24 Flashcards
describe function of mouth
- digestion of food begins in the mouth
- saliva moistens and lubricates mouth and food (important for taste and swallowing)
- mechanical breakdown by teeth
- enzymatic breakdown (salivary amylase to digest complex carbs)
name the 3 main pairs of salivary glands in humans
- parotid
- submandibular
- sublingual
what units make up salivary glands
- salivons = acini and ducts
salivary acini are made up of what types of cells
- mucous cells: produce mucin = lubricant
- serous cells: produce watery secretion and proteins (ie salivary amylase)
describe mucous cells
- mucus contains mucin proteins (forms a protective barrier and acts as lubricant)
- mucins are heavily glycosylated proteins = N and C term contain cysteine residues > allow polymerization
- central region contains serines and threonines = glycosylation > blocks degradation by peptidases
- mucins retain large amounts of water on the epithelial surface
describe serous cells
- salivary amylase catalyzes the hydrolysis of starches
- hydrolyze up to 75% of ingested starches
- amylase is degraded in the stomach by high acidic environment
- produces initial saliva
describe where aquaporins are expressed and their functions
- aquapoirins present in the acini (but not in ductal cells)
- allow water to flow into tubules
describe how saliva is formed
- saliva begins as fluid secreted by acinar serous cells
- composition of this fluid changes as it travels thru ductal system to reach mouth
- ductal cells reabs Na and Cl and secrete K and bicarb
how does ductal cells modify initial saliva produced by acinar cells
- epithelium of duct is largely impermeable to water
- more Na and Cl removed than K and HCO3 added = saliva is hypotonic with respect to plasma
what innervate salivary glands
- salivary glands receive inputs from SNS and PNS
- but PNS inputs predominates
- salivation rate can be increased by variety of stimuli
how is salivation regulated (what factors affect salivary flow)
- by the salivatory nuc of the medulla
- food in mouth - Acidic taste stimulates salivation; Sweet tastes are least potent stimulators
- psychic factor - Thinking about or smelling food stimulates salivation
- circadian rhythm - Highest in afternoon, lowest during sleep
- dehydration
describe PNS input to salivary glands
- primary input via M3R = increases fluid and protein secretion (activation of M3R > cal dependent translocation of aquaporins from intracellular vesicles to the plasma mem in the parotid glands)
- similar to vasopressin in renal interstitium
describe SNS input to salivary glands
- indirect effect on salivary glands via a1R expressed in vasculature
- vasocon decreases blood flow to salivary glands > less production of saliva
describe function of stomach
- chemical breakdown of ingested food
- Acid is pumped into lumen of stomach = Pepsinogen is released > Converted into active peptidase by acid in stomach > Pepsin
- mechanical breakdown of ingested food
- Chyme is formed = Liquified material that will be released into small intestine
name the specialized cells of the stomach (brief function)
Parietal cells = Secrete HCl
Chief cells = Secrete pepsinogen
Mucous neck cells = Secrete mucus that helps protect the stomach lining
Enteroendocrine cell
how does parietal cells secrete HCl
- CO2 in bloodstream enters parietal cells > combines with water > form H2CO3
- carbonic anhydrase convert the carbonic acid into H and HCO3
- HCO3 transported out as Cl transported in
- H is transported into lumen of the stomach while K transported in via ATP transporter
- Cl diffuses into lumen of stomach where it combines with H
describe role of chief cells
- secrete pepsinogen (precursor to active pepsin - group of proteases)
- pepsinogen is release by exocytosis from vesicles
- pepsinogen is converted to active pepsins by acidic condition in stomach (most active ph 3 and below)
describe the role of mucous neck cells
- located in narrow neck of the gastric gland
- mucins are heavily glycosylated peptides > the sugars protect them from pepsins = line gastric mucosa = trap bicarb released by epithelial cells of the stomach > bicarb trapped in mucous at surface of stomach lining to form alkaline barrier to acidic environment of stomach = prevent stomach from digesting itself
describe how food in the stomach causes release of gastrin
food in stomach stim IPANs > causes release of gastrin from G cells in stomach > either
- short-loop reflex = excitation of IPANs activates intrinsic ENS reflex > secretomotor neurons release gastrin releasing peptide (GRP)
- gastrin released into circulatory system > increases the release of H+ from parietal cells
describe gastrin relationship with histamine
- gastrin released into the circulatory system > increases the release of histamine from ECL cells
- histamine = strongest activator of HCl release from parietal cells
describe the long-loop reflex when food in stomach stim IPANs
- long-loop reflex involves PNS input
- excitation of IPANs increase PNS input to ENS > vagus release ACh to increase histamine release from ECL cells + vagus releases ACh to increase H+ release from parietal cells
describe somatostatin release and action
- presence of highly acidic contents of the distal stomach increases release of somatostatin from D cells
- somatostatin binds to inhibitory receptors > inhibit the further release of gastrin from G cells > inhibition of acid secretion
what happens with entry of food into the stomach and gastrin
- food buffers concentration of H+
- decreased H+ inhibits somatostatin release
- decreased somatostatin > less inhibition on gastrin release
- increased release of gastrin causes increased secretion of HCl
what happens with no food in the stomach and gastrin
- pH in lumen is low > increased concentration of H+ increases release of somatostatin from D cells = inhibits secretion of gastrin > HCl secretion into stomach is decreased
what happens if stomach acid comes into contact with surface of GI lumen?
- ulcers may develop
- gastric, esophageal, duodenal
describe the components of gastrointestinal barriers
- has two components
- extrinsic barrier - made up of secretions from epithelium = help maintain barrier function
- intrinsic barrier - made up of the epithelial cells lining the digestive tube (tight junctions between cells)
how does SNS act on ENS
- SNS inhibits ENS functions
- vasoconstriction = decreased blood flow limits removal of excess protons
- decreased secretions = decreased secretion of mucus
- stress may contribute to erosion of the gastric mucosa > gastrointestinal hemorrhage
describe how chyme is emptied from the stomach
- chyme is emptied from the stomach into the duodenum of the small intest via peristalic contraction > pushes chyme towards pyloric sphincter (which is partially relaxed) > small quantity of the chyme is forced through the sphincter into the duodenum
describe anatomy of small intestine
- 3m long divided into sections
- duodenum proximal to stomach
- jejunum between duodenum and ileum
- ileum final section of small intest which leads to colon
how does anatomy of intestine help aid absorption of nutrients
- very large surface area from:
- villi + microvili which contains brush border that has some digestive enzymes
- crypts
what is the main function of the small intestine
- further liquefaction and digestion of ingested food: Water and NaCl are increased in the lumen + Pancreatic juices and bile from liver are introduced
- absorption of nutrients into blood
- distal reabsorption of water and electrolytes ( water and sodium removed from lumen)
what happens when theres a decrease in fluid volume or plasma sodium levels? (think sympathetic inputs)
- increases activity of SNS input to sevretomotor neurons > decreases secretion of Cl- > causes decrease in amount of water and sodium released into lumen of GI tract
