As mentioned several times
throughout this unit,
water makes up a large
percentage of the human body.
It plays an important role in a
number of ways within the body.
The first is that it
acts as a solvent.
So it allows for the various
things found within our body
to actually be dissolved
within it so that it can then
be moved around the body or
be involved in reactions.
That being said, water sometimes
acts as a reactant itself.
So it's part of the
various chemical reactions
that go on in our body.
Not just in the
sense of dissolving
the fluid, as mentioned before
when it acts as a solvent,
but actually as one of the
reactants that then undergoes
chemical changes and is
formed into new chemicals.
It's involved in
temperature control.
And so we discussed this when we
discussed the skin and the role
it plays with evaporation
in terms of perspiration
and cooling the body.
It's involved in transportation.
Because lymphatic fluid,
blood plasma, as well as
just the fluid found
between and within cells
allows for the chemicals
that are within our body
to be moved around, or allows
for cells to actually move
around in the case of blood.
It also acts as a lubricant.
So we've talked about
various membranes that
allow for the
reduction of friction
that there is mucus, which
is made up mostly of water.
And that mucus acts as a
lubricant, reducing friction.

Water and the
dissolved electrolytes
are distributed into two major
compartments within the body.
The first is the
intracellular fluids.
This accounts for 63% of
the water in the body.
"Intra" is referring to
"inside," and "cellular"
referring to "cells."
So this is ultimately
fluid found
within all the
cells of our body,
making up the 63%
of all of our water.
Extracellular fluid makes up
the remaining 37% of fluid.
"Extra," meaning "outside,"
and "cellular" again
referring to the cell.
So these are all fluids
found outside of cells.
This fluid can be
further broken down
into subcategories of areas
where the fluid is found.
Water located between
cells within tissues
is called interstitial fluid.
Plasma is referring
to blood plasma.
And then an amount
found in lymph.
And finally,
transcellular fluid.
This is extracellular
fluid, including categories
like cerebrospinal fluid, and
the aqueous and vitreous humors
in the eyes, the
synovial fluid found
in joints, and the serous
fluids that we've discussed
being found in body cavities.

So now let's discuss
water balance.
Water balance is
referring to the fact
that the intake of
water in our bodies
needs to be equal to the output
of water from our bodies.
As we can see here,
the normal intake
is 2.5 liters or
2,500 milliliters.
And our normal output
on a daily basis
is 2.5 liters or
2,500 millimeters.
There are various
ways that we actually
take in and release water.
Let's start by taking
a look at intake.
1.5 liters of water that
we bring into our body
is through water
gained in beverages.
Another 30% of water is
gained by eating moist foods
and getting the water
that is found within them.
A final 10% is through
water gain from metabolism.
Earlier in the course,
we learned about water
being a waste product ultimately
from cellular metabolism.
And about 10% of our water
comes from this process.
Our output of 2.5 liters,
1.5 liters of that
is water loss in urine.
28% of it is water loss
through the skin and also
water vapor loss
through the lungs
as mentioned
earlier in the unit.
Finally, 12% is water loss
through feces as well as
through sweat.

Ultimately, as
mentioned earlier,
the balance is found that our
intake is equal to our output.
And we should know
these major ways
that we ultimately
gain or lose water.

One important reason to
maintain water balance
is because, as we
mentioned, water
acts as a solvent for
the various chemicals
within our body.
And so let's take a look
at some of those chemicals,
namely the electrolytes,
found within the water portion
of our body.
One of the major
electrolytes, as we've
seen throughout this
course, is sodium.
Its function is that it's the
major extracellular cation,
so found outside of cells.
It regulates
extracellular volume,
because water is
attracted toward sodium.
And as we've seen in
the nervous system unit,
it participates in nerve
muscular functions.
Potassium is the major
intracellular cation
found on the inside of cells.
Again, it participates in
nerve and muscular function
as we looked at
earlier in the course.
Calcium helps to
strengthen bones and teeth,
is also involved in
muscle contraction
and helps blood clotting.
All of these are important
electrolytes within the body
and are found dissolved
in water within the body.

Another electrolyte, this time
a negative anion, is chloride.
It's a major
extracellular anion.
And is involved in
extracellular volume
control along with sodium.
And finally, we
have bicarbonate,
HCO3 with a one minus charge.
This is part of the
bicarbonate buffer system
and participates in
acid-base balance
that we've looked at
previously in this course.
But because we're talking
about the urinary system,
another main function
of the urinary system
is to maintain the body's pH,
we've looked at pH previously.
Blood pH wants to maintain
at a 7.35 to 7.45 range.
There are three main
systems involved
in this regulation of pH.
The first is buffers
within the blood.
The second are the lungs.
And we've previously looked at
this reaction of carbon dioxide
reacting with water in the
blood, forming carbonic acid,
and then breaking down into
hydrogen ions and bicarbonate
ions.
A third system for
the regulation of pH
that we've mentioned
previously is
the kidney, the urinary system.
And the kidney play a major
role in the regulation
of pH in two ways.
The first is that the kidney
causes chemical reactions
between free hydrogen
ions in the blood such
that they are converted to
other chemicals that can then
be excreted in the urine that
is formed through the kidney.
Another function that the
kidneys play in regulation
of pH is to retain as much
bicarbonate ions as possible,
making sure that
they're not urinated off
when they can be maintained
within the body in order
to buffer any acid or basic
changes that go on in the body.

In order to maintain the
appropriate amount of water
in our body, our body
has the sense of thirst,
such that when the volume
of water drops in our body,
our body becomes thirsty
and we seek out water.
So how does this work?
It actually involves a region
that we've looked at earlier
in the course.
So within the diencephalon
we have the hypothalamus.
The hypothalamus plays a major
role in the body's homeostasis.
And the hypothalamus has sensory
receptors for the concentration
of our blood.
As the concentration
of our blood
becomes higher and
higher because we
have less and less
water, the hypothalamus
has chemical receptors that once
they sense that concentration
change within our blood,
causes us to seek out water.
Give us senses like
experiencing dry mouth such
that we feel like we need
water and then seek it out.
In this way, our hypothalamus
controls our sense of thirst
and allows for us to
maintain homeostasis,
because it indicates
to us that we
need an increase in
our water intake.

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