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Lecture 12

PHYL 2044 Lecture Notes - Lecture 12: List Of Lakes By Volume, Body Fluid, Peritubular Capillaries


Department
Physiology
Course Code
PHYL 2044
Professor
Alex Quinn
Lecture
12

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PHYL 2044 Lecture 12 February 26, 2019 Renal Physiology III
Renal Physiology III
Water balance
Control of body fluid osmolarity
Sodium balance
Control of body fluid volume
We are going to talk how the kidney control body water balance and also Na balance, which as we have
seen, is the main regulator for overall body fluid.
Water Balance
First of all, talking about water balance, we bring fluid into our body via different routes. We want to
match water intake with water excretion in the urine so that we are not suffering from edema or
dehydration. If we are sweating a lot or losing water from other routes, such as the lungs, the amount of
urine excreted by the kidney will down appropriately. If you were to drink a litre of water, what we do
not want to have happened is to have all of our body fluid volume to increase by a litre and to have the
osmolarity of these body fluid to decrease.
Water Diuresis (Formation of Dilute Urine)
As shown in the diagram, plasma osmolarity and urinary excretion
of solutes does not change very much. What does change in less
than a hour after drinking 1.0L of water is that we start to
produce much larger volume of urine. At the same time as we
produce more urine, the osmolarity of the urine will decrease. So
we start to produce large volume or relatively dilute urine so that
we can get rid of the excess water without losing the excess
solute. If the blue line is multiplied by the black line, you can get
an idea of how much solute is lost in the urine and overall urinary
solute excretion does not change very much (as shown by the
bottom curve). The kidneys regulate water balance independently
of other solute balances.

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Water Excretion is Controlled by ADH
(Antidiuretic Hormone, Vasopressin, AVP)
Kidney regulates water excretion by the
use of ADH. ADH regulates water
permeability on the most distal part of the
nephron (distal tubule and the collecting
duct) so thereby regulating the amount of
water that get reabsorbed from those
parts of the nephron back into the
bloodstream.
To the right shows what happens when
the ADH level changes. The bottom x-axis
shows the increase of plasma ADH from 0
up to a maximum level.
As ADH level increases, the amount of urine produced will go down and the osmolarity of the urine will
go up. Conversely, if the ADH level decreases, the urine produced will be dilute with a low osmolarity.
Under water diuresis (when we are trying excrete water), we would be producing high volume, low
osmolarity urine. Again, the product of the two lines should tell you roughly the amount of solute loss
(total solute excretion) in the urine, which is quite constant throughout.
ADH Secretion From Posterior Pituitary
Synthesized in hypothalamus and release into the
bloodstream to act on the kidney and nephron.
Secretion regulated by osmoreceptors and
baroreceptors
ADH is a hormone that is secreted in the brainstem
from the posterior pituitary. ADH is synthesized by
neural-secretory cells which have their cell bodies in
the hypothalamus. ADH travels along the axons and
are released from these cells from the posterior
pituitary into the bloodstream and travels through the
blood to act on the kidney.
The way in which the ADH released in the brainstem is controlled is by osmoreceptor (in the
hypothalamus themselves), which would sense the osmolarity of the blood that is dependent on water
intake and excretion, and baroreceptor, that are in the cardiovascular system, residing in the heart and
in the vasculature.
ADH acts by decreasing the flow of urine so we want to release ADH under conditions where we are
water deprived so there is preservation of water. We then want to decrease ADH release under
conditions where we have excess water that we want to try and get rid of by diuresis.

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ADH Secretion From the Posterior Pituitary
ADH secretion stimulated by:
Plasma osmolarity (sensed by osmoreceptors in
hypothalamus)
Blood volume or pressure (sensed by baroreceptors in the
left atrium, pulmonary vessels and aortic arch, carotid sinus)
Circulating ADH degraded within minutes (rapid response)
Under water restricted conditions, plasma osmolarity would be high (we
do not as much water; water volume goes down and osmolarity goes
up). This increase in osmolarity would be sensed by the osmoreceptors
in the hypothalamus, which would lead to a stimulation of ADH
secretion.
The other thing that is sense is blood volume or blood pressure. If we
are water deprived, the amount of fluid in the blood system will go
down, then the both the volume of the blood in the circulatory system
and the pressure of that blood will decrease, resulting in signaling to the
baroreceptors. In response to the decrease in blood volume/pressure, the baroreceptor will stimulate ADH
secretion, which will decrease the flow of the urine, causing it to become more concentrated. As a result, the
kidney would reabsorbed more of the water, which will go back into the bloodstream to bring osmolarity back
down and bring blood volume back up again. This is what will happen under water deprivation conditions.
If we have a lot of water, the activity of these receptors will go down and the ADH secretion will decrease. Because
ADH in the bloodstream is degraded very rapidly, this means that the plasma ADH level will go down so that the
water permeability of the nephron and water reabsorption from the nephron will be regulated and changed over
the course of minutes. If there is water intake, plasma osmolarity will decrease, blood pressure will go up, and ADH
secretion will be decreased and urine flow will be increased to rid our body of the excess water.
Effect of ADH on Water Permeability
ADH acts on the distal part of the
nephron to regulate water
permeability. In the absence of ADH,
the highlighted regions of the
nephron are water impermeable so
that water cannot be reabsorbed
back into the bloodstream, resulting
it to be all excreted in the urine. This
causes high volume, dilute urine
production. When we secrete ADH
in response to high plasma
osmolarity or low plasma volume,
ADH will act on the water
impermeable region of the nephron to allow water to be reabsorbed (which is what the arrows mean).
Most of the water would come back out of the nephron into the interstitial fluid and into the
bloodstream. What is excreted would be a high osmolarity, low volume urine.
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