Each of the two KIDNEYS ﬁlters a large volume of blood – 25 per cent of cardiac output, or around 1,300 ml – through its two million glomeruli (see GLOMERULUS) every minute. The glomeruli ﬁlter out cell, protein, and fat-free ﬂuid which, after reabsorption of certain chemicals, is excreted as urine. The rate of this ultraﬁltration process, which in health is remarkably constant, is called the glomerular ﬁltration rate (GFR). Each day nearly 180 litres of water plus some small molecular-weight constituents of blood are ﬁltrated. The GFR is thus an indicator of kidney function. The most widely used measurement is CREATININE clearance and this is assessed by measuring the amount of creatinine in a 24-hour sample of urine and the amount of creatinine in the plasma; a formula is applied that gives the GFR.
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