Plasma osmolality - Wikipedia
STI Tests · Screening Tests · Blood Tests · Liver Function Tests · Am I Pregnant ? Osmolality is an estimation of the osmolar concentration of plasma and is measure of the difference between the actual osmolality (measured by the A low serum osmolality will suppress the release of ADH, resulting in. Blood samples were taken directly after catheter placement and at 30 and 60 min of recovery. . Changes in plasma protein concentration were almost identical to changes in plasma solids. Renal Function After Dehydration and Rehydration. Relationship between recoveries in plasma volume (rPV) and total body. Looking for online definition of plasma protein in the Medical Dictionary? plasma A protein present in blood plasma, such as albumin or globulin. a therapy- response relationship determined only by experiment; the plasma level plasma osmolality; plasma protein; plasma protein fraction · plasma proteins · plasma.
Note that the size of the particle is unimportant so that a single ion, eg sodium, contributes as much to the serum osmolality as a single large protein molecule, eg albumin. The osmolality of physiological fluids tends to be dominated by small molecules which are present in high concentrations.
For example in serum, sodium, potassium, chloride, bicarbonate, urea and glucose are the only components present in high enough concentrations to individually affect the osmolality. Large serum components contribute little to the overall osmolality. For example the molar concentration of albumin, the most abundant serum protein, is only about 0. Only a few exogenous compounds such as ethanol, methanol, ethylene glycol and manitol can be present in the blood at high enough quantities to significantly affect the osmolality.
In response to even small increases in plasma osmolality usually rises in plasma sodiumADH release from the pituitary is increased causing water resorption in the distal tubules and collecting ducts of the kidney and correction of the increased osmolality. The opposite happens in response to a low plasma osmolality with decreased ADH secretion and water loss through the kidneys.
Note that ADH is also secreted in response to hypovolaemia and this stimulus will over-ride any response to serum osmolality.
Osmolality, Osmolarity and Fluid Homeostasis | Patient
The urine osmolality is the best measure of urine concentration with high values indicating maximally concentrated urine and low values very dilute urine. The main factor determining urine concentration is the amount of water which is resorbed in the distal tubules and collecting ducts in response to ADH.
In a dehydrated patient with normally functioning pituitary and kidneys, a small volume of highly concentrated urine will be produced.
In a patient with fluid overload the opposite will be an appropriate response. Note that there is no reference interval "normal range" for urine osmolality as the interpretation depends on the clinical condition of the patient to determine an appropriate response. For this reason, both terms are often used interchangeably, even though they refer to different units of measurement.
This is what is used when values are measured by a laboratory. Osmolality is measured by clinical laboratories using an osmometer - either a freezing point depression osmometer or a vapour pressure depression osmometer. This is what is used when a calculated value is derived. The osmolarity is unreliable in various conditions - eg, pseudohyponatraemia such as hyperlipidaemia in nephrotic syndrome, or hyperproteinaemia.
The following equations can be used to calculate osmolarity: The doubling of sodium accounts for the negative ions associated with sodium and the exclusion of potassium approximately allows for the incomplete dissociation of sodium chloride. The term osmolarity has largely been superseded by osmolality, even when discussing calculated values.
Osmolality is used for the rest of this article. Osmotic gap The osmotic gap also called osmolal gap is an arbitrary measure of the difference between the actual osmolality measured by the laboratory and the calculated osmolality.
Where the osmotic gap is increased, it indicates the presence of other osmotically active solutes which are not taken into account in the calculated osmolality - eg, in methanol or ethylene glycol ingestion.
Clinical relevance of osmolality As cell membranes in general are freely permeable to water, the osmolality of the extracellular fluid ECF is approximately equal to that of the intracellular fluid ICF.
Therefore, plasma osmolality is a guide to intracellular osmolality. This is important, as it shows that changes in ECF osmolality have a great affect on ICF osmolality - changes that can cause problems with normal cell functioning and volume may even induce cytolysis.
In normal people, increased osmolality in the blood will stimulate secretion of antidiuretic hormone ADH. This will result in increased water reabsorption, more concentrated urine and less concentrated blood plasma. Diabetes insipidus is a condition caused by hyposecretion of, or insensitivity to, the effects of ADH.