Osmosis and Body Fluids

Study Notes

Osmosis is the diffusion of water molecules across a semipermeable membrane into a region with a high concentration of solutes, regulated by cells through the addition or removal of aquaporins.
Aquaporins are channel proteins in the plasma membrane that facilitate osmosis.

Osmotic pressure is an attractive force between solute particles and water molecules that drives water movement from a solution with low solute concentration to one with high solute concentration.
The magnitude of osmotic pressure depends on the number of solute particles in a solution.
Oncotic pressure, or colloid osmotic pressure, is a type of osmotic pressure exerted by large molecules like proteins in blood vessels' plasma or interstitial fluid.

Osmolarity is the concentration of osmotically active particles in one liter of a solution (mol/L H2O).
Osmolality is the concentration of osmotically active particles in one kilogram of a solution (mol/Kg H2O).
Osmotically active particles are molecules or particles that attract water to them.
The number of osmotically active particles in a fluid is determined by the number of moles per liter (or kg) of water and the number of particles released into solution when the solute is dissolved.

The osmolality of plasma and extracellular fluid (ECF) is mainly due to sodium ions, chloride ions, bicarbonate ions, and other ions, urea, glucose, and proteins.
The osmolality of intracellular fluid (ICF) is mainly due to potassium ions, magnesium ions, organic phosphates, proteins, and other nitrogen-containing solutes.
Despite differences in composition, the body fluids have essentially identical total osmolalities of about 290-300 mOsmol/kg H2O.
The osmolality of other body fluids can vary significantly, such as peritubular interstitial fluid of the renal medulla and certain transcellular fluids like urine.