WATER BALANCE
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Kidneys maintain water balance in the body.
Humans' water reserves are depleted faster than their
food reserves. Under normal activity levels we lose 2.0
L of water a day in perspiration, urine and exhaled air.
Increased exercise leads to increased water loss.
To maintain fluid levels we must consume 2L of fluids
daily. A drop in fluid intake by 1% of your body mass
will cause thirst, a 5% decrease will bring extreme pain
and collapse, and a decrease of 10% will cause death.
More water in means more urine out and vice versa.
Water balance is regulated by our nervous and
endocrine systems.
ENDOCRINE REGULATION OF WATER BALANCE
ADH (anti-diuretic hormone) regulates the osmotic pressure*
of body fluids by causing the kidneys to increase water
reabsorption (at the collecting ducts).
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*Osmotic Pressure: A hydrostatic pressure caused by a
difference in the amounts of solutes between solutions that
are separated by a semi-permeable membrane.
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If osmotic pressure is increased in a solution it means it is
hypertonic and water will move in (pressure to do osmosis). If
osmotic pressure is decreased in a solution there is less
pressure for osmosis – it it near to equilibrium. In our body
when we become dehydrated, either by decreasing water
intake or increasing water loss (ie. Sweating) – water is lost
from our bloodstream, this increases it's osmotic pressure.
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When ADH is released, water reabsorption is increased to
decrease
osmotic
pressure and
is more concentrated
⇧ADH
--> ⇧ Water
Reabsorption
-->urine
Concentrated
Urine
due to increased reabsorption of water back into the body.
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NERVOUS CONTROL of WATER BALANCE
Osmoreceptors in the
hypothalamus detect changes
in osmotic pressure (sweating
/ dehydration cause the blood
to be more concentrated which
increases osmotic pressure,
pulls water into the bloodstream
from the cells, the cells of the
hypothalamus shrink).
Message sent to the pituitary
gland to release ADH into the
blood.
At kidneys, ADH makes the
collecting ducts and distal
tubules more permeable to
water, increasing reabsorption
into the blood and urine more
concentrated.
Osmotic pressure lowers.
⇧ Osmotic Pressure
OSMORECEPTORS
(HYPOTHALAMUS)
NERVES
PITUITARY GLAND
ADH
COLLECTING DUCTS
(KIDNEYS)
⇧ Water Reabsorption
(-) Lowers
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The body also has the
sensation of thirst which
is a behavioural
response. Drinking fluids
will also help to decrease
osmotic pressure.
Once the osmotic
pressure decreases, the
cells of the hypothalamus
swell and the pituitary
gland stops the release
of ADH. (negative
feedback!)
Page 356 #1-5, 9, 10 (6)
Normal Osmotic Pressure
OSMORECEPTORS
(HYPOTHALAMUS)
NERVES
PITUITARY GLAND
ADH
COLLECTING DUCTS
(KIDNEYS)
⇧ Water Reabsorption
http://people.bu.edu/fgarcia/lectures/renal1/img007.jp
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REGULATING BLOOD PRESSURE
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Kidneys are also a part in the blood pressure feedback loop:
Blood pressure is too low (dehydration / blood loss)
Blood pressure receptors in the juxtaglomerular apparatus detect
low blood pressure.
Special cells in the apparatus release renin (enzyme used to
convert the blood plasma protein angiotensinogen to angiotensin).
Angiotensin causes blood vessels to constrict (decrease volume
increases pressure) and causes the adrenal glands in the cortex of
the kidneys to release aldosterone
Aldosterone causes the distal tubule and the collecting ducts to
increase Na+ and water reabsorption.
LOW Pressure
Blood Vessels Constrict
JGA
Angiotensinogen
Renin
Angiotensin
Adrenal Gland
Collecting Ducts
Increase Water Reabsorption
Aldosterone
⇧ Pressure
REGULATING BLOOD pH
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Kidneys are important in regulating blood pH:
Despite the variety of foods and fluids consumed with varying
pH levels, the pH of the body remains relatively constant,
between 7.3 and 7.5.
During cellular respiration our cells produce carbon dioxide
which diffuses into the blood and combines with water to form
carbonic acid. Carbonic acid then ionizes to release protons
which lowers the pH.
Excess protons are buffered by the bicarbonate ion in the
blood.
H2O + CO2
H2CO3
Carbonic acid
HCO3- + H+
Bicarbonate ion
H2O + CO2
H2CO3
HCO3- + H+
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Due to this buffering system, the process is reversed, the
bicarbonate ion combines with protons to form carbonic acid,
which breaks down into water and carbon dioxide.
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CO2 will be eliminated by the lungs. Excess protons will be
eliminated by the kidneys. The buffering system helps maintain
the pH of the blood in the meantime.
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Thus the kidneys help maintain the buffer by:
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ensuring that the bicarbonate ions are returned to the
bloodstream (always reabsorbed) and actively
reabsorbing CO2, which reverses the buffering system
so more HCO3- is produced.
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Using any excess protons in the creation of waste
products that are secreted and subsequently urinated
out.
Homework: p 356 #7,8