Hormone concentration metabolism and negative feedback | NCLEX-RN | Khan Academy

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.