Typically calculated as a proportion of dissolved particles per liter of a fluid, plasma osmolarity can account for concentrations of substances such as sodium, glucose, urea, or chloride in blood. Osmolality is a similar measurement except it is generally measured in kilograms. Solute concentration is usually measured by the number of units called osmoles of it in the plasma. Salts and various other ions are regularly passed through the body in levels that can increase or decrease rapidly. An increase in plasma osmolarity can be a sign of dehydration or disease, while a significant decrease often signifies other medical issues.
While the two are expressed in different volume sizes, plasma osmolarity can be mathematically calculated from osmolality using an equation. The calculation of the osmaolarity of a solution generally includes a number that accounts for the dissociation of the solution from the solute particles. Also included is the number of particles, the concentration of solute, and a value that represents the actual type of material dissolved in the solution. An instrument called an osmometer is used to measure the property and other characteristics of a fluid.
Plasma osmolarity generally influences the passage of water in and out of cell membranes. The semi-permeable membrane of a cell is typically regulated by the osmolarity of the fluid outside them being equal to that in between. When osmolarity increases, Antidiuretic Hormone (ADH), normally secreted by the hypothalamus in the brain, is usually secreted. It can trigger the body to re-absorb water, resulting in a lower concentration of blood plasma and higher concentration of urine. The hormone is sometimes secreted in larger quantities than normal, especially in people with some forms of diabetes.
Changes in ADH levels typically affect how the kidneys control the excretion of water, and changes in plasma osmolarity can be adapted to within 20 minutes. In addition to plasma osmolarity, the level of dissolved particles in urine and stool can be measured. Conditions such as dehydration, kidney disease, heart failure, and hyperglycemia are sometimes diagnosed by monitoring the solute concentration of these. Significant changes in osmolarity can interfere with cell function and internal volume, and cells may even die if the effect is great enough.
If plasma osmolarity rises as little as 2%, it can cause thirst. An additional measurement, called the osmotic gap, is performed by comparing the difference between a laboratory measurement and the actual calculation. When this increases, it can indicate the ingestion of various other compounds, such as methanol.