00:00
These hormones that control calcium and phosphorus or phosphate are parathyroid hormone
and where does parathyroid come from? The thyroid gland has 4 nodules in it, and these
are the parathyroid nodules. These parathyroid hormones are going to do a few functions
for us. The 1st thing it does it acts on the kidney to reabsorb calcium, to not reabsorb
phosphate, and to activate vitamin D. So vitamin D has a few forms converted to its most
active form of vitamin D which is known as D3. D3 is going to have hormonal actions on its
own, but let's put that aside just for a second and talk about what parathyroid hormone is
doing. Since it is reabsorbing more calcium from the kidney, you're going to be able to be
putting more calcium back into the circulation. It is dumping phosphate or not reabsorbing it,
so you are losing some phosphate. Parathyroid hormone also acts on the bone to cause
calcium to be reabsorbed. So this is working on your stores of calcium in the body, breaking
them down, for what purpose? So you're reabsorbing it from your kidney, you're reabsorbing
it from the bone. Why are you doing all this work? The big issue is you need to maintain
plasma calcium levels somewhere in between about 8.5 to 10.6 mg/dL. How do you do that?
You are reabsorbing bone from these 2 locales and moving it into the plasma. This movement
of calcium from its stores in the bone and when it's been reabsorbed increase plasma calcium
levels. If you increase plasma calcium levels enough, you can decrease parathyroid hormone.
02:07
So, really parathyroid hormone is stealing calcium from the bone by having it undergo
reabsorption. You might ask well I really thought we wanted to save our calcium, we want to
store it up in the bones like we do our money in the bank. Right? That is true, but you need to
always have some cash flow and our cash flow in this case is the plasma levels of calcium. You
need to always have a certain amount of calcium in the plasma or you will die. So therefore
some of that stealing of calcium from the bone is to make sure you have enough circulating
around in the body. Now, I told you that we had active vitamin D3 also produced from the
parathyroid. Let's now talk about what it is doing for plasma calcium levels. The 1st thing is it
also causes bone resorption to occur. It also stimulates the kidney to reabsorb more calcium,
but it does one thing that's different, it also reabsorbs phosphate. So, our phosphate levels
can increase if we have active D3 around. That's important because remember eventually
we will want to store back up some of that extra calcium, right. We can only do that if it's in
relation to phosphate because we need the calcium phosphate crystals. The last thing that
the D3 will do will act as a negative feedback to stop the conversion to its most active form.
03:51
There's one last effect that we have left off this particular diagram, and that is you do change
the amount of intestinal absorption of calcium. So, if you were to eat a meal and had high
D3 levels around, and let's say that meal involved calcium whether it would be eating your
spinach or drinking your milk, those will allow for more intestinal absorption of calcium, if D3 in
its active form is around, therefore that will also feed into maintaining or increasing the
plasma levels of calcium. So we have a lot of mechanisms involved here, either reabsorbing it
from the bone, using it in the kidney, or reabsorbing it from the intestine all in an effort to
maintain plasma calcium levels at high enough levels that we can undergo normal
physiological function. The 2 hormones that are primarily involved here, PTH and active vitamin
D. We should also discuss the negative feedback inhibition loop of calcium homeostasis. In
the case of elevated calcium levels, calcium will directly inhibit the secretion of parathyroid
hormone. Calcitonin is secreted by the parafollicular cells or C-cells of the thyroid. This
hormone opposes the effects of PTH. It inhibits the reabsorption of calcium by the bone which
will lower the amount of calcium in the blood. It also inhibits renal reabsorption of calcium and
phosphate allowing them to be excreted in the urine. As a result, calcitonin lowers serum
calcium levels. Additionally if the effects of PTH lead to an increased level of active vitamin D,
this will stimulate the osteoclasts within the bone to further inhibit the renal reabsorption of
phosphate. These are the negative feedback loops that help regulate our levels of calcium and
phosphorus.