Osmosis and Osmoregulation: Understanding Water Transport and Osmotic Pressure, Schemes and Mind Maps of Biology

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Osmosis and Osmoregulation

Robert Alpern, M.D.Southwestern Medical School

Osmotic Pressure

-^ If Jv = 0, then^ ∆

P = RT

C^ ∆s^

van’t Hoff equation ∆Π^ = RT

∆C^ s^

Osmotic pressure

-^ ∆Π is not a pressure, but is an expression of adifference in water concentration across amembrane.

Osmolality

-^ ∆^ Π

= RT^

Σ^ ∆Cs

Osmolarity •

• solute particles/liter of water Osmolality •
  • solute particles/kg of waterOsmolality =

Σ^ aC^ ss

-^ Colligative property

Concept of Effective Osmoles• Effective osmoles pull water.• Ineffective osmoles are membrane permeant, and do notpull water• Reflection coefficient

• an index of the effectiveness(σ)(σ)(σ)(σ)^

of a solute in generating an osmotic driving force.^ ∆^ Π

= RT^ Σ

C σ∆s s

-^ Tonicity

• the concentration of effective solutes; the ability of a solution to pull water across a biologic membrane.• Example: Ethanol can accumulate in body fluids atsufficiently high concentrations to increase osmolality by1/3, but it does not cause water movement.

Components of Extracellular

Fluid Osmolality

-^ The composition of the extracellular fluid isassessed by measuring plasma or serumcomposition.•^ Plasma osmolality ~ 290 mOsm/l Na salts

2 x 140 mOsm/l Glucose

5 mOsm/l Urea^

5 mOsm/l

-^ Therefore, clinically, physicians frequently referto the plasma (or serum) Na concentration as anindex of extracellular fluid osmolality andtonicity.

Cells Defend Volume

-^ Extracellular hypertonicity increases expression ofgenes encoding proteins that increase intracellularosmoles. Membrane transportersEnzymes that synthesize intracellular solutes -^ Clinical correlate: Rapid changes in extracellularfluid tonicity alter cell volume and causeneurologic complications, while slow changes arewithout an efect on cell volume and without anapparent clinical effect.

Regulation of Extracellular Fluid

Osmolality

• Excess body water/low plasmaosmolality

→^ Increased renal water excretion• Low body water/high plasmaosmolality

→^ Increased thirst and low renal water excretion