>>Dr. Ketchum: This segment on the endocrine system will focus on the hormones that are
involved in the regulation of blood calcium levels. And so once again, we return to the
outline. And in the outline, the hormones that we are going to be focusing on in regulation
of our blood calcium levels are calcitonin and parathyroid hormone. So let’s begin
our discussion. First off, it’s important to understand why calcium is necessary in
the body. And we have several different functions for calcium depending upon whether it’s
in the extracellular fluid or the intracellular fluid. So let’s just draw a cell to begin
with, and we’ll indicate where the extracellular fluid and the intracellular fluid is located.
And from this table, you will notice that you have a higher concentration of calcium
within the cell or in the intracellular fluid and a lower concentration of calcium outside
of the cell, or in the extracellular fluid. Regardless of this concentration gradient,
calcium still has very important functions in both the extracellular fluid and in the
intracellular fluid. For example, with tight junctions—and we’ve spoke about tight
junctions before—it provides the cement for those tight junctions. Remember that tight
junctions force solutes through cells or across cells rather than between those cells. This
extracellular fluid calcium level is also going to assist in myocardial contractions.
So that’s cardiac or—cardiac muscle or muscle of the heart and also smooth muscle
contraction, and you will see that later in future segments. Calcium is also important
in assistance of neurotransmitter release of the synapse, which you will see in later
segments. And so it also plays a role in exciting neurons and as a cofactor in coagulation cascades,
all of which you’ll see in later segments. Within the intracellular fluid, you’ve already
learned that calcium serves as a second messenger. And it also plays a role in muscle contraction
as well, even if it’s in the intracellular fluid. Again, all of these functions you will
be discovering when we move on to future segments. Thus far, you are responsible for knowing
that calcium functions as a second messenger system within the intracellular fluid.
So let’s look at the hormones then that will control or regulate calcium balance.
There are actually three different hormones. There’s calcitonin, parathyroid hormone
or PTH, and calcitriol. We are not going to focus on calcitriol; we will focus on calcitonin
and PTH. The thyroid gland synthesizes and secrets calcitonin. So recall where the thyroid
gland is located. It’s at the base of your neck, and sits on top of your trachea.
Then we have the parathyroid gland. The parathyroid glands actually sit on the backside of the
thyroid gland. So in this case, you can see four parathyroid glands located on the backside
of the thyroid gland. The parathyroid gland will synthesize and secrete parathyroid hormone.
And please note that both calcitonin and parathyroid hormone are classified as peptides. And hence
knowing that, you will know how they are synthesized and secreted, how they are transported, where
their receptor is located, and what the, the target cell response will be. So let’s look
at the specific functions for calcitonin and PTH.
Now to understand the functions of calcitonin and PTH, you have to understand the functions
of osteoclasts. Osteo means “bone,” and osteoclasts are cells that are found inside
of your bone. And these cells are responsible for breaking down bone; and bone contains
a lot of calcium. So if we break down bone, we release calcium into the plasma. Okay,
so this is something that’s normal. It’s naturally occurring on a daily basis for every
human being. Osteoclasts continually break down bone, and then release calcium into the
plasma. You can also build bone on a daily basis too. So let’s look at calcitonin.
Calcitonin, remember, is synthesized and secreted by the thyroid gland. It targets the bone
and it inhibits the osteoclasts. If we inhibit osteoclasts, bone cannot be broken down. And
if bone cannot be broken down, your plasma calcium levels will decrease. Calcitonin not
only targets the bone, it also targets the kidneys. When calcitonin targets the kidneys,
it causes calcium excretion. So why would we want calcitonin to be synthesized and secreted
from the thyroid gland? If it’s lowering your plasma calcium levels and it’s causing
calcium excretion, the stimulus for calcitonin release must be an excess of calcium in the
plasma. We’ll return to that shortly. Now let’s focus on parathyroid hormone. Parathyroid
hormone also targets the bone like calcitonin does, but this time it activates the osteoclasts.
And if we activate osteoclasts, we will break down bone. And if bone or when bone is broken down,
that calcium will go out into the plasma, thus we increase your plasma calcium levels.
PTH, just like calcitonin, also targets the kidneys. And when it targets the kidneys,
the kidneys will reabsorb calcium. So when we reabsorb calcium that will increase our
calcium plasma levels. So as you can see, calcitonin and PTH do just the opposite. We
want to maintain our calcium levels in both the intracellular, extracellular fluid, as
well as the plasma. We have to maintain those levels; too little or too much calcium can
be a bad thing. And so let’s think about what will cause the synthesis and secretion
of calcitonin and PTH.
So what I have drawn is a capillary or a blood vessel, for example, and this is the plasma.
And let’s assume that you have too high of a level of the calcium in the plasma. What
hormone would be synthesized and secreted at this point? This would be calcitonin. Because
once calcitonin is synthesized and secreted from the thyroid gland, that will return your
calcium plasma levels back to normal. And then that system would shut off via negative
feedback. Once the calcium levels are back to normal, the thyroid gland would stop or
shut down its synthesis and secretion of calcitonin. Now let’s assume that you have too little
calcium in the plasma. What hormone now would be synthesized and secreted? Hopefully you
said parathyroid hormone. And when parathyroid hormone is synthesized and secreted, that
will bring the blood calcium levels back to normal as well. So you have a normal concentration
of calcium in the plasma. So once those levels of calcium are back to normal, then the synthesis
and secretion of parathyroid hormone will stop. So you can see how these two hormones
work together to regulate your blood calcium levels. These two tables give a really nice
summary of both of these hormones. Things that you should be familiar with in this table—and
and it looks like a lot of information. However, once you understand that calcitonin and PTH
are peptide hormones, a lot of this information will become second nature. So be sure to use
these two tables as a summary for these hormones.