WEBVTT

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When we talk about the endocrine
organs and the endocrine glands

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and we talk about hormones
flying all throughout the body,

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it's pretty easy to
develop this mental image

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of that process happening
pretty haphazardly.

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And so you can imagine
hormones just coursing all

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throughout the body, being
fired at will and to everywhere.

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But if you think
about the effects

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of the endocrine glands,
like in the adrenal glands

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with the fight or
flight hormones,

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it becomes pretty
important that the

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effects being stimulated
by these hormones

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be well controlled
because our body is

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pretty sensitive
to those effects.

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And so it turns out that
the hormone concentration

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in our blood at any given time
is pretty tightly controlled.

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And one of the ways
that it's controlled

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is through this idea of
metabolism and excretion.

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And so for every hormone
that reaches its receptor,

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thousands more are swept
up and removed by the body.

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And one of the ways that to
remove this through the liver.

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And the liver will
metabolize extra hormones

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and turn them into bile,
which is ultimately

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excreted in the
digestive system.

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And another organ is the kidney.

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And you have two of these.

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And they're filtering your
blood all of the time.

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And they're removing waste
products from the blood

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through urine.

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And then some hormones are
actually just broken down

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in the blood.

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And then at the products
of that breakdown flow

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into the liver or the kidneys.

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And then sometimes you can
even sweat these hormones out.

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But the idea here is that all
of the time for all the hormones

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reaching the
receptors, a lot are

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swept up and removed
from the body.

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And another way
that concentrations

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of hormones in the
body are controlled

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are through feedback loops.

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And the majority
of feedback loops

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are what we consider to be
negative feedback loops.

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And the idea behind
negative feedback loops

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is that conditions resulting
from the hormone action

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suppress further releases
of those hormones.

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And that can be a
pretty confusing idea.

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So I'm going to draw an example.

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So we have the
hypothalamus here.

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I'm going to draw it in.

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And I'll write it down.

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And the hypothalamus
releases a hormone,

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thyroid-releasing
hormone-- so TRH.

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And it releases it.

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And it goes down to
the pituitary gland,

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which I'll drawn
in, in right here.

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And in response to TRH,
the pituitary gland

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releases thyroid-stimulating
hormone or TSH.

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And TSH goes down to
the thyroid glands,

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which would be about right here.

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And the thyroid gland
releases its hormones, T3,

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or triiodothyronine,
and throxine.

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And these thyroid
hormones travel all

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throughout the body in
search of the receptors

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in order to, let's say,
up-regulate metabolism.

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That's one of the major
jobs of the thyroid glands.

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And so here's where the
idea becomes pretty cool.

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Because some of the receptors
are located on the pituitary

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gland and the hypothalamus.

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And as the thyroid hormones
reach the pituitary

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and the hypothalamus, they
signal the hypothalamus

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and pituitary gland to
stop making their hormones.

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And the hypothalamus
and pituitary gland

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see that we have enough
thyroid hormones in the blood

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and that they don't need
to make any anymore.

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And so this is a major way
that the thyroid hormone

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levels in the body
are controlled.

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And you might say, hey, that
sounds a little bit redundant.

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I mean if the hypothalamus can
be turned off by the thyroid

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hormones and it's upstream
from the pituitary gland, then

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why does the
pituitary gland even

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have to have these receptors?

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But the redundance is
really just a reflection

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of how important
feedback control is

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and how important the
concentration of hormones

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in the body is.

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And so hopefully
what we can see is

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that the hormone
levels in the body

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aren't haphazard and
aren't willy-nilly.

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And that concentration
is important.

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