Detected by the thyroid gland, specifically C-cells.
Control Center Activation:
C-cells activate a specific gene to produce output.
Output:
Hormone: Calcitonin
Function: "Tones down" calcium levels in the blood.
Effectors and Actions
Osteoclasts:
Decrease activity to stop breaking down bone and releasing calcium.
Osteoblasts:
Increase activity to take calcium from the blood and store it in bones.
Kidneys:
Excrete excess calcium in urine.
Result
Calcium levels drop back to normal (negative feedback system).
Low Calcium Levels
Detection:
Detected by the parathyroid cells.
Control Center Activation:
Increase production of cyclic AMP.
Activates the parathyroid hormone (PTH) gene.
Output:
Hormone: Parathyroid Hormone (PTH)
Function: "Picks up" calcium levels in the blood.
Effectors and Actions
Osteoclasts:
Increase activity to release stored calcium from bones into the blood.
Osteoblasts:
Decrease activity to prevent new bone formation which consumes calcium.
Kidneys:
Retain calcium, do not excrete in urine.
Produce calcitriol (active vitamin D).
Calcitriol
Target: GI tract.
Function: Increases absorption of calcium from food.
Result
Calcium levels increase back to normal (negative feedback system).
Recap
High Calcium: Calcitonin tones it down by decreasing osteoclast activity, increasing osteoblast activity, and excreting calcium via kidneys.
Low Calcium: PTH picks it up by increasing osteoclast activity, decreasing osteoblast activity, retaining calcium in kidneys, and increasing dietary calcium absorption via calcitriol.