Renal Water Balance Flashcards
What is most important in regulating plasma tonicity?
Water
How much water is absorbed by the proximal convoluted tubule?
65%
How much water is absorbed by the descending loop of henle?
10%
How much water is absorbed by the collecting duct?
5-25%
How is water reabsorbed by the nephron?
Osmosis through aquaporins
Which aquaporin is regulated by ADH?
AQP2 of principal cells in the collecting duct
How is water reabsorbed in the proximal tubule?
AQP1 (80%) and paracellular route (20%)
How is water reabsorbed in the descending thin segment of the loop of henle?
AQP1
What three things regulate water reabsorption from tubular lumen?
- Osmotic gradient
- Functional water channels
- Anti-diuretic hormone
What are three features of the medullary osmotic gradient?
- Loop of henle
- Vasa recta
- Counter current mechanism
What is a counter current system?
A system in which the inflow runs parallel to, counter to, and in close proximity to the out-flow for some reason
Why do we need a counter current mechanism within the medulla?
To generate the medullary osmotic gradient (which will favour the reabsorption of water from the collecting duct)
What structure releases antidiuretic hormone?
Posterior Pituitary
When is ADH released?
- When osmoreceptors sense a higher than normal osmolality
2. When baroreceptors (in carotid bodies and aortic arch) sense a lower than normal blood pressure/volume
Is osmoregulation or volume regulation more sensitive to changes?
Osmoregulation (as little as 1% change, coupled to thirst)
What are four renal effects of ADH?
- Increase water permeability of collecting duct via V2 receptor to input more AQP2
- Increase urea permeability of collecting duct
- Increase Na absorption and K secretion
- Increase renal prostoglandin production (vasodilation)
What are three extra renal effects of ADH?
- Systemic vasoconstriction via V1 receptor
- Increase release of von Willebrand factor
- Stimulate ACTH release via V3 receptor
What happens when plasma osmolality increases?
Increase thirst and increase ADH release to cause water retention
When osmolality and volume do not correspond, what takes precedence?
Volume
(activation of RAAS and ADH despite low plasma osmolality and hyponatremia –> ongoing water retention despite low plasma osmolality)
Describe SIADH
Syndrome of Inappropriate ADH
Chronic activation of ADH leading to excessive water retention (but NOT sodium); osmolality changes not volume changes
Describe Diabetes Insipidus
Disease in which ADH is not effective; polyuria, polydipsia, dehydration, hypernatremia
What is the difference between central and nephrogenic diabetes insipidus?
Central - problem with ADH secretion, responsive to exogenous ADH
Nephrogenic - problem with V2 receptor on AQP2, unresponsive to exogenous ADH
Impermeability to water is essential to the medullary interstitial gradient in which segment of the nephron?
Thick ascending limb of the loop of henle
What molecules are important in generating an interstitial concentration gradient within the medulla?
Urea, sodium, chloride
