Plasma osmolarity and osmolality measure the concentration of dissolved particles in a fluid, affecting water passage in and out of cell membranes. Changes in osmolarity can indicate dehydration, illness, or other medical problems. Antidiuretic hormone regulates water reabsorption, and significant changes in osmolarity can interfere with cell function. An increase in plasma osmolarity can cause thirst and an increase in osmotic gap can indicate the ingestion of other compounds.
Typically calculated as the proportion of dissolved particles per liter of a fluid, plasma osmolarity can explain the concentrations of substances such as sodium, glucose, urea or chloride in the blood. Osmolality is a similar measure, except it is usually measured in kilograms. The concentration of the solute is usually measured by the number of units called osmoles of it in the plasma. Salts and various other ions pass through the body on a regular basis in levels that can rise or fall rapidly. An increase in plasma osmolarity can be a sign of dehydration or illness, while a significant decrease often indicates other medical problems.
While the two are expressed in different volume sizes, plasma osmolarity can be mathematically calculated from osmolality using an equation. Calculating the osmaolarity of a solution usually includes a number that takes into account the dissociation of the solution from solute particles. Also included is the number of particles, solute concentration, and a value that represents the actual type of material dissolved in the solution. An instrument called an osmometer is used to measure the properties and other characteristics of a fluid.
Plasma osmolarity generally affects the passage of water into and out of cell membranes. The semipermeable membrane of a cell is typically regulated by the osmolarity of the fluid outside it equal to that in between. When osmolarity increases, antidiuretic hormone (ADH), normally secreted by the hypothalamus in the brain, is usually secreted. It can stimulate the body to reabsorb water, resulting in a lower concentration of blood plasma and a higher concentration of urine. The hormone is sometimes secreted in larger amounts than normal, especially in people with some forms of diabetes.
Changes in ADH levels typically affect how the kidneys control water excretion, and changes in plasma osmolarity can be accommodated within 20 minutes. In addition to plasma osmolarity, the level of dissolved particles in urine and faeces can be measured. Conditions such as dehydration, kidney disease, heart failure, and hyperglycemia are sometimes diagnosed by monitoring the concentration of these solutes. Significant changes in osmolarity can interfere with cell function and internal volume, and cells can even die if the effect is large enough.
If plasma osmolarity increases up to 2%, it can cause thirst. An additional measurement, called the osmotic gap, is done 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.
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