Water and Solutes I

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Water and Solutes I

• Concepts
• Water and ion balance in aquatic organisms
• Why it is important
• Osmoregulatory cells
• Diadromous animals
• Nitrogenous waste
• Its forms
• Who makes what
• Why does it matter to water balance?
• The MIGHTY kidney
• How it works
• Its role in hydration state and blood pressure
• Comparative renal physiology

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Review - Omoregulation

• Both freshwater and saltwater animals face
  osmoregulatory challenges
• Water flux
• Ion flux
• Osmoregulatory cells (ion balance)
• Chloride cells in gills
• Freswater form
• Saltwater form
• Hormonally mediated
• Euryhaline invertebrates
• Osmoregulators
• Osmoconformers

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Nitrogen excretion

• Animals produce waste N as ammonia/ammonium
• Excrete that waste N in different ways
• Ammonotelic animals
• Excrete ammonia
• Generally aquatic
• NH3 diffuses out
• Ureotelic animals
• Synthesize urea
• Takes ATP, less toxic
• Urea concentrated in kidney
• Uricotelic animals
• Synthesize uric acid
• Take more ATP, non toxic
• Uric acid concentrated in kidney
• Can precipitate as a solid

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Excretory organs in animals

• Basic idea removing toxins from blood
• Two main ways this is achieved in animals
• Ultrafiltration
• Force blood contents into a tube and then recover
  what you want only.
• E.g. most vertebrate kidneys
• Active secretion
• Selectively remove what you dont want from blood
  via specific pumps
• E.g. insect malphigian tubules
• Some fish kidneys

Q which is more selective?

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Urea uric acid are synthesized in the liver
Ammonia
Urea Cycle
NH3
Urea

• Urea ( ornithine) cycle
• Set of reactions
• Makes waste NH3 into urea
• Occurs in liver
• Requires ATP!

Amino acid (aspartate)
Urea enters blood to be later excreted!
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The Kidney
KIDNEY WORKS LIKE A BLOOD FILTRATION SYSTEM
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What does a kidney need to do?

• Remove waste
• Urea
• Any other unrecognized compounds
• Recover back what you need
• Water
• Ions
• Nutrients
• Too much water loss
• Decreased blood volume
• Decreased blood pressure
• Too little water loss
• Increased blood volume
• Increased blood pressure

HORMONALLY REGULATED! Aldosterone Angiotensin
Vasopressin
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The nephron
2) PCT Nutrient recovery
4) DCT Adjusts ion (Na) uptake
300 mOsm
CORTEX
1) Bowmans capsule Plasma filtration
MEDULLA
3) Loop of Henle Water recovered Salts recovered
5) Collecting Duct Adjusts urine
Concentration (water recovery)
To ureter!
1200 mOsm
There are gt 1 million of these in each of your
kidneys!
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The basics of blood filtration

• Blood pressure squeezes plasma (and solutes) into
  nephron tubule
• (Glomerulus)

2) Nutrients are selectively returned to blood
(glucose, amino acids, bicarbonate) (PCT)
3) 90 of the water and ions are recovered too
(PCT, Loop of Henle, DCT, collecting duct )
4) Final adjustments to salt and water uptake
(DCT, Collecting duct) (hormonally controlled)
5) Pee out water and remaining compounds (urea)
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The sieve Glomerulus
High hydrostatic pressure Dense capillary net
within renal arterioles
Hi Permeability Capillaries are porous Creates a
molecular sieve
Pushes water and solutes (salt, glucose, etc)
into Bowmans capsule
Blood cells and big proteins stay behind
Creates sludgy blood High osmotic potential!
These filter about 180 liters/day 20 of your
blood flow 99 of the water is reabsorbed later!
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Nephron is closely associated with blood vessels
Post-filtration blood has high osmotic
potential Capillaries happily slurp up excess
water leaving the nephron
Glomerulus
Renal artery
Renal vein
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Measuring glomerular filtration rate (GFR)

• Inject inulin into an organism (inert, small
  molecular wt.)
• Inulin is not secreted or reabsorbed
• Therefore, total inulin filtered total inulin
  in urine
• Measure rate of urine production
• Collect a blood sample ( plasma inulin)
• Collect a urine sample ( urine inulin)

Total inulin in filtrate total inulin in urine
RfiltrateX filtrate Rurine X urine
Rfiltrate (Rurine X urine)/ filtrate
R ml/min
Volume and inulin of urine easy to measure
in filtrate in blood.
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2) Proximal convoluted tubule Active transport of
nutrients (amino acids, glucose), bicarbonate,
salt H2O 75 of blood H2O recovered here
Filtrate
Only molecules with specific transporters get
taken up Urea increases 4 fold
Lots of villi (Similar to small intestine)
High surface area For nutrient transport!
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Diabetes mellitus honey urine

