Water balance Flashcards
What is osmolarity?
A measure of the solute concentration in a solution that depends upon the number of dissolved solutes present
What is normal plasma osmolarity?
285-295 mosmol/L
Each day we assume excess water and salt (20-25%) so what must be done?
- remove excess volume (or hypertension)
- remove excess water (or cells will swell)
- remove excess salt (or cells shrink)
How can we get rid of excess water/volume?
- sweat (450ml/day)
- faeces (100ml/day)
- respiration (350ml/day)
- urine (1500ml/day, variable)
What are water levels and salt levels used to determine?
- Water balance is used to regulate plasma osmolarity
- Salt levels are used to determine the ECF volume
How much fluid is intracellular an how much extracellular?
intracellular - 25L, 65%
extracellular - 15L, 35%
How much blood is filtered daily by the kidneys and how much is produced as urine?
180L (only 1-2L is urine)
What % of water reaches the loop of Henle and DCT and out as urine?
LoH - 30% (60-70% of everything is reasborbed at PCT)
DCT - 20% (10% is absorbed at LoH)
out of CD as urine - from less than 1% to 10%
Is the LoH longer or shorter in animals that need to conserve more water?
longer
Urea in the establishment of the countercurrent gradient
- Water comes out of the tubule in the descending limb because of the higher interstitial osmolarity
- Urea doesn’t follow it, so the concentration of urea increases in that region
- When it gets to the urea permeable region at the collecting duct, the high concentration urea comes out into the interstitial space, where the concentration then becomes high, so it moves into the permeable region at the bottom of the LoH
- The concentration of urea increases at the bottom of the loop because water has been taken out of the tubular system
- This is the second component in creating the gradient
Give examples of urea transporters
UT-A1, UT-A3 – Inner medullary collecting duct
UT-A2 – Thin descending limb
UT-B1 – Descending vasa recta
What happens if UT A1/A3 is inhibited?
- severe reduced urea in inner medulla
- reduction in ability to concentrate urine
- increased water intake
- no ability to produce urine if water restricted for 24hrs
What happens if UTA2 is inhibited?
mils phenotype only observable on low protein diet
What happens if UTB is inhibited?
- Increased urine production
- reduced concentrating ability
- weight loss
Give examples of mutations in urea transporters in humans and the effects
- In humans no UTA1/3 mutations
- Those with point mutations in U2 A2 have reduced blood pressure
- Those with loss of function mutation in UT B have reduced urine concentration ability
Why doesn’t the medullary blood flow eliminate the countercurrent flow?
- The vasa recta are fully permeable to both water and solutes.
- As the vasa recta descends, the blood loses water and solutes enter so the concentration of solute rises
- As the vasa recta ascends, the opposite occurs so the gradient isn’t removed
- Blood flow in the vasa recta is countercurrent too
ADH - peptide length, where is it synthesised and secreted and where does it bind?
- Vasopressin is a peptide hormone (9aa)
- Synthesised in the hypothalamus and secreted in the neurohypophysis
- Binds to receptors on the basolateral membranes of principal cells in the collecting ducts
What are the effects of ADH?
- Insertions of Aquaporin2 (AQA2) molecules into the luminal membrane
- Stimulates urea transport from IMCD into thin ascending LoH and interstitial tissue by increasing localisation of UT-A1 and UT-A3 in the CCD
What are some trigger for ADH release?
- Osmolarity rise above 300mosmol/L (osmolarity)
- Marked fall in blood volume/pressure (baroreceptor)
- Ethanol inhibits ADH release = dehydration
What is the permeability of the CD, ascending limb and descending limb to water?
CD - ADH dependent
ASCENDING LIMB - impermaeble to water
DESCENDING LIMB - permeable to water
What happens when plasma osmolarity goes down?
Water Diuresis
- Low ADH, so minimal water reabsorption so the urine remains diluted and so has a low osmolarity
- This results in a more hypotonic urine leaving the LoH and a more hypotonic urine entering the LoH
- Decreased plasma osmolarity -> Less ADH release (osmoreceptors) -> reduction in collecting duct permeability to water -> increased urine flow rate
What happens when you are dehydrated?
Maximal Anti-Diuresis as plasma osmolarity increases
- At high ADH, there is maximal water reabsorption so the urine becomes concentrated and so has a high osmolarity
- This results in a less hypotonic urine leaving the LoH and a much more hypertonic urine entering the LoH
- Increased plasma osmolarity -> increased thirst & increased ADH release (osmoreceptors) -> more ADH release -> increased collecting duct water permeability -> decreased urine flow rate
Disorder of water balance
Feedback control via ADH keeps plasma osmolarity in normal range
No/insufficient ADH – Central DI
No detection of ADH (mutant ADH receptor) – Nephrogenic DI
- No response to ADH signal (mutant aquaporin)
Leads to always producing dilute urine, excrete lots of water a date, very thirsty and gives polyuria. DIABETES INSIPIDUS
What is the main extracellular solute and major component of the body?
Water is the major component of the body and
sodium is the major extracellular solute
What is one osmole?
1 mole of dissolved solutes per liter
What is the main determinant of ECF volume?
number of osmoles present - majority is sodium hence sodium is a main determinant of ECF volume
What is normal plasma osmolarity and sodium osmolarity?
285 to 295mosmol/L
Na+ = 140mosmol/L
Water and sodium in the countercurrent multiplier
- 200mMol/l of sodium is pumped out of the ascending limb
- Because the thin descending limb is permeable to water, water follows into the interstitial space by osmosis to try to balance the hyperosmolarity created by the ascending limb
- filtrate down the descending limb has more solute so at the medullary end the ascending limb can pump more ions out
- gradient of Na+ from cortex to medulla
- This whole sequence is repeated again and again. Pumping salt like this can generate a maximum interstitial osmolarity of about 600mMol/L of the total 1200mosmol/L osmolarity of the interstitial fluid
