Negative  feedback  in calcium
   homeostasis.
<P>
   A  drop  in  blood  Ca^2+  causes  the  parathyroid  gland  to produce
   parathyroid hormone (PTH), which has several effects:
<P>
<BR><LI> Ca^2+ is released from bone.
<BR><LI> Ca^2+ is reabsorbed in the kidney.
<BR><LI> Kidney  in  turn activates Vitamin D, which promotes the uptake of
Ca^2+ in intestines.