08 - Filtration and GFR

role of kidney

• regulates volume

• regulates solute concentration

  • maintains homeostasis by regulating electrolyte concentrations

  • impacts long-term regulation of blood pressure

  • removes waste, toxins, and certain drugs

• regulates pH

• serves as endocrine organ

  • releases calcitriol (active vitamin D), renin, prostaglandins

• erythropoeisis

  • releases erythropoietin, which goes to bone marrow to trigger hematopoietic stem cells to make red blood cells

equations for GFR

• for governing fluid and solute filtration

  • SNGFR = k_f (P_gc - P_T - π_gc)

  • solvent drag (J_s) = (SNGFR) (C) (1 - σ)

• for quantify facets of kidney function

  • excretion rate = [U]_x x V

  • clearance = ([U]_x x V) / [P_x]

  • GFR = ([U]_x x V) / [P]_x

  • filtered load = GFR x [P]_x

kidney filtration

kidney receives 20-25% of cardiac output, but only 20% of plasma gets filtered

• filtration barrier is size and charge selective

• only small molecules are filtered → water, urea, electrolytes, glucose, amino acids

• large proteins and RBCs are not filtered

filtration forces

• fluid filtration → SNGFR = k_f (P_gc - P_T - π_gc)

  • SNGFR = single nephron glomerular filtration rate

  • k_f = filtration coefficient (filtration barrier permeability)

  • P_gc = hydrostaic pressure of gloom cap → favors filtration

  • P_T = hydrostatic pressure of tubule → opposes filtration

  • π_gc = oncotic pressure of gloom cap → opposes filtration

• solute filtration → J_s = (SNGFR) (C) (1-σ)

  • J_s = solvent drag, pulling small solutes along

  • C = log mean concentratio nacross membrane

  • σ = reflection coefficient for solute

    • σ = 0 → solutes that cross as easily as water

    • σ = 1 → solutes that cannot cross membranes

excretion rate

rate at which solute/substance is excreted into urine

• unit: mass/time

• excretion rate = urine concentration x urine flow rate = [U]_x x V

clearance

measure of hypothetical volume of plasma that is cleared of that solute/substance per unit of time

• unit: volume/time

• clearance = excretion rate of substance / plasma concentration of substance = ([U]_x x V) / [P]_x

glomerular filtration rate (GFR)

volume of plasma that is filtered across glomerular capillaries into tubules per unit of time

• unit: volume/time

• ultimate measure of kidney function

• GFR = clearance = ([U]_x x V) / [P]_x

• clearance of only few solutes can be used to calculate GFR

  • have low reflection coefficient → freely filtered

  • neither reabsorbed nor secreted by nephron

  • true for inulin or creatinine

kidney excretion

excretion = filtration + secretion - reabsorption

• 20% of plasma perfusing glomerulus gets filtered

• >99% of water and solutes are reabsorbed in nephron by transporters

• secretion occurs in nephrons mainly by transporters

• <1% of what is filtered gets excreted

filtered load

amount of solute/substance filtered across glomerular capillaries into tubules per unit of time 

• unit: mass/time

• filtered load = GFR x [plasma]_x = GFR x [P]_x

• calculated only for small molecules with a small reflection coefficient

• small molecules are freely filtered, so increase in plasma concentration will increase their filtration

• cannot calculate filtered load for large molecules

clinical measurements

• GFR → > 120 mL/min

  • stage 1 CKD → 90-120

  • stage 2 CKD → 60-90

  • stage 3 CKD → 30-60

  • stage 4 CKD → 15-30

  • stage 5 CKD → <15

• serum creatine → 0.6-1.2 mg/dL

• blood urea nitrogen (BUN) → 7-18 mg/dL

• protein in urine → <0.15 g/day

• blood in urine → none