Water balance Flashcards
What is meant by osmolarity
measure of the solute concentration in a solution (osmoles/liter; 1 Osmole = 1 mole of dissolved solutes per liter); depends on the number of dissolved solutes present. The greater the number of dissolved particles, the greater the osmolarity
What does the kidney need to do
Maintain plasma osmolarity under lots of different conditions
Some- high conc urine
Others- low conc urine
Describe the flow of water
Water flows across a semi permeable membrane from a region of low osmolarity to a region of high osmolarity
What is the consequence of a permeable system
Increase salt reabsorption- water will also move to balance it- thus increasing volume of the ECF
Describe the inter-relation between salt and water regulation
Increasing salt increases water reabsorption, which increases plasma volume and maintains osmolarity
§ Water balance is used to regulate plasma osmolarity.
§ Salt levels are used to determine the ECF volume.
What is the major salt
Na+- most prevalent solute in plasma and ECF
On average, what do we consume daily
On an average day we consume 20-25% more water and salt than we need to replace that lost.
o Must get rid of excess volume – or become hypertensive and oedematous.
o Must get rid of excess water – or cells will swell in the body (dilute salt in body).
Why is it important to get rid of the excesses
Must get rid of any excess water
To keep osmolarity up
Must get rid of any excess salt
To stop osmolarity going too high
If you don’t get rid of excess water you will dilute your body salt and the cells will expand, if you don’t get rid of excess salt the cells will shrink.
Describe the osmolarity that we should maintain the plasma at
o Plasma osmolarity = 285-295mosmol.L-1 (greatest proportion is ~140mmol.L-1 Na).
What are the most abundant components and solute of the plasma and ECF
Water: most abundant component of the plasma and ECF
Sodium: most prevalent solute in the plasma and ECF
What is the key difference in terms of the movement of water and salt
Water can move freely, but salt cannot- therefore we balance salt.
Describe the proportions of water found in different compartments of the body
In the body, 25L (65%) is found intracellularly and 15L (35%) is found extracellularly (e.g. plasma and interstitial).
What compensates for a low IC cl- conc
A high IC HPO42-
Describe some other relative concs of ions
HCO3- lower in cells than ECF or plasma
Ca2+ higher in cells
Describe how we get rid of water
· Sweat – 450ml.days-1 – UNCONTROLLABLE (variable: fever, climate, activity)
· Faeces – 100ml.days-1 – UNCONTROLLABLE (Diahhroea up to 20L/day with cholera)
· Respiration – 350ml.days-1 – activity- UNCONTROLLABLE.
· Urine - ~1500ml.days-1 – CONTROLLABLE (variable).
Where is water reabsorbed
All along the nephron except for the ascending limb
Summarise the reabsorption of water
§ Even though 180L.days-1 is filtered, only 1-2L.days-1 is produced as urine. § ~60-70% reabsorbed at PCT. § ~30% reabsorbed at LoH. § ~20% reabsorbed at DCT. § ~1-10% reabsorbed at collecting duct.
Regulation occurs at the collecting duct
What is maximum urine osmotic concentration proportional to
Relative medullary area
Longer descending limb= more concentrated urine- important in animals that need to reabsorb more water
Also depends on activity of the transport system too- not just the length.
How can you concentrate urine above normal plasma osmolarity?
Produce a region of ‘hyperosmolar’ interstitial fluid
What is essential to remember about the movement of water
We cannot pump it, therefore we must create a gradient for it to move to produce urine that is more concentrated than the plasma.
Outline how this gradient is established
Use schematic too!
Initially no gradient
Salt pumped out of ascending limb- decreasing osmolarity in the ascending limb (increasing it in I.F)
Descending limb detects this- water moves out to equilibrate- increasing osmolarity in the descending limb
More fluid comes in - osmolarities move around i.e high osmolarity of descending limb enters ascending limb
Same happens again
Only this time water at bottom of descending limb has a higher osmolarity and water leaving ascending limb has a lower osmolarity
Ascending limb capable of generating a 200 mosmol/L difference between it and the I.F (i.e loses 100 to increase the I.F by 100
Can this movement generate the full 1200mosmol/L gradient
No
How do we get to this 1200mosmol/L gradient
§ Concentration of the urea in the tube becomes higher as it goes up the ascending limb of the LoH and back down in the collecting duct as more and more water is removed (while the membrane is IMPERMEABLE to urea).
§ This means when it gets to the inner medulla CD, the membrane becomes permeable and urea passes down it’s concentration gradient into the bottom of the descending limb
Urea then follows this path until max osmolarity of urine- 1200momol/L is reached.
Describe the roles of the different urea transporters
o UT-A1, UT-A3 – Inner medullary collecting duct.
o UT-A2 – Thin descending limb.
o UT-B1 – Descending vasa recta.
