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Calcium comes from the kidneys, intestines, and bone. Kidney maintains calcium and phosphate levels. PTH, calcitriol, and calcitonin facilitate the movement of ions. 99% of calcium is stored in bone, 1% in ICF, 0.1% in ECF.
Calcium can easily shed hydrated cell and react with other species so it is stored in organelles in the ICF. In ECF, calcium is either half bound or freely ionized. 45% of bound calcium is either attached to albumin or complexed in ions. Acidosis increases ionised Ca2+, while alkalosis decreases it.
Calcium homeostasis depends on total amount of Ca2+ in the body and distribution between ECF and bone.
Thyroid glands: have parafollicular cells that secrete calcitonin when calcium levels are high, reduces secretion of calcium from bone
Parathyroid glands: have principal cells that secrete PTH when calcium levels are low, increasing breakdown of bone tissue.
Vitamin D is converted into calcitrol in PCT cells, which in turn increases calcium and phosphate reabsorption in the small intestine, increase calcium and phosphate release from bone, and reduces calcium and phosphate excretion
Calcium Reabsorption in PT:
Most calcium transport is paracellular due to Na+/K+ ATPase and transepithelial voltage creating solvent drag. 20% is transcellular, Ca2+ channels on apical surface bring calcium in and binds to calbindin. Ca2+-ATPases on basolateral surface move calcium out.
Calcium Reabsorption in DCT and CD:
Ca2+ channels on apical surface. Ca2+ binds to calbindin. NCX1 channels exchange calcium for three sodium on basolateral surface. PTH stimulates opening of calcium channels on apical side.
Low Calcium --> muscle spasm, high calcium --> arrhythmias
Phosphate Homeostasis:
86% of phosphate stored in bone, 14% in ICF, .03% in ECF.
Most reabsorption is transcellularly in PT, little amounts in LoH.
PTH decreases phosphate reabsorption.
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