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Normal : 280-300 mOsm/L.
Fluctuation of even 1% (3mOsml/L) will be detected by Osmoreceptors in Anterior Hypothalamus (Organum Vasculosum of Lamina Terminalis don't have blood brain barrier) & they will try to return it towards the normal point through various hormonal mechanisms like ADH, Aldosterone.
The osmolality test measures the body’s ability to make urine, control urine and control water or fluid balance in blood. The most common cause for ordering the osmolality test is low blood sodium levels.
Serum osmolality can be calculated by measuring the amounts of sodium, glucose, and blood urea nitrogen (BUN) in the blood by using following formula :
Serum Osmolarity = 2[Na+] + [Glucose]/18 + [ BUN ]/2.8 where [Glucose] and [BUN] are measured in mg/dL.
The osmolality of urine may be measured and compared to blood osmolality. This can help determine how well the kidneys are working.
High serum osmolality levels may be caused by:
Results from a serum osmolality test may be affected by:
Serum Osmolarity decreases in Pregnancy & that is Perfectly Normal. It will be Between 275-280 mOsm/Kg. Infusion of hypertonic fluids is usually not Required.
The osmolar gap is the difference between the measured osmolality and the calculated osmolarity.Clinically the osmolar gap is used to detect the presence of an osmotically active particle that is not normally found in plasma, usually a toxic alcohol such as ethanol, methanol or isopropyl alcohol.
Types of Fluids:
Hypotonic solutions have an osmolality of less than 240 mOsm/liter. They exert less osmotic pressure than the fluid in the extracellular compartment which allows water to be drawn from the extracellular fluid. Blood cells will draw these solutions into them causing the blood cells to swell and burst. There is only one hypotonic solution in common use and that is 0.45% sodium chloride (1/2 Normal Saline). It has an osmolality of 155, a pH of 5.6, and contains 77 mEq of sodium and 77 mEq of chloride. Continuous infusion can cause dilution and depletion of electrolytes because of the small amount of sodium in this particular mixture resulting in hyponatremia. Because there are no calories in the solution, the patient is going to become calorie depleted as well if it is infused for a long period. Isn't it interesting that adding 5% Dextrose to it to make 5% Dextrose in 0.45% Normal Saline makes it a hypertonic solution?
Lactated Ringers solution has an osmolarity of 275 mOsm/liter and a pH of 6.6. It contains 130 mEq of sodium, 4 mEq of potassium, 3 mEq of calcium, 109 mEq of chloride, and 28 grams of lactate. It is an isotonic solution. It is also called Hartmann's solution. It is primarily used to treat hypovolemia and when the patient's oral intake is limited, absent or fluid losses are very high. It does not, however, supply enough electrolytes for maintenance and does not contain any magnesium. The lactate is a buffer that when metabolized produces bicarbonate. Complications connected with the infusion of LR (Lactated Ringers) are overhydration, electrolyte excess (particularly sodium), electrolyte dilution, and calorie depletion. Patient can also develop metabolic alkalosis if LR is run over long periods of time. It shouldn't be used in patients with liver disease because the lactate is metabolized in the liver. You will commonly see LR used for surgical patients. Adding 5% dextrose to LR makes the solution hypertonic.
Isotonic solutions have the same tonicity as plasma so that when they are infused into a vein, water neither enters or leaves the cells. These kinds of IV solutions are used to expand the extracellular fluid volume and do not cause any fluid to move from into or out of the blood cells. Isotonic solutions have an osmolality of 240 to 340 mOsm/liter. Isotonic solutions are: