Ca-PO4 - Calciumandphosphatehomeostasis I.DavidWeiner,M.D....

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Figure 2. Calcium transport along the nephron. Figure 1. An overview of calcium homeostasis. Calcium and phosphate homeostasis I. David Weiner, M.D. 374 6102 David.Weiner@Medicine.ufl.edu www.RenalLectures.com Calcium and phosphate play a central role in maintenance of the skeletal, cell signaling and function and provision of energy, as ATP, to cells. Excessive blood calcium levels leads to nausea, vomiting,polyuria,dehydration,hypotension,mentalconfusionandevencoma. Excessphosphate leads to precipitation with calcium in skeletal tissues, including the cardiac conduction system and arteries, leading to failure of normal electrical conduction and vascular disease, respectively. Lack of phosphate leads to ATP deficiency, due to an inability to form ATP from ADP, and complications such as muscular weakness, respiratory failure and skeletal muscle death. Normal calcium homeostasis reflects a balance between oral calcium intake, intestinal excretion and urinary excretion. Figure 1 summarizes the major features of whole organ calcium homeostasis. Dietary calcium intake averages 1000 mg/d and ~800 mg/d is excreted in the stool. GI tract calcium absorption is regulated primarily by 1,25 (OH) 2 vitamin D 3 , also known as calcitriol. Calcitriol is formed through enzymatic hydroxylation of 25 (OH) vitamin D 3 by renal tubular cells; this process is stimulated by parathyroid hormone (PTH). Plasma calcium is ~35 40% protein bound, ~10% complexed with bicarbonate and other anions, and only ~50% is “free” in solution, and thus filterable at the glomerulus. The primary long term regulation of calcium balance is through renal calcium excretion. Figure 2 summarizes the relative transport of calcium along the nephron. The proximal tubule reabsorbs the majority of filtered calcium,~70%. Themajorityoccursthroughparacellularroutes, driven by the lumen positive voltage that develops as a result of paracellular chloride reabsorption. A small amount of calcium absorption occurs via transcellular routes. Once again, the loop of Henle is a major site for ion
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This note was uploaded on 09/18/2011 for the course CHM 3218 taught by Professor Stewart during the Spring '08 term at University of Florida.

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Ca-PO4 - Calciumandphosphatehomeostasis I.DavidWeiner,M.D....

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