Calcium balance: PTH and calcitonin
Blood calcium
normal
range
Parathyroid glands
(on the back of the thyroid)
PTH into the blood
Osteoblast
Osteoclast
Indirect – via osteoblasts
Vitamin D → active vitamin D
Small intestine
If calcium is too high
Parafollicular cells
release calcitonin
Osteoclast
less active
PTH
PTH
active vitamin D
1
Blood calcium falls
Humoral stimulus – sensed directly
2
PTH released
3
Bone: PTH acts on
osteoblasts → osteoclasts
4
Kidneys keep calcium;
make active vitamin D
5
Gut absorbs
more calcium
6
Calcium back in range →
PTH falls
(if too high: calcitonin)
–
negative feedback
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