People who chronically drink excessive amounts of water either by choice or due to a psychological condition may have chronic hyponatraemia. excessive drinking) or retention of fluid by the kidneys (observed as decreased urine output). Increased fluid may be due to either increased intake of fluids (e.g. in the urine) or increased fluid volume in the bloodstream. Hyponatraemia can occur either due to loss of sodium (e.g. Serum and urine osmolality are also used together to help investigate causes of hyponatraemia (low sodium concentration). Serum and urine osmolality can be used to investigate for conditions that affect the secretion of ADH, such as diabetes insipidus, or the syndrome of inappropriate ADH secretion (SIADH). This results in a decrease in the amount of water in the body, and so blood osmolality rises to normal. If, on the other hand, blood osmolality decreases (for example following a large drink of water) then ADH secretion is suppressed and the kidneys excrete increased amounts of dilute urine. This retention of water dilutes the blood causing a decrease in osmolality back to normal levels. ADH signals for the kidneys to conserve water, resulting in formation of concentrated urine (which has a high osmolality). When blood osmolality increases, for example in dehydration, the hypothalamus secretes the hormone antidiuretic hormone (ADH). In health, the osmolality of blood is very closely regulated. Osmotic sensors in the body sense and react to changes in the amount of water and particles in the bloodstream (i.e. Water balance in the body is a dynamic process that is regulated by controlling the amount of water excreted in the urine and by regulating the sensation of "thirst". Measurement of osmolality in serum and urine is used to assess water balance, and whether the body is reacting appropriately to changes in water balance. Is any test preparation needed to ensure the quality of the sample? You may be asked to provide a urine sample as well. The more concentrated the urine is, the higher its osmolality.Ī blood sample is taken by needle from a vein in the arm. Urine osmolality is largely due to the presence of sodium, potassium and urea. Osmolality of the urine is used as a measure of the kidneys ability to concentrate urine. An increase in the osmotic gap (greater than 10) indicates the presence of other substances such as toxic alcohols, aspirin, or mannitol. The difference between measured and calculated (estimated) osmolality is called the "osmotic gap" or "osmolal gap". The calculated and measured osmolality can be compared to determine if there are other particles present (in addition to sodium, potassium, glucose and urea) that contribute to the measured serum osmolality. Osmolality is measured in the laboratory using an osmometer. This simple calculation is useful because sodium, potassium, glucose and urea are more frequently and more easily measured than osmolality. Normally, the major particles contributing to osmolality are sodium and potassium salts, glucose and urea and the osmolality can be approximated by the following equation: (2 x sodium) + glucose + urea (all measured in mmol/L). Osmolality and osmolarity values are approximately the same. Osmolarity is the number of particles in a litre of fluid. Osmolality is a measure of the number of particles dissolved in a kilogram of fluid.
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