Concentration of body fluids is expressed in three


1. Osmolality

2. Osmolarity

3. Tonicity.


Measure of a fluid’s capability to create osmotic pressure is called osmolality or osmotic (osmolar) concentration of a solution. In simple words, it is the concentration of osmotically active substance in the solution. Osmolality is expressed as the number of particles (osmoles) per kilogram of solution (osmoles/kg H2O).


Osmolarity is another term to express the osmotic concentration. It is the number of particles (osmoles) per liter of solution (osmoles/L). Osmotic pressure in solutions depends upon osmolality. However, in practice, the osmolarity and not osmolality is considered to determine the osmotic pressure because of the following reasons:

i. Measurement of weight (kilogram) of water in solution is a difficult process

ii. Difference between osmolality and osmolarity is very much negligible and it is less than 1%.

Often, these two terms are used interchangeably. Change in osmolality of ECF affects the volume of

both ECF and ICF. When osmolality of ECF increases, water moves from ICF to ECF. When the osmolality decreases in ECF, water moves from ECF to ICF. Water  movement continues until the osmolality of these two fluid compartments becomes equal.

Mole and Osmole

A mole (mol) is the molecular weight of a substance in gram. Millimole (mMol) is 1/1000 of a mole. One osmole (Osm) is the expression of amount of osmotically active particles. It is the molecular weight of a substance in grams divided by number of freely moving particles liberated in solution of each molecule. One milliosmole (mOsm) is 1/1000 of an osmole.


Usually, movement of water between the fluid compartments is not influenced by small molecules like

urea and alcohol, which cross the cell membrane very rapidly. These small molecules are called ineffective osmoles. On the contrary, the larger molecules like sodium and glucose, which cross the cell membrane slowly, can influence the movement of water. Therefore, such molecules are called effective osmoles. Osmolality that causes the movement of water from one compartment to another is called effective osmolality and the effective osmoles are responsible for this. Tonicity is the measure of effective osmolality. In terms of tonicity, the solutions are classified into three


i. Isotonic fluid

ii. Hypertonic fluid

iii. Hypotonic fluid.

i. Isotonic Fluid

Fluid which has the same effective osmolality (tonicity) as body fluids is called isotonic fluid. Examples are 0.9% sodium chloride solution (normal saline) and 5% glucose solution. Red blood cells or other cells placed in isotonic fluid (normal saline) neither gain nor lose water by osmosis. This is because of the osmotic equilibrium between inside and outside the cell across the cell membrane.

ii. Hypertonic Fluid

Fluid which has greater effective osmolality than the body fluids is called hypertonic fluid. Example is 2%

sodium chloride solution. When red blood cells or other cells are placed in hypertonic fluid, water moves out of the cells (exosmosis) resulting in shrinkage of the cells (crenation).

iii. Hypotonic Fluid

Fluid which has less effective osmolality than the body fluids is called hypotonic fluid. Example is 0.3% sodium  chloride solution. When red blood cells or other cells are placed in hypotonic fluid, water moves into the cells (endosmosis) and causes swelling of the cells. Now the red blood cells become globular (sphereocytic) and get ruptured (hemolysis).


Body has several mechanisms which work together to maintain the water balance. The important mechanisms involve hypothalamus and kidneys.

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