Urinary System 5 - Control of water balance Flashcards
What does your body need to remove in urine?
- Excess volume to prevent bloating
- Remove excess water to keep osmolarity up
- Remove excess salt to stop osmolarity getting too high
What is the normal plasma osmolarity?
285-295mosmol/l
How is water lost?
- Sweat (uncontrollable)
- Faeces (uncontrollable)
- Respiration (Uncontrollable)
- Urine output (controllable)
List the places where water is reabsorbed in the kidney
- Proximal tubule
- Descending loop of Henle
- Distal tubule
- Collecting duct
List the places where salt is reabsorbed in the kidney
- Proximal tubule
- Ascending limb of the loop of henle
- Distal tubule
List the fraction of filtered load reaching the different parts of the nephron
- 30% reaches descending loop of henle
- 20% reaches the distal convoluted tubule
- 1-10% enters the urine
How is water reabsorption varied in different animals?
- Length of the loop of henle
- Activity of the loop of henle
Describe how the gradient is established by the loop of henle with regards to sodium
- Salt is pumped out of the ascending limb to generate an osmotic gradient
- Water follows from the descending limb
- As more fluid enters the loop of henle, more salt is pumped out to get the same concentration gradient
- The osmolarity is more different at the bottom of the loop than at the top
Describe how urea increases the osmotic gradient
- Collecting duct and the bottom of the loop of Henle are permeable to urea
- Urea diffuses out of collecting duct and into the bottom of the loop of henle, then into the interstitial fluid increasing the osmolarity
List the different urea transporters and their locations
- UT-A1 is in the inner medullary collecting duct
- UT-A2 In the thin descending limb
- UT-A3 in the inner medullary collecting duct
- UT-B1 in the descending vasa recta
What happens if there is a fault in urea transporter A1 or 3?
- Reduced urea in the inner medulla
- Reduction in ability to concentrate urine
- Increased water uptake by 20%
- No ability to reduce urine output
What happens if there is a fault in urea transporter B?
- Increased urine production
- Reduced urine concentrating ability
- Weight loss
How does blood flow through the vasa recta create a concentration gradient?
- Permeable to water and solutes
- Water diffuses out of the descending limb and salts diffuse in
- In the ascending limb the reverse happens
- This allows oxgen and nutrients to be delivered
How is water reabsorption controlled in the collecting duct?
- Vasopressin secreted from the posterior pituitary gland
- Binds to V2 receptors on the basolateral membrane of principal cells in the collecting duct
List the actions of vasopressin in the kidney
- Increases insertion of aquaporins in the cell membranes
- Stimulates urea transport from the collecting duct to the ascending limb of loop of henle cy increasing UTA1 and UTA3 in the collecting duct
