Page 34 - The Cell in 40 Topics

The Cell in 40 Topics

Figure 20.
When necessary, the parathormone borrows
some calcium from the bones.

Although under normal circumstances, the bones are reluctant to release cal-
cium, they do release a specific amount when they come into contact with
parathormone.

order not to re-absorb the calcium, the intestine cells need activated
Vitamin D. But the Vitamin D obtained through food is not in an active
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state And so, at this point, a major problem emerges that must be resolved
in order for the intestine to absorb more calcium, and thus to raise the level
of calcium in the blood. In other words, a very special molecule is needed to
alter the chemical structure of the non-activated Vitamin D. This molecule
that activates Vitamin D, once again, is parathormone (Figure 22).
We now need to reflect with great care. There are three different ways
of raising the level of calcium in the blood, yet the key that starts these three
very different systems is the same in each case. This key turns the three sys-
tems on. Even more amazingly, when these three systems, with their very
different structures and methods of operation, are turned on, the result is
exactly the same: a rise in the level of calcium in the blood.
How did the parathyroid cells arrive at the formula for this triply-ef-

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