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14-September-2008 18:02:48 - blood flow In the physiology of the kidney, renal blood flow RBF is the volume of blood delivered to the kidneys per unit time. In humans, the kidneys together receive roughly 25% of cardiac output, amounting to 1 L/min in a 70-kg adult male. RBF is closely related to renal plasma flow RPF, which is the volume of blood plasma delivered to the kidneys per unit time. While the terms generally apply to arterial blood delivered to the kidneys, both RBF and RPF can be used to quantify the volume of venous blood exiting the kidneys per unit time. In this context, the terms are commonly given subscripts to refer to arterial or venous blood or plasma flow, as in RBFa, RBFv, RPFa, and RPFv. Physiologically, however, the differences in these values are negligible so that arterial flow and venous flow are often assumed equal. Contents 1 Calculation 2 Renal plasma flow 3 Measuring 4 References Calculation Renal blood flow calculations are based on renal plasma flow and hematocrit HCT. This follows from the fact that hematocrit estimates the fractional volume of blood consumed occupied by red blood cells. Hence, the fraction of blood that is in the form of plasma is given by 1-HCT. RPF = RBF1 - HCT Alternatively: RBF = \fracRPF1 - HCT Renal plasma flow Renal plasma flow is given by the Fick principle: RPF = \fracU_x VP_a - P_v This is essentially a conservation of mass equation which balances the renal inputs the renal artery and the renal outputs the renal vein and ureter. Put simply, a non-metabolizable solute entering the kidney via the renal artery has two points of exit, the renal vein and the ureter. The mass entering through the artery per unit time must equal the mass exiting through the vein and ureter per unit time: RPF_a \times P_a = RPF_v \times P_v + U_x \times V where Pa is the arterial plasma concentration of the substance, Pv is its venous plasma concentration, Ux is its urine concentration, and V is the urine flow rate. The product of flow and concentration gives mass per unit time. As mentioned previously, the difference between arterial and venous blood flow is negligible, so RPFa is assumed to be equal to RPFv, thus RPF \times P_a = RPF \times P_v + U_x V Rearranging yields the previous equation for RPF: RPF = \fracU_x VP_a - P_v Measuring Main article: PAH clearance Values of Pv are difficult to obtain in patients. In practice, PAH clearance is used instead to calculate the effective renal plasma flow eRPF. PAH para-aminohippurate is freely filtered, and it is not reabsorbed by the kidney so that its venous plasma concentration is approximately zero. Setting Pv to zero in the equation for RPF yields eRPF = \fracU_xP_a V which is the equation for renal clearance. For PAH, this is commonly represented as eRPF = \fracU_PAHP_PAH V Since the venous plasma concentration of PAH is not exactly zero in fact, it is usually 10% of the PAH arterial plasma concentration, eRPF usually underestimates RPF by approximately 10%. This margin of error is generally acceptable considering the ease with which eRPF is measured. References Boron, Walter F., Boulpaep, Emile L. 2005. Medical Physiology: A Cellular and Molecular Approach. Philadelphia, PA: Elsevier/Saunders. ISBN 1-4160-2328-3. Eaton, Douglas C., Pooler, John P. 2004. Vander's Renal Physiology, 8th ion, Lange Medical Books/McGraw-Hill. ISBN 0-07-135728-9. v d e Urinary system, physiology: renal physiology and acid base physiology Filtration Renal blood flow - Ultrafiltration - Countercurrent exchange Hormones affecting filtration Antidiuretic hormone ADH - Aldosterone - Atrial natriuretic peptide Secretion/clearance Pharmacokinetics - Clearance of medications Reabsorption Solvent drag - Na+ - Cl- - urea - glucose - oligopeptides - protein Endocrine Renin - Erythropoietin EPO - Calcitriol Active vitamin D - Prostaglandins Assessing Renal function/ Measures of dialysis Glomerular filtration rate - Creatinine clearance - Renal clearance ratio - Urea reduction ratio - Kt/V - Standardized Kt/V - Hemodialysis product - PAH clearance Effective renal plasma flow - Extraction ratio Acid base physiology Fluid balance - Darrow Yannet diagram - Body water - Interstitial fluid - Extracellular fluid - Intracellular fluid/Cytosol - Plasma - Transcellular fluid - Base excess - Davenport diagram - Anion gap - Arterial blood gas Buffering/compensation Bicarbonate buffering system - Respiratory compensation - Renal compensation Retrieved from http://en..org/wiki/Renal_blood_flow Categories: Kidney | Physiology Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page This page was last modified on 3 May 2008, at 18:51
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