8. Salivary and gastric secretions Flashcards
Which 4 structures are responsible for secreting into the GI tract?
Salivary glands (in saliva)
Cells of the gastric mucosa (gastric secretion)
Exocrine cells of the pancreas (pancreatic secretion)
Liver (bile)
Addition of fluids, enzymes and mucous to the lumen of the GIT
Saliva:
Characteristics?
Factors that increase secretion?
Factors that decrease decrease?
Characteristics:
- High HCO3-
- High K+
- Hypotonic
- Alpha-amylase and lingual lipase
Factors that increase secretion:
- Parasympathetic (primarily)
- Sympathetic
Factors that decrease decrease:
- Sleep
- Dehydration
- Atropine
3 features of the structure of the salivary glands
- Paired
Parotid: Serous cells secreting an aqueous fluid composed of water, ions and enzymes
Sublingual: Mostly mucous cells
Submandibular: Mixed glands containing serous and mucous cells - High blood flow
- Parasympathetic (dominant) and sympathetic supply both stimulate saliva production
Compound in mucous responsible for lubrication?
Mucin glycoprotein
Role of acinar cells in salivary glands?
Acinar cells produce an initial isotonic saliva composed of water, ions, enzymes and mucus
Role of myoepithelial cells in salivary glands
Myoepithelial cells in (and near) the acini are stimulated by neural input to eject saliva
Role of ductal cells in salivary glands?
Ductal cells modify the initial saliva by altering electrolyte concentrations
What are the 4 methods of salivary modification?
Combined action
– Absorption of Na+ and Cl-
– Secretion of K+ and HCO-3
– Because more NaCl is absorbed than KHCO3 secreted there is net absorption of solute
– Low water permeability of ductal cells means that the final saliva is hypotonic
What 4 compounds are secreted by acinar cells?
- alpha-amylase: Begins the initial digestion of carbohydrates
- Lingual lipase: Begins the initial digestion of lipids
- Mucus: lubricant
- Kallikrein: Enzymatic cleavage of kininogen to bradykinin (potent vasodilator)
What is the neural regulation of salivary secretion?
- Salivary secretion is exclusively under neural control by the ANS (no hormonal regulation)
- Salivary secretion is increased by both the parasympathetic and sympathetic nervous system
- Neural stimulation results in an increased saliva production, HCO-3 production, enzyme secretion and myoepithelial cell contraction
Difference between PS and sympathetic innervation on salivary secretion?
PS: Produce fluid/ protein in saliva by binding to muscarinic receptor
Symp: Produces mainly protein in saliva by binding to beta adrenal receptor
What are the 4 components of gastric juice?
- HCl: For protein digestion
- Pepsinogen: for protein digestion]
- Intrinsic factor: carries Vit B12 to ileum for absorption
- Mucus: Proteins the gastric mucosa and lubricates food
Gastric secretions:
Characteristics?
Factors that increase?
Factors that decrease?
Characteristics:
HCl
Pepsinogen
Intrinsic factor
Factors that increase: Gastrin ACh Histamine Parasympathetic
Factors that decrease: H+ in stomach Chyme in duodenum Somatostatin Atropine Cimetidine Omeprazole
3 secretors gastric cells
Parietal/ oxyntric cells (found in stomach body)
Chief/peptic cells (found in stomach body)
G cells (found in stomach Antrum )
Structure of an oxyntric/gastric gland?
Compounds secreted by cells?
Surface epithelium
Mucous neck cells: Secrete mucous
Oxyntric/ parietal cells: Secrete HCl and intrinsic factor
Peptic/chief cells: Secrete pepsinogen
What cells secrete HCl?
What is the consequence of HCl secretion?
Parietal cells secrete HCl which acidifies the gastric contents to pH 1 – 2.
Secretes H+ (via ATPase protein) and Cl-, to form HCl
Low gastric pH converts inactive pepsinogen to pepsin (active form)
Pepsin is a protease which initiates protein digestion
How is the H+ ion produced in parietal cells?
Carbonic anhydrase catalyses the combination of metabolic CO2 and H2O
CO2 + H2O –(carbonic anhydrase)–> H2CO3 –> H+ and HCO3-
What are the different processes that occur at the apical and basolateral surfaces of parietal cells to secrete HCl
Basolateral surface:
HCO-3 (into blood) is exchanged for Cl- (into cell) via the chloride-bicarbonate exchanger (alkaline tide)
Eventually HCO-3 that has been put into blood is secreted back into the GI tract in pancreatic secretions
Apical surface:
H+ is secreted into the lumen via the H+-K+ ATPase
Cl- follows by diffusion through an apical channel
What are the two phases of gastric HCl release?
Cephalic phase
- 30% of secretion
- Smell, taste, chewing, swallowing, conditioned reflex in anticipation of food
- Direct stimulation of the parietal cells by the vagus
- Indirect stimulation of the parietal cells by gastrin
Gastric phase
-60% of secretion
-Distension of the stomach and the presence of breakdown products of proteins, amino acids and small peptides in the stomach
Amino acids and small peptides on G cells stimulates gastrin release
Intestinal phase
- 10% of secretion
- Presence of breakdown products of proteins in the duodenum
What does vagal GRP stimulate?
Vagal gastrin releasing peptide (GRP) stimulates gastrin
release from G cells. Gastrin hormone enters the circulation and stimulates parietal cells to release HCl
Distension causes what 3 things?
– Direct vagal stimulation of parietal cells
– Indirect stimulation via gastrin
– Local reflexes in the antrum that stimulate gastrin
release
What are the direct and indirect methods of regulation of HCl secretion?
Direct vagal pathway:
Vagal release of ACh activates parietal cells
Indirect vagal pathway:
Vagal stimulated released of gastrin into circulation from G cells (by GRP neuropeptide transmitter)
What cells secrete pepsinogen?
Stimulation of secretion?
- Secreted by chief and mucous cells in the oxyntic glands in response to vagal stimulation
- H+ triggers local reflexes which stimulate chief cells to secrete pepsinogen
Intrinsic factor secretion:
What is intrinsic factor?
Absence causes…
Relevance in gastrectomy patients?
What is it?
• Mucoprotein released from parietal cells required for Vitamin B12 absorption in the ileum
• Only essential secretion of the stomach
Absence causes..
• Lack of intrinsic factor causes pernicious anaemia
• Following gastrectomy, patients receive injections of Vitamin B12 to bypass the absorption defect
What is Zollinger-ellision syndrome?
Endocrine tumour known as gastrinoma (gastrin secreting tumour)
Usually non-B cell of the pancreas
Leads to high circulating levels of gastrin, leading to high H+ secretion by parietal cells –> Hypertrophy of the gastric mucosa –> duodenal ulcers + acidification of the intestinal lumen
