There are several considerations in
regulating the body calcium levels.
We’ve talked about the blood but
there’s several things to consider.
First of all, vitamin D in our diet.
Are we getting enough calcium in our diet?
And is our body making
sufficient amounts of vitamin D
to support the calcium that we need.
The parathyroid hormone or
the PTH that I talked about
is active in the kidneys and
the bone as we shall see.
And this helps to either, in the
case of PTH, to release calcium
so that the body has enough calcium
travelling to the blood stream
or in the case of a
protein called calcitonin
to actually absorb that
calcium back into the bone
and reduce the blood levels of calcium.
We’ll see that in just a second.
So the bones as I said are
important reservoirs of calcium.
A place to put excess calcium
when the body has excess calcium
and a place to provide calcium when
the calcium levels are too low.
These two hormones, calcitonin
and PTH, that I have described
play important roles in that process.
The bone making that is the taking
of calcium out of the blood
and putting it into the bone occurs in
specialized bone cells called osteoblasts.
This reduces blood calcium and
stimulated by calcitonin.
On the other, when the
blood needs more calcium,
bones are broken down in specialized
cells called osteoclasts.
These cells are stimulated
by the hormone known as PTH.
Now, another important
variable in calcium levels
in the blood is phosphate concentrations.
Calcitonin decreases phosphate levels.
Calcitonin acts on the kidneys
and it stimulates the process
of phosphate reabsorption
that is preventing the phosphate
from being excreted in the urine.
As a consequence phosphate concentration
in the blood supply goes up
and with phosphate concentration, the blood
supplies goes up, bone making is favored.
On the other hand, PTH
favors the opposite process.
It favors the excretion of
phosphate by the kidneys.
So the blood levels
of phosphate fall
and the falling blood phosphate levels
favors the release of calcium from bones.
In the addition to the stimulation
of increasing the calcium levels
from the bone by PTH,
PTH also stimulates the absorption
by stimulating the formation
of active vitamin D, okay?
The increased levels of vitamin D favors
the taking of calcium from the diet
and putting it into the blood supply,
thus favoring the increase in
concentration of blood calcium.
PTH synthesis, however,
is negatively regulated
by the amount of ionized calcium
that’s in the blood serum.
Meaning that the higher the levels
of calcium that’s in the blood,
the less PTH is made.
High serum calcium reduces this
secretion because PTH is made
by the parathyroid gland.
And by negatively impacting the
parathyroid gland, less PTH is made.
So conversely low calcium, however, favors
PTH released by the parathyroid gland
and stimulation of the processes
that I’ve described here.
Well, that’s a mouthful of things.
Let’s look at the cellular level
and see what’s actually happening
with these individual hormones.
On the left, I show what happens in
the body with respect to calcitonin.
So we can imagine we have
a certain amount of free
calcium ions that are
present inside the cell.
In this case, I’ve shown
an abundant level of
calcium ions that are present
in the blood supply.
When this happens, calcitonin
will favor the making of bone.
The making of bone will
reduce the levels of calcium
that’s occurring inside of the blood
supply and more bone cells will be made.
Another way that calcitonin affects this
is by changing the phosphate levels.
Remember that what calcitonin does
is it raises the levels of phosphate
by stimulating the reabsorption
of phosphate by the kidney.
When phosphate levels increase,
then more of that calcium is gobbled
up and converted into bone.
This is the way that calcitonin
actually makes the bone.
So when calcitonin is present,
blood supply of
calcium will go down,
the increase in bone concentration
or bone density will actually go up.
PTH works in the opposite way.
Bone breaking occurs when the
calcium levels fall too low.
The release of calcium from
the bone increases the
blood concentration of
calcium as you can see here.
The other thing that happens
with PTH, remember,
is that PTH favors the excretion
of phosphate by the kidneys.
So as phosphate is being
excreted by the kidneys
more calcium remains
in the blood supply
because less of it is being
converted into bone by calcitonin.
Now, I said that PTH is
affected by the levels
of calcium that’s present
in the blood supply.
High calcium levels will
favor lower amount of PTH
being released by the
This causes a lower level
of vitamin D to be made
and lower level of
vitamin D being made
reduces the amount of calcium that is
taken from the intestine in our diet.
On the other hand, when there are low
concentrations of calcium present in the blood,
then a higher level of PTH is made.
The higher level of PTH favors a
higher level of vitamin D being made
and a higher level of vitamin D being made
favors additional absorption
of calcium from the diet.
So PTH is ultimately the
molecule or the protein
that is most important in
controlling these overall levels
because it’s actually sensing the calcium
levels affecting vitamin D accordingly.