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clearance

clearance (kler´ans)

1. (C with a subscript indicating the substance removed) Removal of a substance from the blood, e.g., by renal excretion, expressed in terms of the volume flow of arterial blood or plasma that would contain the amount of substance removed per unit time; measured in ml/min. Renal c. of any substance except urea or free water is calculated as the urine flow in ml/min multiplied by the urinary concentration of the substance divided by the arterial plasma concentration of the substance; normal human values are commonly expressed per 1.73 m2 body surface area. 2. A condition in which bodies may pass each other without hindrance, or the distance between bodies. 3. B>Removal of something from some place; e.g., "esophageal acid c." refers to removal from the esophagus of some acid that has refluxed into it from the stomach, evaluated by the time taken for restoration of a normal pH in the esophagus.
p-aminohippurate c. a good measure of renal plasma flow, which it slightly underestimates; when a low plasma concentration of p-aminohippurate (PAH) is maintained by intravenous infusion, the kidney extracts and excretes almost all of the PAH from the plasma before it reaches the renal vein.
creatinine c. measurement of the clearance of endogenous creatinine, used for evaluating the glomerular filtration rate (GFR).
endogenous creatinine c. a term distinguishing measurements based on the creatinine normally present in plasma; since no infusion is necessary, an average value may be obtained by collecting urine for a long period, e.g., 24 hours.
free water c. the amount of water excreted in the urine beyond that which would accompany the excreted solutes if the urine were isosmotic with plasma; it represents the loss of body water in excess of solute tending to raise body osmolality and making urine hyposmotic. Unlike other c.'s, it is calculated by subtracting the osmolal c. from the actual volume of urine excreted per minute. A negative value for free water c. represents the amount of water that the body has reclaimed from isosmotic tubule fluid to make the urine hyperosmotic and to lower body osmolality.
interocclusal c. freeway space
inulin c. an accurate measure of the rate of filtration through the renal glomeruli, because inulin filters freely with water and is neither excreted nor reabsorbed through tubule walls. Inulin is not a normal constituent of plasma and must be infused continously to maintain a steady plasma concentration and a steady rate of urinary excretion during the measurement. Inulin c. in a normal adult person is about 120 ml/min (range 100-150) per 1.73 m2 body surface area.
isotope c. the rate at which an isotope is removed (usually by blood flow) from a tissue or organ such as the brain.
maximum urea c. the urea c. when the urine flow exceeds 2 ml/min; normal value is about 75 ml blood/min per 1.73 m2 body surface area.
occlusal c. a condition in which the opposing occlusal surfaces may glide over one another without any interfering projection.
osmolal c. the volume of urine that would be excreted per minute if the urinary solutes were accompanied by just enough water to make the urine isosmotic with plasma, i.e., so that the solute excretion did not change the osmolality of body fluids. To calculate it, the volume of urine excreted per minute is multiplied by the urinary osmolality (usually measured by freezing point depression) and divided by the plasma osmolality. Osmolal c. is less than actual urine flow when urine is hyposmotic and exceeds it when urine is hyperosmotic.
standard urea c. the value obtained when the square root of the urine flow (when below 2 ml/min) is multiplied by the urine urea concentration and divided by the whole blood urea concentration; represents an old empirical adjustment for the effect of low urine flow on urea excretion; sometimes corrected for body size by dividing by some function of body weight or surface area. Later, plasma concentration was substituted for blood concentration in the calculation. The normal value is about 54 ml/min per 1.73 m2 in an adult person.Van Slyke's formula;
urea c. the volume of plasma (or blood) that would be completely cleared of urea by one minute's excretion of urine; originally calculated as urine flow multiplied by urine urea concentration divided by concentration of urea in whole blood rather than plasma, representing blood urea c. rather than plasma urea c.

 

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