if you have CKD, UW states that bc your filtered load of p04 goes down, its level rises. so why doesnt ca2+'s level rise? Vit D is decreased in CKD or calcitriol, so both ca and p04 should be decresaed, but your p04 remains high, and bc the po4 is high, it drives ca2+ into the tissues, is that the sole reason why calcium is decreased? bc shouldnt ca2+ filtered load also be decreasing?
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Step I CKD Phosphate
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Effects due to tubular dysfunction:
Decreased renal activation of vitamin D > hypocalcemia
Phosphate retention > hyperphosphatemia > phosphate binds free calcium > hypocalcemia
Renal tubular/metabolic acidosis > dissolution of hydroxyappatite > demineralization of the bone matrix > osteomalacia
Secondary Effects:
Hypocalcemia > secondary hyperparathyroidism
Secondary hyperparathyroidism > osteoclastic activation > bone resorption
Decreased renal activation of vitamin D > hypocalcemia
Phosphate retention > hyperphosphatemia > phosphate binds free calcium > hypocalcemia
Renal tubular/metabolic acidosis > dissolution of hydroxyappatite > demineralization of the bone matrix > osteomalacia
Secondary Effects:
Hypocalcemia > secondary hyperparathyroidism
Secondary hyperparathyroidism > osteoclastic activation > bone resorption
vit d deficeincy wouldnt cause phophate deficiency too? why is there phosphate retention with the kidneys failing.
Because phosphate is excreted by the kidneys, and with reduced kidney function less will be getting to the kidneys and more will be retained. PTH is responsible for getting rid of phosphate in the kidney, but there are fewer nephrons for it to act on in CKD. You are correct that loss of vitamin D will lead to reduced GI absorption of phosphate, but that doesn't lead to hypophosphatemia because your body has no way of getting rid of the phosphate it already has due to the CKD.
In primary vitamin D deficiency with intact renal function you would see low calcium and low phosphate because both are being excreted renally but neither can be absorbed from the GI tract.
