When we talk about the endocrine organs and the endocrine glands
and we talk about hormones flying all throughout the body,
it's pretty easy to develop this mental image
of that process happening pretty haphazardly.
And so you can imagine hormones just coursing all
throughout the body, being fired at will and to everywhere.
But if you think about the effects
of the endocrine glands, like in the adrenal glands
with the fight or flight hormones,
it becomes pretty important that the
effects being stimulated by these hormones
be well controlled because our body is
pretty sensitive to those effects.
And so it turns out that the hormone concentration
in our blood at any given time is pretty tightly controlled.
And one of the ways that it's controlled
is through this idea of metabolism and excretion.
And so for every hormone that reaches its receptor,
thousands more are swept up and removed by the body.
And one of the ways that to remove this through the liver.
And the liver will metabolize extra hormones
and turn them into bile, which is ultimately
excreted in the digestive system.
And another organ is the kidney.
And you have two of these.
And they're filtering your blood all of the time.
And they're removing waste products from the blood
through urine.
And then some hormones are actually just broken down
in the blood.
And then at the products of that breakdown flow
into the liver or the kidneys.
And then sometimes you can even sweat these hormones out.
But the idea here is that all of the time for all the hormones
reaching the receptors, a lot are
swept up and removed from the body.
And another way that concentrations
of hormones in the body are controlled
are through feedback loops.
And the majority of feedback loops
are what we consider to be negative feedback loops.
And the idea behind negative feedback loops
is that conditions resulting from the hormone action
suppress further releases of those hormones.
And that can be a pretty confusing idea.
So I'm going to draw an example.
So we have the hypothalamus here.
I'm going to draw it in.
And I'll write it down.
And the hypothalamus releases a hormone,
thyroid-releasing hormone-- so TRH.
And it releases it.
And it goes down to the pituitary gland,
which I'll drawn in, in right here.
And in response to TRH, the pituitary gland
releases thyroid-stimulating hormone or TSH.
And TSH goes down to the thyroid glands,
which would be about right here.
And the thyroid gland releases its hormones, T3,
or triiodothyronine, and throxine.
And these thyroid hormones travel all
throughout the body in search of the receptors
in order to, let's say, up-regulate metabolism.
That's one of the major jobs of the thyroid glands.
And so here's where the idea becomes pretty cool.
Because some of the receptors are located on the pituitary
gland and the hypothalamus.
And as the thyroid hormones reach the pituitary
and the hypothalamus, they signal the hypothalamus
and pituitary gland to stop making their hormones.
And the hypothalamus and pituitary gland
see that we have enough thyroid hormones in the blood
and that they don't need to make any anymore.
And so this is a major way that the thyroid hormone
levels in the body are controlled.
And you might say, hey, that sounds a little bit redundant.
I mean if the hypothalamus can be turned off by the thyroid
hormones and it's upstream from the pituitary gland, then
why does the pituitary gland even
have to have these receptors?
But the redundance is really just a reflection
of how important feedback control is
and how important the concentration of hormones
in the body is.
And so hopefully what we can see is
that the hormone levels in the body
aren't haphazard and aren't willy-nilly.
And that concentration is important.