Osmolality and its Measurement

Vocabulary flashcards covering the concepts of moles, equivalents, osmolarity, osmolality, and their physiological significance as detailed in the lesson on solute concentration measurement.

Mole

The gram-molecular weight of a substance, representing the molecular weight in grams; each mole consists of $6 \times 10^{23}$ molecules.

Millimole (mmol)

$1/1000$ of a mole.

Micromole (umol)

$1/1,000,000$ of a mole.

Equivalent (Eq)

$1\,mol$ of an ionized substance divided by its valence; it represents the concept of electrical equivalence in physiology.

Milliequivalent (mEq)

$1/1000$ of $1\,Eq$.

Osmolarity

The number of osmoles of solute per one liter of solution, used to describe the concentrations of osmotically active particles.

Osmotically active particle

Each liberated ion from a solute that dissociates in an ideal solution, such as $Na^+$ and $Cl^-$ from $NaCl$, or a non-ionizable organic substance like glucose.

Semi-permeable membrane

A membrane that is permeable to the solvent but not to the solute, such as cell membranes and capillaries.

Osmosis

The migration of solvent resulting from the difference in osmolarity between two compartments separated by a semi-permeable membrane.

Osmotic pressure

The pressure necessary to prevent solvent migration or to prevent osmosis.

Osmolality

The number of osmoles per one kilogram of solvent; it is more accurate than osmolarity because mass is constant while volume is affected by temperature and pressure.

Plasma osmolality

The osmolality of plasma, which is between $280-300\,mosm/kg$, largely determined by sodium ($Na^+$) and its anions.

ECF osmolality determinant

Sodium ($Na^+$) and its associated anions, which are responsible for most of the osmolality of the Extracellular Fluid.

ICF osmolality determinant

Potassium ($K^+$) and its associated anions, which are responsible for most of the osmolality of the Intracellular Fluid.