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GI Tract.doc

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Physiology 3120
Tom Stavraky

GI Tract (I) - Influences CV system (diverts 1/3 blood), has its own nervous system (enteric), largest endocrine organ (by weight and by number of enzymes), contributes to body pH and total body ion content. - More gut cancer deaths than any other cancer (pancreas, liver, esophageal, etc) So both clinically and economically important, most surgeries performed on this organ system and most days are lost from work because of gut problems. - 3 glands -> parotid, submandibular, sublingual Functions: lubricate bolus, slight digestion (amylase), lubricate oral cavity (protective), also some antibacterial actions by lysozymes. - salivary glands have adopted to the diet of the animal being studied - secretions of the 3 glands are different - Salivary gland is converging duct system. The less ducts the larger the diameter. - Salivon is primary unit of the gland. - Myoepithelial cells have ability to contract and can HELP (not primary reason for movement) fluid move out of acinus - Primary movement is by pressure gradient - Salivon behaves like a nephron, has pressure gradient, fenestrated membrane, capillary bed surrounding it. - Primary salivary fluid is secreted by the acinus cells of the salivon. - In the acinar, tonicity is isotonic. Although there is a HCO3- ATPase pump, fluid is not hypertonic because the acinus is very permeable to water so water will follow the HC03- into the acinus making it isotonic. (cells also have carbonic anhydrase) - The striated duct has microvilli on the basolateral side full of mitochondria - This indicates active transport and is where the juice is modified - Na+/K+ an electrogenic-unbalanced exchanger (more (2) Na+ removed than (1) K+ added) in the striated duct that is tonically active. - Unlike the acinus, the striated duct is a tight epithelium. - The solution now is hypotonic because ions are being removed without water following. - All of this occurs without extrinsic stimulation (forms 2L in 24 hours different levels of secretion throughout the day) Concentration Flow Relationship in Salivon (Qualitative) - Hyperbolic reaches a plateau - Na/K pump is not regulated, functions at one rate regardless of stimulation (pump becomes less efficient) o E.g. 3 Na out 2 K in per minute - Regular Na concentration is 150 mosmol/L so it plateaus can it can only reach a level at which it was formed. - HCO3 produces optimum pH for enzymes. HCO3 primary pump is not well regulated, but the secondary pump in the striated duct is regulated (stimulation increases activity) - Qualitatively, looks the same as the Na/K pump exchanged - Plateaus because active transport has maximum velocity of transport II - HCO3- is transported by active transport (requires ATP but we have finite amount of ATP so it is saturable) (only have certain amount of pumps). Qualitatively same flow/concentration relationship as Na+ even though Na+ is unregulated pump (remember Na+ is hyperbolic because efficiency of transporter decreases as flow rate increases). Regulation of Salivary Glands- not under hormonal control. Both sympathetic and parasympathic increase saliva secretion (but different types). Sympathetic (dry, gummy) mucous (glycoprotein) secretion. Decrease in blood flow due to vasoconstriction. Higher final concentration of HCO3- Parasympathetic Ach causes vasodilation which causes increase in blood flow which causes more saliva being formed. Ach also increases permeability of acinus. Therefore increase in salivary volume. 10-fold increase in volume. - Both sympathetic and parasympathetic have same stimulatory effect on the HCO3- pump in the striated duct. (Ach and Epi have same effect on pump). BUT different final concentrations because smaller volume in sympathetic. - Vasoconstriction will cause release of organic molecules including an enzyme called kallikrein (not secreted, absorbed into the extracellular matrix and then into the blood) acts on a peptide making bradykinin which causes vasodilation. - Parasympathetic cause only vasodilation to some extent some of the dilation is due to kallikrein release Stomach
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