_IMG_45_11CalciumHomeostasis_L.jpg
<strong>Negative feedback in calcium homeostasis.</strong>

A drop in blood <span class="GREEN">Ca<sup>2+</sup></span>
causes the <strong>parathyroid</strong> gland to produce <b>parathyroid</b> hormone (<b>PTH</b>),
which has several effects:

<UL>
<li><span class="BLACK">Ca<sup>2+</sup></span> is released from <b>bone</b>.

<li><span class="BLACK">Ca<sup>2+</sup></span> is <strong>reabsorbed</strong> in the
_fig_../ch44/nephron_urine.html	<b>kidney</b>.

<li><b>Kidney</b> in turn activates <span class="BLACK">Vitamin D</span>, which promotes the uptake of
<span class="BLACK">Ca<sup>2+</sup></span> in intestines.
</UL>