Serum osmolarity is the effective solute concentration of blood serum and is measured in osmoles. Blood serum is considered to encompass everything in blood except red blood cells, or erythrocytes. Serum osmolarity is an important diagnostic tool in medicine because osmotic pressure can affect all cells in the body, due to the power of osmosis and cell wall permeability to water but not to many solutes.

Osmolarity is the measurement of how many solute molecules are contained within a certain volume of liquid. The unit of osmolarity measurement is called the "osmole" and indicates the molecular molarity of a solute dissolved in solution. One reason osmolarity is an important measure when working in biological systems is the fact that water moves through osmosis, being attracted through diffusion and concentration gradients to areas with a higher ionic or solute density, rather than areas with lower solute densities. Osmosis can force water into cells or draw it out of them, depending on the relative concentration of ions and solutes inside cells to the concentration of solutes in extracellular fluid like blood serum.

Cell walls function as, among other things, semi-permeable membranes. While there are some channels in cells called "aquaporins" that allow water to pass freely in and out of cells, ion concentrations inside cells are regulated with channels, gates and ion pumps. Sodium, calcium and potassium are common ions found inside cells that are heavily regulated in concentration.

If serum osmolarity is too high, that indicates that the solute concentration of blood serum will tend to leach water from inside cells due to the effects of osmosis. Water will be attracted to the extracellular space, where solute concentration is higher due to osmosis. This can cause cell shrinkage and, in extreme cases, necrotize some cells.

Too low serum osmolarity indicates a lack of sufficient ions or solutes in blood serum, which creates a problem of hypotonicity. When this happens, cells draw too much water in due to relative solute concentration gradients across the cell membrane. In extreme cases, this can cause cell death through apoptosis.

Serum osmolarity can be used to diagnose dietary deficiencies or dangerously high intakes of certain foods or minerals. Too few ions or solutes in blood serum can be dangerous, which is why it's important to drink electrolytic fluids during periods of heavy water loss through sweating. Serum osmolarity measures can also be used as indicators for other health anomalies, like an inability or deficiency in the ability to process sugar.