum by pyloric valve
what do gastric pits do?
increase SA
composition of gastric juice
- HCl - parietal cells
- pepsinogen to pepsin - chief cells
- mucus and bicarbonate - mucus neck and surface epithelial cells
- intrinsic factor - parietal cells
how much gastric juice a day?
2 litres
what are parietal cells stimulate by?
ACh
gastrin
histamine
what happens in parietal cells?
H2O & CO2 converted to H+ and HCO3- by carbonic anhydrase
H+ is exchanged for K+ by K+/H+ ATPase into lumen of the stomach
Na+/K+ ATPase on basolateral membrane
potassium leaky channels allow K+ recycling
HCO3- exchanged for Cl-
H+ and Cl- combine to make HCl
what do proton pump inhibitors (PPIs) do?
they block the K+/H+ ATPase so H+ cannot be pumped into the lumen of the stomach to make it more acidic
what happens on the apical membrane of parietal cells?
H+/K+ ATPase - primary proton pump
K+ channels - supplies K+ for recycling through proton pump
Cl- channels - supplies anion for HCl
what happens on the basolateral membrane of parietal cells?
Na+/K+ ATPase
Cl-/HCO3- exchange
what do chief cells do?
secrete pepsin and gastric lipase
digest proteins and fats
stimulated by:
• ACh
• acid
• secretin
what is pepsin inactivated and denatured?
inactivated > pH 3.5
denatured > pH 7.2
what do mucous neck cells and surface cells do?
secrete mucus and HCO3-
mucus provides physical barrier between lumen and epithelium
HCO3- buffers gastric acid to prevent damage of epithelium
how does the stomach stop auto digestion?
gastric mucosal protection
mucus-HCO3- barrier
control of parietal cell function
direct and indirect chemical regulators • histamine from ECL cells • somatostatin from D cells • gastrin from G cells • Ach from vagus and enteric neurones
what do ECL cells do?
in body only
secrete histamine
stimulate gastric acid secretion
what do D cells do?
in body and antrum
secrete somatostatin
inhibition of gastric acid
what do G cells do?
in antrum only
secrete gastrin
stimulate gastric acid secretion
how does histamine act?
binds to H2 receptors on parietal cells in the body
increases cAMP
leads to HCl secretion
what is cimetidine?
a H2 antagonist
what are G cells activated by?
ACh
peptides
amino acids
