LO2: Osmolarity

What is osmolarity?

Osmolarity is the total concentration of all solute particles (ions and molecules) in a solution, measured in osmoles per litre (Osm/L).

What types of solutes contribute to osmolarity?

Both electrolytes (e.g., Na⁺, K⁺, Cl⁻) and non-electrolytes (e.g., glucose, urea), but electrolytes contribute more due to their ability to dissociate into multiple particles.

Why do electrolytes have a greater impact on osmolarity than non-electrolytes?

Because they split into charged ions in water, increasing the number of solute particles.

Is osmolarity the same in the ECF and ICF?

Yes, under normal conditions, osmolarity is equal between the extracellular and intracellular fluid compartments.

What is osmosis?

Osmosis is the passive movement of water across a semi-permeable membrane from low to high solute concentration.

What is the main mechanism for maintaining osmolarity?

The movement of water across cell membranes via osmosis and the regulation of electrolytes by organs like the kidneys.

What organ is primarily responsible for maintaining osmolarity?

The kidneys, which regulate water and solute (especially sodium) balance

What hormone helps regulate osmolarity by controlling water retention?

Antidiuretic hormone (ADH) – it increases water reabsorption in the kidneys to dilute high osmolarity.

What happens when the body is dehydrated?

Osmolarity increases, triggering ADH release, which conserves water by reducing urine output and reabsorbing more water.