Fluid Physiology

1.1 Properties of Water

Water is one of the two major solvents in the body. It is a remarkable substance with several important properties, in particular, it has:

Its concentration in biological systems is very high: 55.5 Molar at 37°C (see Box below). This is almost 400 times the concentration of the next most concentrated substance in the body (ie [Na+] in ECF = 0.14M, [K+] in ICF = 0.15M). The significance is that water provides an inexhaustible supply of hydrogen ions for the body.

Calculation of Water Concentration

Molecular weight of H2O = (1 + 1 + 16) = 18, so one mole is 18 grams

One ml of liquid H2O weighs about 1 gram (so 1 litre weighs 1,000 grams)

Therefore: [H2O] = 1000/18 = 55.5 moles/liter

The large dielectric constant means that substances whose molecules contain ionic bonds will tend to dissociate in water yielding solutions containing ions. This occurs because water as a solvent opposes the electrostatic attraction between positive and negative ions that would prevent ionic substances from dissolving. The ions of a salt are held together by ionic forces as defined by Coulomb’s Law.

Coulomb's Law

F = (k . q1 . q2) / D . r2


  • F is the force between the two electric charges q1 and q2 at a distance r apart
  • D is the dielectric constant of the solvent.

The large dielectric constant of water means that the force between the ions in a salt is very much reduced permitting the ions to separate. These separated ions become surrounded by the oppositely charged ends of the water dipoles and become hydrated. This ordering tends to be counteracted by the random thermal motions of the molecules. Water molecules are always associated with each other through as many as four hydrogen bonds and this ordering of the structure of water greatly resists the random thermal motions. Indeed it is this hydrogen bonding which is responsible for its large dielectric constant.

Water itself dissociates into ions but the dissociation constant is very small ( Kw = 4.3 x 10-16 mmol/l). The paradox here is that though this is incredibly small, it has an extremely large effect in biological systems. Why? Because the dissociation produces protons (ie H+). These are very reactive and have a biologic importance out of all proportion to their minute concentration. (Why? See Importance of Intracellular pH)

Physiological Significance of Water’s Unusual Properties



High molar concentration

Provides inexhaustible supply of H+

Large dielectric constant

Allows Ionic substances to dissolve producing charged species

Very small dissociation constant

Produces extremely small but biologically significant [H+]

Key Point: Water makes it happen!

Water is often treated as though it was just a bland and simple solvent that happens to holds the various solutes in the body. The truth is that it is a solvent with properties unusual enough to allow the situation to occur in the first place.