Answer: PARATHORMONE (PTH), secreted by the PARATHYROID GLANDS
About the parathyroid glands:
FOUR tiny glands — two pairs — embedded on the back surface of the THYROID gland in the neck.
HOW PTH RAISES BLOOD CALCIUM — it acts in three ways:
1. On the BONES
It stimulates the osteoclasts to DISSOLVE BONE, releasing stored $Ca^{2+}$ into the blood. (This is called bone resorption.)
2. On the KIDNEYS
It INCREASES the REABSORPTION of $Ca^{2+}$ from the glomerular filtrate, so less is lost in the urine.
3. On the INTESTINE
It INCREASES the ABSORPTION of $Ca^{2+}$ from the digested food, acting through vitamin D.
Net effect: PTH is a HYPERCALCEMIC hormone — it RAISES the blood calcium level.
THE OPPOSING HORMONE — CALCITONIN (TCT)
Secreted by the THYROID gland, calcitonin LOWERS blood calcium by doing the opposite.
$$\textbf{PTH RAISES } Ca^{2+} \qquad \textbf{CALCITONIN LOWERS } Ca^{2+}$$
Together they form a perfect antagonistic pair that keeps the blood calcium steady — a classic example of homeostasis by negative feedback.
WHY CALCIUM MATTERS SO MUCH:
- MUSCLE CONTRACTION — $Ca^{2+}$ binds to troponin
- BLOOD CLOTTING — it is an essential clotting factor
- Nerve impulse transmission — it triggers neurotransmitter release
- Bone and tooth formation
DISORDERS:
| Condition | Cause | Effect |
|---|
| Hypoparathyroidism | Too little PTH | Low $Ca^{2+}$ → TETANY (severe muscle cramps and spasms) |
| Hyperparathyroidism | Too much PTH | Bones become soft and brittle; kidney stones |
