1. Sodium flux during dialysis
Sushrut S. Waikar, MD, MPH
Renal Division
Brigham and Women’s Hospital, Harvard Medical School

2. 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

3. 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)

4. 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)

5. 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

6. 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

7. 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)

8. Plasma versus dialysate
Sodium (meq/L)
140
Potassium (meq/L)
5.0
3.0
Chloride (meq/L)
114
110
Bicarbonate (meq/L) 20 35
Calcium (meq/L)
2.5
Urea nitrogen (mg/dL) 40

10. Urea 70 mg/dL
Urea 30 mg/dL

11. Sodium 142 meq/L
Sodium ? meq/L

12. Sodium in dialysate: 140 meq/L
Sodium 142 meq/L
Sodium ? meq/L

13. 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)

14. 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

15. Sodium 142 meq/L
Sodium ? meq/L

16. 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

17. 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

18. 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

19. Dialysis procedure Blood flow ~ 400 ml/min Dialysate flow ~ 800 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)