So calcium in the blood is
carried in two different ways.
First of all, it can be carried
by proteins like albumin,
that’s the most common
way it’s carried.
And there are other proteins that
can bind and carry calcium as well.
That’s not our most important
consideration for calcium, however.
It’s the free ion that is
the unbound form of calcium
in our bloodstream that
your doctor measures
to determine the level of calcium that you
have and if that’s the proper amount.
It's the uptake of
calcium by cells
that occurs through calcium pumps that
gives us the calcium that we have.
But calcium is something that cells
are very, very careful with.
Calcium ions are
used in signalling.
Calcium can cause muscular
contraction and a variety of things.
So cells have to be careful
how much calcium they take up
and when they do take it up,
they tend to gobble it up
and hold it inside the proteins.
Another way that cells protect
themselves from the effects of calcium
are to sequester the calcium
in an organelle known as
the endoplasmic reticulum
in normal cells or in the sarcoplasmic
reticulum of muscle cells.
Another way that
calcium can be stored
is by being bound to a
protein called calmodulin.
And calmodulin is a way of
telling other proteins that,
“Hey, I’ve gotten a signal
that calcium is here.”
And that communicates to those proteins
that they need to take action.
Now, the ionized calcium
is regulated by vitamin D.
And that’s what we’re
going to focus on here.
Another protein that’s very
important in this process,
in fact, more important than vitamin D
is parathyroid hormone or
PTH as I will call it here.
A third protein that’s
involved is calcitonin
and I’m going to show a scheme
whereby this regulation now occurs.
So vitamin D happens in
our body or in our blood,
gets there via the calcium being absorbed
from our diet, by the cells in our gut.
PTH and calcitonin balance
the ionized calcium
by controlling different processes
in the bone and in the kidney.
And finally, bodily calcium
is stored in bones.
Now, we think of bones as very important
for giving us arms and legs and so forth.
And they are very important
for that process.
But a second function of bone
that’s critical for the body
is to serve as a reserve of
calcium for the rest of the body.
Now, calcium as I said has
many functions in the body
and it’s very carefully
controlled by cells.
First of all, the bones hold about
99% of the calcium in the body.
About a kilogram in the body is the
amount of calcium that we have,
that we walk around with everyday.
Calcium is involved in the
process of signalling,
telling cells to do something or
telling cells not to do something.
Calcium is the impetus for
Muscle cells know to start
the contraction process
when calcium stores are
released inside of them.
We’ve already seen how calcium
affects the visual process
by the closing of the calcium gates
to stop the calcium concentration
from accumulating inside the eye.
That lowered calcium concentration
stops the neurotransmitter release,
and tells the brain that
they eye detected light.
And last, calcium is important for
the process of blood clotting.
Without calcium’s involvement in blood
clotting, blood clotting will not occur.
Now, let’s look at that actual
absorption process that happens
in getting calcium from our
diet into our blood stream.
I’ve shown on the screen here a
depiction of the intestinal lumen,
that is the inner part of
our intestines on the left.
And intestinal cell that is in the middle,
and the blood supply that’s on the right.
So let’s follow the movement of calcium
ions through these individual units.
So we eat a meal that
has dietary calcium.
It appears in the lumen of our
intestine as you can see here.
Specialized calcium pumps move the calcium
into the intestinal cell as shown here.
Now, calcium is a problem
because we don’t want to
have too much calcium
or this intestinal cell is going
to have difficulties as well.
So the intestinal cell uses
a protein called calbindin.
Specifically, this form of
calbindin is called calbindin D9k.
Calbindin D9k quickly
grabs those calcium ions
and takes them to the
Now, this helps the sequestration
that is to keep the calcium
from activating processes
in the rest of the cell.
The endoplasmic reticulum then
moves to the basal membrane
on the other side of the cell
to deposit those calcium ions
so they can be pumped out
into the bloodstream as
it happens right here.
So the calcium in the bloodstream then
gets there from the intestinal cells
and is the amount of free calcium that’s
there that is important for our body.
Remember the calcium is also bound to
albumin as it travels through the body.
The rate limiting step in this overall
process is the amount of calbindin
that is available to bind
to the calcium ions.
The amount of calbindin that’s there
is regulated by the level of vitamin D
and we’ll see that the
level of vitamin D is
ultimately controlled by
the protein known as PTH.
Now, calbindins are actually families
of calcium-binding proteins.
There are several
and they appear to be
unrelated to each other,
but each have the important features
that they have what are called EF hands.
Now, EF hands are common
we see in proteins that bind to calcium.
And they literally have
the structure of a hand.
The calcium ions fitting in the palm
of the hand as I describe it here.
Some proteins can have
multiple EF hands
and therefore bind to
multiple calcium ions.
The proteins as I said are not
closely related to each other
except for that
The level of calbindin gene
expression as I noted earlier
is thought to be
controlled by calcitriol,
the form of active vitamin
D that we’ve seen.
So controlling the level of calcitriol
helps us to control the amount
of calbindin that’s made
and the amount of calbindin that’s
made will determine how much
calcium is actually
absorbed from our food.