5. Control of Water Balance Flashcards
What are the units for osmolarity?
osmoles/litre (mosmol/l)
What happens to a hyperosmotic cell in solution?
- Water moves in
* Cell swells
What happens to a hypoosmotic cell in solution?
- Water moves out
* Cell shrinks
What does it mean by the body operating in a ‘constant osmolarity environment’?
- If you increase salt concentration, osmolarity increases
* However, water follows the salt so osmolarity returns to the original level
What are the 3 reasons for urine production?
- Removal of excess volume (to prevent oedema + hypertension)
- Removal of excess water (prevent ECF from becoming hypoosmotic - cell swelling)
- Removal of excess salt (prevent ECF from becoming hyperosmotic - cell shrinkage)
What is the most prevalent solute in plasma/ECF?
Sodium
How do we get rid of water?
- Urine output - 1500ml/day (controllable)
- Sweat - 450ml/day (uncontrollable, depends on fever, climate and activity)
- Respiration - 350ml/day (uncontrollable, varies with physical activity)
- Faeces - 100ml/day (uncontrollable, depends on solidity)
Where does water removal occur in the tubular system?
All regions apart from the ascending limb
How much water passes through the glomerular fenestrae?
125ml/min
Where is the most water reabsorbed and why?
- PCT
* Most solute pumping occurs here, water follows
How does urine maintain its concentration (above normal plasma osmolarity)?
Osmolarity increases from 290-1200mosmol/l from the cortex to the inner medulla
What is the difference in osmolarity between the ascending limb and interstitial fluid, and how does this affect the osmolarity of fluid at the end of the Loop?
- 200mosmol/l => 400mosmol/l
- Created by active pumping of salts into the interstitial fluid
- When water moves out from the descending limb, more salt leaves at the bottom of the loop, generating a further difference of 200mosmol/l
- Therefore, the tubular fluid that leaves the Loop of Henle to the DCT is hypoosmolar (as a lot of salts have been lost)
How does urea play a part in generating a concentration gradient in the nephron?
• The bottom of the Loop is permeable to urea
- urea enters the tubular fluid making it hyperosmolar (UT-A2)
- urea enters the descending vasa recta in this area too (UT-B1)
• The collecting duct is permeable to urea, which is removed (UT-A1, UT-A3)
What urea transporter problems have been observed in humans?
- Point mutations in UT-A2 - reduced blood pressure
* Loss of function mutations in UT-B - reduction in urine concentrating ability
Describe the renal medullary blood flow
- All tubular cells require oxygen and nutrients
- Ordinary blood supply - loss of hyperosmotic IF gradient, as excess salt would move into capillaries
- Vasa recta is permeable to water and solutes
- In the descending limb, water diffuses out/solutes diffuse into the limb from the vasa recta
- Reverse happens in the ascending limb