• An excessively high blood glucose level makes
  urine sweet
• Term comes from the observation that bees were
  attracted to the urine of diabetics!
• Why might too high a BGL mean glucose gets
  excreted in urine?
• Q How is glucose recovered?
• A Glucose transporters in PCT

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Glucose transport
Glucose transporters are saturated!
maximum

Q How else could a high glucose load affect
kidney function?
concentration
Transport rate of glucose back to blood (ml/min)

glucose
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Loop of Henle dips into osmotic gradient
300 mOsm
Q Is filtrate more or less concentrated after
Loop of Henle?
3) Loop of Henle
NaCl
A Slightly less! But, a lot of fluid And salt
has been recovered
Water diffuses out (returns to capillaries) NaCl
actively pumped out From ascending limb
impermeable to water
1200 mOsm
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4) Distal Convoluted Tubule Hormone-regulated
sodium transport
300 mOsm
More transport more salt in blood less salt in
urine Controlled by adrenal steroid
hormone Aldosterone
Q How does salt in filtrate affect water
balance?
1200 mOsm
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Hormonal control of blood pressure and blood
osmolarity

• Specific cells in glomerulus are pressure
  sensitive
• Low pressure stimulates RAAS pathway
• renin-angiotensin-aldosterone system
• signals Aldosterone production
• Increases NaCl in blood
• Decreases NaCl in urine
• Angiotensin also acts on hypothalamus
• Increases thirst
• Produces Vasopressin (increases water resorption)
• Increased plasma osmolarity
• Sensed directly by adrenal gland
• Aldosterone production is inhibited

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300 mOsm
5) Collecting duct Adjusts water loss by
adjusting water permeability!
Q How?
Permeability HIGH urine is concentrated (1200
mOsm)
1200 mOsm
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300 mOsm
5) Collecting duct Adjusts water loss by
adjusting water permeability!
Q How?
Permeability LOW urine is dilute (300 mOsm)
1200 mOsm
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Water balance

• Concentration of urine is adjusted by Vasopressin
• Also called anti-diuretic hormone (ADH)
• Vasopressin stimulates the production of porins
  in collecting duct
• Presence of porins causes water to diffuse into
  medulla fluid be taken up by blood
• Absence of porins causes diuresis
• excessive urination!
• Vasopressin is inhibited by alcohol
• Hangovers are largely dehydration!

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Solutes in filtrateSalt and Sugar revisited
300 mOsm
Q What is the effect of low solute load in
collecting duct?
Low solutes in filtrate (urine) Big
gradient (urine vs surrounding fluid) More
water recovery
H20
Salt pumping in DCT (aldosterone) enhances water
recovery into blood by making urine less
concentrated!
1200 mOsm
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Solutes in filtrateSalt and Sugar revisited
300 mOsm
Q What is the effect of high solute load in
collecting duct?
Higher solutes in filtrate (urine) Smaller
gradient (urine vs surrounding fluid) Less
water recovery More dilute urine (EVEN WHEN
VASPORESSIN IS ON!)
Excess glucose in urine decreases water recovery
- High quanitity of urine (Diabetes used to be
called the pissing evil)
H20
1200 mOsm
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What maintains this gradient?
300 mOsm

• Solute gradient in kidney fluid is due to Na and
  urea
• Active sodium pumping in loop of Henle helps
  maintain gradient
• Collecting duct is fairly permeable to urea
• If urea in duct is higher than medulla, urea
  will leak out
• This also helps maintain gradient!

1200 mOsm
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Questions?
2) PCT Nutrient recovery
4) DCT Adjusts ion (Na) uptake
300 mOsm
CORTEX
1) Bowmans capsule Plasma filtration
MEDULLA
3) Loop of Henle Water recovered Salts recovered
5) Collecting Duct Adjusts urine
Concentration (water recovery)
To ureter!
1200 mOsm
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Kidneys and water conservation!
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Reptile type kidneys

• Have no loop of Henle!
• Not as good as concentrating urine (1-2X of
  blood)
• However, both birds and reptiles have a cloaca
• Urine is released into lower large intestine
• Water can be resorbed in cloaca
• Uric acid excreted as a paste
• Good at water conservation
• But not because of their kidneys
• Birds can have a mix of mammal-type and reptile
  type nephrons
• Depends on need for water conservation!

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MAMMALS
Excrete most waste N as urea in water
However, some mammals are very good at conserving
H2O - Extremely hyperosmotic urine
Concentration of urine depends on Solute
gradient in kidney
Desert kangaroo rat
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Concentration gradient in kidney determines urine
Determined by capability for active ion
transport in tubule
300 mOsm
A convenient index of this gradient is the ratio
of medulla to cortex width!
9000 mOsm!!!
Australian marsupial mouse (Notomys) Produces
urine that is 9000mOsm!
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Medullacortex ratio and urine
Note that the best urine concentrators are small
mammals!
These are also the animals with high rates of
evaporative water loss for their body size (high
MR/g)