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.
Is 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 signaling,
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 of 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.
In calmodulin is a way of telling other proteins that, hey, I've got this 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 gonna 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 gonna show a scheme whereby this regulation now occurs.
So vitamin D happens in our body or in our blood gets there by 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 bones 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 signaling, telling cells to do something or telling cells not to do something.
Calcium is the impetus for muscular contraction.
Muscle cells noticed throughout 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 lower calcium concentration stops the neurotransmitter release
and tells the brain that the 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,
an 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 wanna have too much calcium or this intestinal cell is gonna 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 endoplasmic reticulum.
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 blood stream as happens right here.
So the calcium in the bloodstream then gets there from the intestinal cells
and as the amount of free calcium that’s there that is important for our body,
remember that 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 different calbindins 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 structural features we see in proteins that bind the 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 they said are not closely related to each other except for that calcium binding portion.
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.