Changes

PTH leads to increased calcium levels in the blood by actions on:

PTH leads to increased calcium levels in the blood by actions on:

* '''Bone'''

1. Bone

**'''Fast Phase''' - This phase begins in minutes and progressively increases for hours. PTH acts on existing osteoblasts and osteocytes to increase calcium uptake from the ''bone fluid''. Nearby calcium phosphate crystals replace the calcium which has been removed from the bone fluid.

*'''Fast Phase''' - This phase begins in minutes and progressively increases for hours. PTH acts on existing osteoblasts and osteocytes to increase calcium uptake from the ''bone fluid''. Nearby calcium phosphate crystals replace the calcium which has been removed from the bone fluid.

**'''Slow Phase''' - This phase involves activation of osteoclasts and creation of new osteoclasts, and thus takes ~48 hours to activate. There are no receptors for PTH on osteoclasts, so the signal comes from existing osteoblasts and osteocytes. This results in progressive depletion of bone mineral.

*'''Slow Phase''' - This phase involves activation of osteoclasts and creation of new osteoclasts, and thus takes ~48 hours to activate. There are no receptors for PTH on osteoclasts, so the signal comes from existing osteoblasts and osteocytes. This results in progressive depletion of bone mineral.

* '''Kidneys''' - PTH increases the calcium reabsorption at the level of the late distal tubules and collecting ducts. It also increases [[Magnesium|magnesium]] reabsorption. This occurs at the expense of [[Phosphorus Homeostasis - Anatomy & Physiology|phosphorus]] in the proximal tubule. Thus Mg and Ca are reabsorbed, P is excreted in the urine.  

2. Kidneys - PTH increases the calcium reabsorption at the level of the late distal tubules and collecting ducts. It also increases [[Magnesium|magnesium]] reabsorption. This occurs at the expense of [[Phosphorus Homeostasis - Anatomy & Physiology|phosphorus]] in the proximal tubule. Thus Mg and Ca are reabsorbed, P is excreted in the urine.  

* '''GI tract''' - Indirect effect via activation of Vitamin D3.

3. GI tract - Indirect effects occur via activation of Vitamin D3.

===Calcitriol (Active Vitamin D3)===

===Calcitriol (Active Vitamin D3)===