What is osmolarity?

• A. A condition where the body loses more water than sodium, prompting conservation of water
• B. The rate at which the kidneys filter plasma
• C. A measure of how quickly solutes are excreted in urine
• D. A condition where the body loses sodium faster than water, prompting sodium conservation?

Answer: (A)

Osmolarity is a measure of how concentrated a solution is with osmotically active particles—how many solute particles are present per liter. This concentration drives water movement across cell membranes: higher osmolarity in the extracellular fluid pulls water out of cells, while lower osmolarity can draw water into cells. In the body, extracellular osmolarity is mainly determined by sodium-containing salts, with additional contributions from glucose and urea. Normal serum osmolarity is about 275–295 mOsm/L. It isn’t a condition, nor a rate of kidney filtration or a rate of solute excretion, and it isn’t defined simply by sodium loss.