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Organization of the Urinary System, Clearance, GFR, and Renal Blood Flow Jamie Mitchell, MSc, PhD Physiol 210

Distribution of CO

Renin

Basement Membrane

3 Basic Renal Functions:

Assessing Renal Function: Renal Clearance
• Clearance measurements - compare the rate at which the glomeruli filter a substance (water or a solute) with the rate at which the kidneys excrete it into the urine • The “virtual” volume of plasma from which a substance is completely removed by the kidney in a given amount of time (usually a minute). • Measurements indicate/evaluate renal function • Used clinically to estimate renal plasma flow (RPF) and glomerular filtration rate (GFR)

Px = plasma concentration of X; RPF = renal plasma flow Ux = urine concentration of X; V = urine flow

Clinical Uses of Clearance: The Clearance Equation

(p-aminohippurate (PAH))

Glomerular Filtration Rate (GFR)

Note: Ideal glomerular marker would fit into the clearance equation

The Clearance of Inulin is a Measure of Glomerular Filtration Rate Criteria for Ideal Marker: 1. Substance must be freely filterable in the glomeruli. 2. Substance must be neither reabsorbed nor secreted by the renal tubules. 3. Substance must not be synthesized, broken down, or accumulated by the kidney. 4. Substance must be physiologically inert (not toxic and without effect on renal function).

Clearance of Creatinine is a Useful Clinical Index of GFR

CCr = GFR= UCr x V -----PCr

– If Cx > GFR, then substance was secreted

– If Cx < GFR, then substance was reabsorbed

GFR and Renal Blood Flow

Glomerular Filtration Rate (GFR) 125ml/min [180 L/24 hours]

Urine= 1.5 L/24 hours

Filtration Fraction of Plasma = 125/625 = 20%

Renal Plasma Flow (RPF) 625 ml/min

Molecular Size, Electrical Charge, and Shape Determine the Filterability of Solutes Across the Glomerular Filtration Barrier

- Charge - Charge

UF = Ultrafiltrate

Filterability

- Charge -...
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