Lecture 35- Wednesday, April 7 , 2010th
Osmolarity: total solute concentration of a solution; measure of water concentration
in that the higher the solution osmolarity, the lower the water concentration
Hypoosmotic: having total solute concentration less than that of normal extracellular
fluid (300 mOsm)
Isoosmotic: having total solute concentration equal to that of normal extracellular
Hyperosmotic: having total solute concentration greater than that of normal
Renal Regulation of Water Balance
• Water is freely filtered but ~99% is reabsorbed
• The majority of water reabsorption (~2/3) occurs in the proximal tubule.
• But the major hormonal control of reabsorption occurs in the CD.
Water Reabsorption Depends on Na Reabsorption (promimal tubule)
1. Na is reabsorbed from the tubular lumen to the interstitial fluid across the
2. The local osmolarity in the lumen decreases, while the local osmolarity in the
3. This difference in osmolarity causes net diffusion of water from the lumen into the
via tubular cells’ plasma membranes
via tight junctions
4. From the interstitium, water, sodium, and everything else dissolved in the
interstitial fluid move together by bulk flow into peritubular capillaries. Maintenance of Water Balance
The body has to maintain water balance.
• When the water intake is small, the kidney reabsorbs more water (e.g. urine
output 0.4 L per day).
• When the water intake is large, the kidney reabsorbs less water (e.g. urine output
25 L per day).
This dynamic regulation takes place in CD and there are two critical components:
1. High osmolarity of the medullary interstitium.
2. Permeability of CD to water (regulated by vasopressin)
Urine Concentration: Countercurrent Multiplier System
• The kidney has the ability to concentrate urine up to 1400 mOsm/L.
• Urinary concentration takes place as tubular fluid flows through the medullary
• Urinary concentration depends on the hyperosmolarity of the interstitial fluid. In
the presence of vasopressin, water diffuses out of the ducts in