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Local osmosis and isotonic transport
1Department of Physiology and Biophysics, SUNY at Stony Brook, NY 11794-8661, USA. Richard.mathias@sunysb.edu
The Journal of Membrane Biology
|April 6, 2006
Summary
Osmotically driven water flow can approach isotonic conditions when membrane permeability ratios are low. External boundary conditions significantly influence fluid transport, with optimal conditions found in the renal proximal tubule.
Area of Science:
- Physiology
- Biophysics
- Membrane Transport
Background:
- Osmotically driven water flow is crucial for biological fluid transport.
- Theoretical maximum isotonic water flow is dependent on salt transport rate and solution osmolarity.
- Experimental studies often show water transport indistinguishable from isotonic.
Purpose of the Study:
- To investigate the conditions under which osmotically driven water flow approaches isotonicity.
- To analyze the influence of membrane properties and external boundary conditions on fluid transport.
- To provide a hypothesis for isotonic fluid transport in epithelia like the renal proximal tubule.
Main Methods:
- Theoretical modeling of osmotically driven water flow across series membranes.
- Analysis of fluid transport under conditions with and without externally imposed boundary conditions.
- Evaluation of the role of membrane salt/water permeability ratio (epsilon) and lateral space dimensions.
Main Results:
- In the absence of external boundary conditions, transport is near isotonic, with secretion osmolarity approximately (1 + 2*epsilon)c(o).
- External boundary conditions imposing isotonicity on both sides can reduce fluid flow significantly if lateral spaces are wide.
- Optimal water flow occurs when apical and basolateral membrane water permeabilities are equal.
- In the renal proximal tubule, fluid reabsorption approaches optimal conditions, with a predicted length constant of ~0.3 cm for osmolarity changes.
Conclusions:
- Near-isotonic fluid transport is achievable under specific conditions, particularly when external boundary conditions are absent or when membrane properties are optimized.
- The model provides a plausible explanation for how epithelia, such as the renal proximal tubule, maintain isotonic fluid transport.
- Transmembrane osmotic gradients are adjusted by membrane transport to ensure water flow is isotonic within the order of epsilon.
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