Salivary and gastric secretion Flashcards
Three functions of saliva
Lubrication
Protection
Digestion
Lubrication
Moistening the mouth to aid swallowing
Facilitates movement of the mouth and tongue for speech
Helps to dissolve chemicals within food for its presentation to the taste receptors
Protection
Reduces the adverse effects of oral bacteria
Alkalinity of fresh saliva neutralises acid produced by oral bacteria
Flow of saliva across teeth helps wash away bacteria
Digestion
Begin breakdown of carbs and fats via alpha amylase and lingual lipase
Sjogren’s syndrome
Autoimmune disease
Destroys exocrine glands, mostly tear and salvia
Dry eyes and mouth
Xerostomia
Lack adequate saliva
Dental caries and haltosis due to bacterial overgrowth
Difficulty speaking or swallowing food
Volume of saliva produced
1.5L per day
Two types of secretions
Serous secretions:
- the main type of protein secreted is ptyalin
Mucous secretions:
- main protein is mucin acting as a lubricant
Three major salivary glands
Submandibular glands secrete approximately 70% of saliva
Parotid glands secrete 25%
Sublingual glands secrete 5%
Structure of salivary glands
Contain blind-ended acini connecting with ducts draining major ducts
Acinar cells histologically distinct from duct cells
Serous acini distint to mucous secreting acinus
Composition of saliva
Functional unit consists of acinar cells, secrete primary saliva into a duct system
Primary saliva secreted by acinus is isotonic solution resembling interstitial fluid
Duct reabsorbs NaCl, causing saliva to become hypotonic
Secretion and modification of saliva by salivary acinar
Cl- uptake by a basolaterally located Na+, K+, 2Cl- cotransporter and release through the calcium activated apical chloride channel
Sustained by Na+/K+/ATPase
Na+ enters acinar lumen paracellularly through leaky tight junctions
H2O follows via aquaporin 5 or paracellularly
Secretion and modification of saliva by ductal cells
Removal of Na+ from saliva via apical sodium channel and basolateral Na+/ K+ ATPase
Cl- removal from saliva via different chloride channels in the apical and basolateral membranes of ductal cells
Removal of salt not accompanied by water since ductal tight junctions are not leaky and aquaporins are not expressed in apical membranes
Secrete bicarbonate and potassium via unidentified apical bicarbonate chloride and potassium proton exchangers
Composition of saliva changes with flow rate
Electrolytes
- Na+ and Cl- < plasma
- HCO3- and K+ > plasma
High flow rate less time for ducts to absorb NaCl so resembles isotonic solution produced by acini
Central control of salivary secretions
Stimulated through thought, smell or taste of food by reflexes and nausea
Sleep, dehydration, fatigue and fear inhibit salivation
Integrated by salivary nuclei in the pons
Efferent nerves reach salivary glands via glossopharyngeal and facial nerves
Acinar secretion stimulated by acetylcholine via muscarinic receptors
Parasympathetic stimulation
Promotes watery secretion myoepithelial cells surrounding acini
Ducts contract and eject preformed saliva
Leads to increased blood flow
Superior cervical ganglion
The only hormonal effect on saliva secretion is from aldosterone which increases ductal Na+ absorption and K+ secretion
Composition of gastric juice
1-2L from several cell types produced per day
- water, electrolytes
- HCl
- pepsins
- mucus
- intrinsic factor
Water, electrolytes
Dissolve and dilute digested food
HCl
Hydrolyses fat and starch
Antiseptic
Converts pepsinogen to pepsin and provides optimum pH
Pepsins
Secreted as inactive pepsinogens away from stomach lining activated to form pepsins at low pH
Mucus
Bicarbonate barrier
Protect surface epithelial cells from acid/ pepsin erosion
Intrinsic factor
Glycoprotein binds vitamin B12 needed for absorption in the ileum
The indispensable substance in gastric juice
Origins of secretion
Stomach has two major regions:
- an exocrine or glandular portion consists of the fundus and body or acid secreting area
- an endocrine or hormone secreting area that is located in the antrum or gastrin secreting area
Structure of gastric gland
Mucous cell
Endocrine cells- D cells
Chief cells
Parietal cells
Histamine cells
Endocrine cells- G cells
Mucous cells
Secrete protective barrier
Endocrine cells- D cells
Secrete somatostatin
- regulateor of gastrin release and gastric acid secretion
Chief cells
Secrete pepsinogen
Converted by gastric acid to active form of pepsin
Pariteal cells
Acid secreting, IF
Endocrine cells- G cells
Produce gastrin
Protection against self digestion and mechanical damage
Lumen
Gartitis
Many causes
Most commonly caused by an infection by the bacteria helicobacter pylori
Other common causes include smoking, use of alcohol and nonsteroidal anti- inflammatory drugs and chronic stress
Surface of epithelium acutely damages
Acid secretion- oxyntic cell
Tubolovesicular membrane contains H+-K+ pumps responsible for acid secretion
Upon stimulation tubulovesicular membrane fuses into canalicular membrane
Fusion accompanied by insertion of H+/K+ ATPase and K+ and Cl- ion channels into canalicular membrane
Dense mitochondria to support high metabolic activity of cell
Acid secretion at the luminal membrane of a stimulated parietal cell
Major component if H+/K+ ATPase which actively exchanges H+ for K+
Enormous active secretory capacity, capable of secreting protons against large electrochemical gradients
Luminal pH can approach 1-2
No other part of the body can approach this level of acidification
Mechanism of gastric acid secretion by oxyntic cells
H+ and HCO3- made inside the cell by carbonic anhydrase
H+ pumped out of the luminal membrane by H+/K+ ATPase
Cl- leaves by diffusion
Cl-HCO3- exchanger in basolateral membrane provides Cl- for HCl and HCO3- exits in large quantities so gastric venous blood becomes alkaline
Omeprazole
Proton pump inhibitor
Binds irreversibly to the H+/K+ ATPase inhibiting H+ secretion until new H+/K+ ATPase protein is synthesised
Three major stimulators of acid secretion
- Gastrin- predominantly secreted by antral G cells
- Histamine- in humans probably arising from mast cells
- Acetylcholine- secreted by postsynaptic vagal fibres innervating the gastric mucosa
Two major paracrine inhibitors of gastric acid secretion
- Somatostatin- secreted from antral and oxyntic gland D cells as well as pancreatic islet cells
- Prostaglandins- from mucosal cells
Atrophic gastritis
Autoimmune
Antibody mediated destruction of gastric parietal cells
Causes hypochlorydria and a deficiency of IF
Loss off IF results in vitamin B12 malabsorption and pernicious anaemia
Three phases of gastric acid secretion
Cephalic: sight, cell, taste, though
Gastric: antral distension, protein content, increase pH
Intestinal: intestinal gastrin, absorption amino acids
Gatrin
G cells of pylorus and duodenum
Endocrine effect
Release triggered by aa and peptides in stomach
Low pH inhibits release
Histamine
ECL cells close to parietal cells
Paracrine effect by local diffusion
ECL cells stimulated by gastrin and acetylcholine
ACh/ gastrin has direct and indirect route of stimulation which amplify signal
Leads to vasodilation of arterioles
Acetylcholine
Acts on parietal and ECL cells to promote acid and histamine secretion
Acts on D cells to inhibit somostatin release
Vagal stimulation- corpus
Via ACh
Increases acid secretion directly via parietal cells and indirectly via ECL and D cells
Vagal stimulation- antrum
Via GRP
Stimulates both G and D cells
Gastrin from antrum promotes acid secretion by two endocrine mechanisms
- directly via parietal cells
- indirectly via ECL cells
