Body water compartments - Water / ion movement


see The consequence of not maintaining serum electrolyte levels

Important concepts that maintain ICF/ECF difference

A disturbance of serum osmolality results in significant alterations in cell volume (and hence function)

Body water values

Water movement

NB water movement across capillary membranes between vascular and interstitial compartments occurs faster than between interstitial (extracellular) and intracellular compartments due to gaps between capillary endothelial cells

The practical outcome is that every 3L of crystalloid fluid injected into the vascular space, only 1L will remain after equilibrium is reached.

Fick's law of diffusion (across cell membranes)

Flux proportional to:

inversely proportional to:

Starling's forces (of filtration) - DO NOT confuse Starling's equation with Starling's law

Ion movement

Depends on

Types of ion transport

Ions will generally follow a chemical or electrical gradient
Ions require energy to go against the gradient
Passive (does not require energy)
Active (requires energy)

Clinical note - the 'chanellopathies'

Genetic abnormalities in ion channel structure can lead to a number of diseases that result in altered ionic fluxes and affect the function in renal tubules, excitable tissues (e.g. skeletal muscle, cardiac muscle and nerve) and other tissues


Osmolarity (qv osmolality)

NB Serum osmolarity determines if cells swell or shrink due to water movement)