Glomerular filtration rate
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Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time.<ref>Physiology at MCG 7/7ch04/7ch04p11 - "Glomerular Filtration Rate"</ref> Clinically, this is often measured to determine renal function.
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[edit] Measurement
There are several different techniques used to calculate or estimate the glomerular filtration rate.
[edit] Measurement using inulin
The GFR was originally determined by injecting inulin (not insulin) into the plasma. Since inulin is not reabsorbed by the kidney after glomerular filtration, its rate of excretion is directly proportional to the rate of filtration of water and solutes across the glomerular filter.
[edit] Estimation using creatine clearance
In clinical practice however, creatinine clearance is used to measure GFR. Creatinine is an endogenous molecule, synthesized in the body, which is freely filtered by the glomerulus (but also secreted by the renal tubules in very small amounts). Creatinine clearance is therefore a close approximation of the GFR. The GFR is typically recorded in milliliters per minute (ml/min).
Example: A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour he excretes 75 mg of creatinine in the urine. The GFR is calculated as M/P (where M is the mass of creatinine excreted per unit time and P is the plasma concentration of creatinine).
<math>\mbox{GFR }= \frac{\frac{75\mbox{ mg}}{60\mbox{ mins}}}{0.01\mbox{ mg}/\mbox{ml}} = 125 \mbox{ ml}/\mbox{min}</math>
[edit] Estimation using Crockroft-Gault formula
The Crockroft-Gault formula may be used to calculate an Estimated Creatinine Clearance, which in turn estimates GFR:<ref>GFR Calculator at cato.at - Crockroft-Gault - GFR calculation (Crockroft-Gault formula)</ref>
<math>\mbox{Creatinine clearance} = \frac { \mbox{(140 - Age)} \times \mbox{Mass (in kilograms)}} {\mbox{72} \times \mbox{Plasma Creatinine (in mg/dl)}} \times \mbox{0.85 if female}</math>
[edit] Calculation using Starling equasion
It is also theoretically possible to calculate GRR using the Starling equation.<ref>Physiology at MCG 7/7ch04/7ch04p12 - "Forces Driving the Glomerular Filtration Rate"</ref> It reads as follows:
- <math>J_v = K_f ( [P_c - P_i] - \sigma[\pi_c - \pi_i] )</math>
In practice, it is not possible to identify the needed values for this equasion, but the equasion is still useful for providing a theoretical underpinning for the above calculations.
[edit] Normal ranges
The normal range of GFR for males and females is:<ref>Creatinine clearance at allrefer.com - The normal ranges of GFR.</ref>
- Males: 97 to 137 ml/min.
- Females: 88 to 128 ml/min.
GFR can decrease due to hypoproteinemia. GFR can increase due to constriction fo the efferent arteriole.
[edit] See also
[edit] References
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