Renal Clearance

Volume of blood that flows through the kidneys per minute (mL/min). Typically 20-25% of total cardiac output. Volume of renal blood plasma that flows through the kidneys per minute (mL/min) Plasma is the aqueous portion of the blood. Volume of plasma from which a substance is removed in a given amount of time. Indicates whether a substance is filtered, reabsorbed, and/or secreted. Renal clearance is calculated according to the Fick Principle, which states: The amount of substance that enters the kidney is equal to the amount of substance that leaves it. Approximately 55% renal plasma

• 93% plasma water, which gets filtered across the glomerular capillaries to create ultra filtrate.
• 7% plasma proteins

Approximately 45% blood cells (hematocrit) Renal blood flow = Renal plasma flow divided by (1 minus Hematocrit). – Renal blood flow averages 1000-1250 mL/min. – Renal plasma flow averages 550-690 mL/min. Clearance of substances varies from 0% to nearly 100%: Renal clearance of albumin, which is a large protein, is 0% because large proteins are excluded from the ultrafiltrate. Renal clearance of glucose, which is freely filtered, is also 0%, because it is completely reabsorbed in the nephron tubule. Renal clearance of para-amniohippuric acid (PAH), which is an organic acid, is nearly 100% because it is both filtered at the renal corpuscle and secreted into the nephron tubule lumen. In other words, nearly all of the PAH that enters the kidney is cleared from the plasma and excreted into the urine. Because it is both filtered and secreted, the clearance of PAH can be used clinically to measure the effective renal plasma flow. Be aware that it does not measure true renal plasma flow because some PAH remains in the blood (approximately 10%). To express this mathematically: The clearance of PAH and effective renal plasma flow are equal to: – The urine concentration of PAH multiplied by the urine flow rate divided by the plasma concentration of PAH. In other words, we apply the Fick principle to compare the amount of PAH that entered the kidney in the blood plasma with the amount of PAH that was excreted in the urine; we ignored the amount of PAH in the renal vein, because it is almost completely excreted in the urine. Let's show how renal clearance is used to calculate effective renal plasma flow and renal blood flow. Urine concentration of PAH is 550 mg/100 mL Urine flow rate is 1 mL/min Plasma concentration of PAH is 1 mg/100 mL Hematocrit is 0.45. With these values, we can estimate PAH clearance, and, therefore, renal plasma flow: 550 mg/100 mL multiplied by 1 ml/min Divided by 1 mg/100 mL. = 550 mL/min. – Then, return to our equation for renal blood flow, and write that: Renal blood flow= 550 mL/min divided by 1-0.45. Thus, we have a renal blood flow of 1000 mL/min. A GFR marker is a substance with a clearance equal to GFR; therefore, its clearance can be clinically determined to evaluate GFR and kidney functioning. Two examples of GFR markers: Inulin has a clearance exactly equal to GFR because it is filtered, but not reabsorbed or secreted. Creatinine has a clearance that is nearly equal to GFR because it is filtered and only minimally secreted. – However, since creatinine is an endogenous substance (and inulin is not), it is the preferred GFR marker in most clinical situations.