Please wait while the transcript is being prepared...
0:00
Hello, and welcome back to
the Fundamentals of
Human Physiology.
My name is Jessica Briffa,
and today we'll be going through
the last lecture in the
Kidney Function series,
which is on water balance.
0:15
Before we delve into
today's lecture content,
let's first start
with a stop-and-think
question to get you thinking
about the lecture content.
What do you think a
negative water balance
will normally result in?
Similarly to a negative
sodium balance,
a negative water
balance means that
water output is greater
than water input.
This means that a
person that has
a negative water balance
will excrete more water,
resulting in dilute urine.
That is, unless there's
another underlying condition,
such as vomiting or diarrhea.
0:53
As depicted here, we have
two separate forms
of water intake,
which is by the food and
beverages we consume,
as well as water
that's generated
from normal metabolic processes.
Water output occurs in two ways,
via insensible and
sensible manners.
Insensible manners
include water loss
via the respiratory
system and skin,
which is so called
because we are
consciously not aware
that it's occurring.
Whereas sensible manners include
urine and feces production,
which is so called
because we are aware
of the water loss
via these processes.
1:29
Now that we've covered
what water balance
is and what a negative
water balance means,
let's consider the
following question.
What do you think a
negative water balance
will be most obvious as?
As previously mentioned, a
negative water balance can
be due to passing large
volumes of dilate urine,
as well as due to
vomiting and diarrhea.
First, I'll go through what
happens in severe water loss,
and then I'll go through
what the correct answer is.
When water loss
exceeds water intake,
blood volume decreases and
plasma osmolarity increases.
This decrease in blood volume
causes a reduction
in blood pressure,
leading to increases in
renin and angiotensin
II secretion.
The latter of which,
along with aldosterone,
promotes sodium and chloride
reabsorption in the kidneys,
and thus water reabsorption,
which ultimately
decreases urinary output.
Increased blood osmolarity
and angiotensin II also
stimulates the hypothalamus
and vasopressin is released,
which promotes renal
water retention
and reduced urinary output.
Increased plasma osmolarity also
stimulates thirst
through osmoreceptors
to replace water loss.
This tells us that the
most obvious sign of
a negative water balance would
be a reduction in plasma volume.
However, if this is a significant
amount of water loss,
it's possible that
cardiac venous return
may also be reduced.
