sodium flux during dialysis
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Sodium flux during dialysis. Sushrut S. Waikar, MD, MPH Renal Division Brigham and Women’s Hospital, Harvard Medical School. Background. Kidneys filter the blood, clearing it of waste products 20% of heart’s output is directed to the kidneys, which filter ~180L plasma per day

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sodium flux during dialysis

Sodium flux during dialysis

Sushrut S. Waikar, MD, MPH

Renal Division

Brigham and Women’s Hospital, Harvard Medical School

background
Background
  • Kidneys filter the blood, clearing it of waste products
    • 20% of heart’s output is directed to the kidneys, which filter ~180L plasma per day
  • Kidneys also regulate water and salt balance
background1
Background
  • Loss of kidney function leads to death within days
    • Excessive buildup of nitrogenous waste, acid, potassium, sodium, water
  • Kidney function can be partially replaced by hemodialysis (~300,000 patients in United States)
hemodialysis
Hemodialysis
  • Kidney function can be partially replaced by hemodialysis
  • First performed by Wilhelm Kolff in early 1940s
  • Procedure is typically performed for 4 hours thrice weekly (e.g., Monday, Wednesday, Friday)
hemodialysis procedure
Hemodialysis procedure
  • Blood flows into tubing that divides into thousands of parallel hollow fibers
  • Each fiber is a semipermeable membrane
  • Outside of the fibers runs the “dialysate” solution
clearance during dialysis
Clearance during dialysis
  • Convection
    • Negative pressure applied, water and dissolved small solutes (< 40 kDa) pass across membrane into the dialysate fluid which is then discarded
  • Diffusion
    • Solutes travel across membrane down concentration gradient
    • Blood and dialysate flow in opposite directions, maximizing concentration gradient
goals of a dialysis procedure
Goals of a dialysis procedure
  • Get rid of the water that was ingested and produced (during metabolism) since the last procedure – usually 3 liters
    • Done by convection
  • Get rid of salt (sodium chloride, potassium chloride)
  • Maintain acid-base balance
  • Get rid of nitrogenous waste products (urea)
slide10

Urea 70 mg/dL

Urea 30 mg/dL

slide11

Sodium 142 meq/L

Sodium ? meq/L

slide12

Sodium in dialysate: 140 meq/L

Sodium 142 meq/L

Sodium ? meq/L

slide13

Sodium in dialysate: 140 meq/L

Sodium 142 meq/L

Sodium ? meq/L

CONVECTIVE CLEARANCE: water and small solutes are removed by negative pressure across membrane

DIFFUSIVE CLEARANCE: sodium moves down its concentration gradient (in either direction, depending on plasma concentration; dialysate sodium usually fixed)

factors that influence sodium flux
Factors that influence sodium flux
  • Dialysate sodium concentration = 140 meq/L
  • Plasma sodium concentration = 142 meq/L, changes during procedure
  • Convection and diffusion occur simultaneously
  • Gibbs Donnan effect
    • Large negatively charged proteins NOT able to pass through membrane; electroneutrality must be maintained
    • Sodium and other cations less “permeable” than anions like chloride, bicarbonate
slide15

Sodium 142 meq/L

Sodium ? meq/L

factors that influence na in plasma
Factors that influence Na in plasma
  • Plasma sodium concentration itself has many determinants, not just mass balance of sodium and water
    • Also affected by potassium mass balance
    • Water and sodium flux in various body compartments
distrbution of salt water in body
Distrbution of salt + water in body
  • Water in a 70 kg man
    • 60% water = 42 liters
      • Intracellular: 28 liters
      • Extracellular: 14 liters
        • Interstitial = 10 liters
        • PLASMA VOLUME = 4 liters
  • Sodium is the primary extracellular cation, 140 meq / liter
  • Intracellular sodium
dialysis membranes
Dialysis membranes
  • Hollow fibers ~12,000 in parallel
    • 20 – 24cm length, diameter 180 to 220 um, 6 to 15 um thickness
    • Pores: avg diameter 30 Angstroms, 10^9 in number
  • Old: Cellobiose (saccharide)
  • New: synthetic membranes (e.g., polysulfone, polyamide, polymethylmethacrylate)
    • Manufactured polymers classified as thermoplastics
dialysis procedure
Dialysis procedure
  • Blood flow ~ 400 ml/min
    • 40% red blood cells, 60% plasma
      • Plasma: 93% water, 7% protein and lipids
  • Dialysate flow ~ 800 ml/min
  • Typically 2-4 liters of “ultrafiltration” (volume removed during 4h procedure)
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