PHYS Gut Secretions I - Week 4 Flashcards
Acidity of salivary, stomach, pancreas, bile, SI & LI secretions.
7-8. 1-3.5, 8, 8, 8.
Saliva composition
- H2O (97-99.5%)
- Na, K, Ca, Mg, PO4, HCO3, Cl
- Salivary amylase
- Mucin
- Immunoglobulins
Functions of saliva
- Lubrication
- Digestion
- Protection
- Control of H2O intake
- Speech
- Absorption (e.g., of nitrates in certain medications)
- Taste sensation
Salivary glands classifications & locations.
Saliva is secreted from salivary/buccal glands @ mucosa (epithelial lining of mouth) = intrinsic salivary glands.
Saliva is also secreted from the sublingual/parotid & submandibular glands @ mucosa = extrinsic salivary glands.
Salivation process
Stimulus -> Olfactory/gustatory/visual/auditory cortex (higher centres) -> excite salivatory nucleus of the medulla -> activation of salivary glands -> salivation.
Factors which inhibit salivation include:
Sleeping, stress
Factors which increase salivation include:
Nausea, increased acidity of the oral cavity, hunger/approach of food.
What is xerostomia?
Dry mouth.
Xerostomia treatment
M receptor agonist.
What is ptyalism?
Excessive saliva production.
Ptyalism treatment.
M receptor antagonist.
What is deglutition?
Process of bolus passing from mouth to stomach.
3 phases of swallowing
- Buccal phase (voluntary ) – skeletal muscles of tongue contract -> push bolus to top of pharynx -> bolus activates touch-sensitive/mechanoreceptors @ pharynx wall.
- Pharyngeal phase (involuntary) – uvula rises & closes off nasal passages and epiglottis lowers & closes off trachea -> upper oesophagus relaxes -> oesophageal peristalsis.
- Oesophageal phase (involuntary) – gastroesophageal/lower oesophageal sphincter relaxes -> bolus enters stomach.
What part of the brain controls the buccal phase?
Cerebral cortex.
What parts of the brain/nerves control the pharyngeal & oesophageal phases?
Swallowing centre @ medulla -> efferent outputs (via vagal nerve)
Both parasympathetic nerves (innervating the smooth muscle @ lower oesophagus & gastroesophageal sphincters) and somatic motor nerves (innervating the skeletal muscle @ pharynx, upper oesophageal sphincter) travel via vagus nerve.
Common causes of dysphasia
- Diseases of the mouth, tongue, salivary glands
- Neuromuscular disorders
- Outpouchings on pharyngeal/oesophageal wall.
Achalasia
Specific type of dysphasia, in which the lower oesophageal sphincter does not relax fully.
Achalasia treatments
Balloon inflation of lower sphincter or botox injection.
Where are gastric glands located?
Under gastric pits.
What do gastric glands include:
- Mucous cells – secrete mucous to lubricate food
- Parietal cells – secrete HCl & intrinsic factor
- Chief cells – secrete pepsinogen (inactive form of pepsin)
- Enteroendocrine cells – release hormones
- ECL – secrete histamine
- G cells – secrete gastrin
Importance of intrinsic factor secretion.
Required for uptake of B12. (B12 is required for RBC manufacturing/synthesis).
Pepsin synthesis process.
pH2. Chief cell releases pepsinogen. Parietal cell releases HCl & intrinsic factor. HCl ensures pH to facilitate pepsinogen-pepsin conversion.
How do parietal cells synthesise HCl in the gut?
CO2 diffuses from ECF/CO2 present in cell already is used in the reaction: CO2 + H2O H2CO3 -> rapidly dissociated into H+ & HCO3- -> bicarbonate ions are exchanged for Cl- ions in veins draining into the stomach -> HCO3- enter veins draining into the stomach (forming alkaline tide).
H+ ions need to be actively pumped out of parietal cells via H/K ATPase pumps & Cl- ions leave via chloride channels.
Receptors present on parietal cell membrane?
G receptors - respond to gastrin
H receptors - respond to histamine released by ECL cells
M receptors - respond to parasymp nerves
S receptors - respond to somatostatin
Activation of which receptors on the parietal cell membrane leads to increased gastric acid secretions and how?
Activation of G, H, M receptors -> second messenger production within parietal cells -> H+ proton pump -> increased gastric acid secretions.
Excessive gastric acid secretion can be treated via what 2 medications?
Anti-histamines or PPIs.
What triggers the 3 x phases of gastric secretion?
Cephalic - occurs before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food; and the greater the appetite, the more intense is the stimulation.
Gastric – triggered by increased pH & stretching of the stomach.
Intestinal - duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes.
