Osmolarity is the measure of osmoles, or moles of a compound that contributes to a solution's osmotic pressure, per litre.
Osmolality is the number of osmoles per kilogram of solvent.
Salts are presumed to dissociate into their component ions; therefore, a mole of NaCl in solution produces two osmoles, as both Na and Cl affet the osmotic pressure.
Tonicity is the ability of a solution to cause water movement, in reference to hypertonic, hypotonic, and isotonic states. It is important to remeber that tonicity is determined by water concentration, not solute concentration. Water will move down its concentration gradient to equalize the solutions.
Normal plasma osmolarity is 280-303 mOsm/kg, mainly due to sodium, chloride, urea, glucose, and other ions in the blood. These solutes determine plasma osmolarity, but they do not control it, as changes in water content instead vary.
Plasma osmolarity can be estimated by multiplying [Na+] by 2.
Increased osmolarity induces ADH secretion and water retention, while a low osmolarity suppresses ADH release. Below 280 mmol/L, ADH secretion shuts down.
As cell membranes are largely freely permeable to water, plasma osmolarity tends to guide intracellular osmolarity. This is important in considering the effects of high or low plasma osmolarity on cells.
The intracellular compartment is 2/3 the total body volume.
Urine osmolarity varies with volume and plasma osmolarity, but can vary between 50-1400 mmol/L, depending on water intake. Often, urine ranges between 500-800 mmol/L.
Urine osmolarity is increased by ADH through water retention.
Urine osmolarity may be decreased
Symptoms