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parathyroid_1_ - 1 2 Calciumhomeostasis Plasmacalcium...

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Calcium homeostasis Plasma calcium 9.0-10.5 mg/dL Ionized (free):  50% Protein bound: 40% Anion bound (citrate, bicarb): 10%   citrate w/ blood transfusion   phosphate w/ crush injury to muscle Alkaline pH more bound than Acid pH 3
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Role of Calcium Nerve & muscle excitability Regulates Na +  permeability & action potential Hypocalcemia: hyperexcitability   opening of membrane Ca ++  channels ↓ threshold potential (more rapid depolarization) Trousseau’s (Carpal spasm, aka. main d’accoucher)  Chvostek’s signs Hypercalcemia: hypoexcitability Neurotransmitter release Excitation-contraction coupling in muscle Intracellular signal transduction Enzyme cofactor 4
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Calcium Homeostatic Pathway GI Net absorption of calcium Active transport in duodenum/jejunum (low intake) Vitamin D 3  regulated Kidney Excretion (1% of filtered calcium) PTH stimulates Ca ++   reabsorption in late distal  tubule Bone Resevoir: balanced deposition & resorption PTH, Calcitonin & Vitamin D effects 5
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Phosphorus Homeostasis Plasma 3.0-4.5 mg/dL Inorganic orthophosphate (PO 4 ) HPO 4 -2 :  80% at pH of 7.4 Daily levels fluctuate more than Calcium Role pH buffer (intracellular) Component: DNA, RNA, phosphoproteins 7
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Phosphorus GI Most PO 4  is absorbed Active transport is major mechanism & is  unregulated Minor amount coupled with Calcium active  transport & regulated by Vitamin D Kidney 85% reabsorbed in proximal tubule PTH inhibits PO 4  reabsorption 8
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