GIT Secretions 1 Flashcards
What is the composition of saliva?
Composition:
– From Parotid, Submandibular and Sublingual glands
– 99% water (large volume)
– 1% salts, enzymes (amylase, lipase) and mucins
– Hypotonic
Whhat is the. Function of saliva?
Function : – Protection – Taste – Lubrication – Digestion
- Control of salivation is mediated almost entirely by nervous system
- Both Sympathetic and Parasympathetic stimulate secretion
Describe the steps of acinar & ductal modifications
Step 1
The acinus produces an initial saliva similar to plasma (isotonic). Contains Na+, K+, Cl-, HCO3-
Step 2
Ducts modify the saliva: absorb Na+, Cl- and secrete K+ and HCO3- (hypotonic)
At low flow rates (basal), contact time allows for reabsorption and secretion. At high flow rates (stimulated) saliva becomes more similar to acinar secretions
Describe acinar secretions
- acinar cells are leaky and secrete digestive enzymes, mucin, electrolytes and water (isotonic NaCl)
- Cl is co-transported with Na into the cell basolateral
- Cl diffuses down the gradient into the lumen (apical)
- channel allows HCO3- to enter the lumen
- Na+ and H2O follow paracellularly
- extent of modification depends on flow rate
-isotonic
Describe the ductal modification of the salivon
Ductal cells are tight (low water permeability)
Acinar secretion is modified in the duct
- K+ and HCO3- are actively secreted
- Na-K ATPase on BL membrane drives active reabsorption of Na+ and K+ secretion
- anionic exchanger reabsorbs Cl
- active secretion HCO3- via
anionic exchanger (Cl- /HCO3-)
Explain regulation of salivary secretion
Both parasympathetic (predominant) and sympathetic ANS regulate release of saliva • Mechanisms of control – Ach, VIP and substance P increase intracellular Ca2+, greatest effect on volume of secretion – NE elevates intracellular cAMP, mainly increasing enzyme and mucous content
• Resting state salivary secretion
– low at 30 ml /hr
– submandibular glands contributing ~ 2/3 resting saliva
• Stimulated glands
– secrete up to 400 ml /hr
– mostly from parotid gland
How does gastric secretions are changed with flow rates?
• Composition changes with flow rates
– Cellular mechanism H+ secretion
– Factors that increase and decrease secretion
– Amounts secreted vary during each GI Event:
cephalic, gastric, intestinal phase
Contrast basal secretions and stimulated gastric secretions
Basal Secretion : similar to ECF • Mainly mucus and water (paracellular diffusion of fluid from ECF and secretions from mucus glands) • Non-oxyntic component • Isotonic (high Na+ and Cl- content)
Stimulated Secretion : change • Parietal cells secrete HCL • Acidic • Oxyntic component • Still Isotonic (high H+ and Cl- content)
What are the steps in gastric acid secretion?
- CO2 + H2O→H+ + HCO3- (catalyst carbonic anhydrase)
- H+ is secreted into the lumen via H-K ATPase
- K+ can leak back into lumen
- Cl- is also secreted
- HCO3- produced absorbed into
the blood stream in exchange for
Cl- (Cl- - HCO3- exchanger) - alkalinetide:increaseinvenous
blood pH - Tight junctions prevent
paracellular transport - 3 stimulants for parietal cells:
ACh, gastrin and histamine
Summarize gastric acid secretion
- Basal secretion is from non-parietal cells, high Na+ and Cl- (resembles plasma)
- Stimulation of parietal cells → production of acid secretion
- Composition now becomes high in H+, K+, and Cl- is also elevated
- Gastric juice becomes acidic
- Overall effect of acid secretion is H+ into stomach lumen, and HCO3- into venous blood. pH of venous blood rises: alkaline tide
What is the stimulus of cephalic secretions ?
Thought of food, smell, taste, chewing and swallowing
Describe the pathway of cephalic secretions and stimulus to parietal cell
-Vagus nerve to parietal cells and G cells
Stimulus to parietal cell: Ach and gastrin (from G cells)
What is the stimulus of gastric secretions?
Stomach distention by food
What is the pathway and stimulus to parietal cells of gastric secretions?
Pathway: local ENS reflexes and vagovagal reflexes to parietal cells and G cells
Stimulus to parietal: Ach and Gastrin
What is the stimulus of intestinal secretions?
Protein digestion products in duodenum
Distension
What is the pathway and and stimulus to parietal cells for gastric secretions?
Pathway:
Amino acids in blood. Intestinal endocrine cell
Stimulus to parietal cell
Amino acids and entero-oxynitin
Describe cephalic acid production
Food: sight, smell, taste stimulates vagus nucleud—> vagus
Vsgus releases Ach to D cells which decreases somatostatin
Vagus releases ACh to stimukate parietal cells to release HCl
Vagus releases GRP to G cell which then releases gastrin
Gastrin stimukates parietal cell to release HCl and ECL cell to release histodine to release parietal
Why is cephalic phase acids necessary?
Necessary to inhibit somatostatin since SST normally decreased gastrin
Summarize gastric phase acid production
Food enters stomach—> neutralizes gastric acid (pH can increase to 6)—>. Gastrin secretion restored—> increased acid secretion
50-60%. Of gastric acid is secreted is secreted during gsstric phase
How does stomach stretch lead to acid prpduction?
Distention of stomach with food —> stimulate afferents—> long loop vagal reflexes and short ENS reflexes—> ACh
ACh stimulate the G cells of eigestive oroducts and increase gastrin, which increases acid release
What phases release their proportional amounts of acid secretion ?
Intedigestive ohase- 15%
Cephalic pgase- 30%
Gastric phase- 50%
Intestinal phase- 5%
Describe the negative feedback of acid secretion
Most gastric acid release ~1hr after meal.
Meal no longer buffers→gastric pH falls
At Stomach pH <3, G cells are inhibited by H+
At low gastric pH, D cells priduce SST (inhibits gastrin)
What are enterogastrones?
Enterogastrones are enteric Hormones inhibiting gastric acid secretion
Give an example 9f an H-K ATPase inhibitor(reduce acid secretion)?
Omeprazole
Lanzoprezole
Give examples of Histamine receptor blockers (H2-blockers)(acid secretion inhibitors)
Cimetidine, Ranitidine
Give examples of Muscuranic antagonist (acid secretion inhibitirs)
Atropine
Give examples of gastrin receptor blockers( acid secretion inhibitors)
Benzotript
What is the function of the gastrin mucosal barrier?
Protects acid damage from gastric acid
Cl- exist into lumen via apical channels—> Creates a lumen negative potential—> Facilitates H+ secretion into lumen—> Trapping H+ in the lumen
How does the gastrc mucousal barrier use alkaline uss acid?
Bicarbonate is secreted and trapped in mucus gel- neutralizes acid in lumen
Tight junctions prevent back diffusion of H+ and so protect the oxynitic cells against high proton concentration
Describe gastro(duodenal) barrier integrity
HCO3^-, mucus, blood flow, growth factors, cell renewal, prostaglandins, increased blood flow
Are all counteracted by
H+, pepsins, ethanol, NSAIDs, bile acids, ischemia, smoking, h. Pylori
Ulcers form when there is imbalance between the
digestive effects of the acid and the ability of
gastric and duodenal mucosa to resist
