Dialysis: A Thermodynamic Perspective

Alyssa Chang, Austin Dosch, Meredith Greeson, Carrie Martin,

Bobby Palmer

Objectives• The Kidney• History of Dialysis• Thermodynamics of Dialysis

• Chemical potential (μ), Affinity, and Gibbs-Free Energy

• Osmosis & Osmotic Pressure• Entropy

Introduction: The KidneyStructure•Renal Vein and Artery•Nephron•Cortex•Inner and outer Medula

Function1. Removes…

–nitrogenous wastes –toxins

2. Regulates…–blood volume–blood solute concentrations–blood pH

The Filtration Process

The Nephron is responsible for:

1. Filtration2. Secretion3. Reabsorption

• Urine is formed through the filtration under pressure, active transport, and osmosis

red arrow = active transport lavender arrow = diffusion or osmosis

Dialysis Machines

History:•1913 Able, Roundtree and Turner use artery of animal•1924 Hass credited for first human dialysis•Today dialysis last 4 hours, 3 times a week

How it works:Contaminated blood Dialyser clean blood returns to body

Thermodynamic Principles Governing Dialysis

• Chemical potential (µ), Gibbs Free Energy, and the Affinity of a diffusion process

• Osmosis and Osmotic Pressure• Entropy

Chemical potential (μ)• The chemical “driving force” which causes movement of molecules from an area

of higher chemical potential to lower potential (affinity)• Solutions of different chemical potentials will flow across a semi-permeable

membrane until their potentials are equal• Chemical potential of a particular solute is dependent on the activity:

This equation can be related to the thermodynamics of a dialysis system by:

In which the dialysis tubing containing contaminated fluid is passed through a bath containing low fluid concentration and therefore is able to be removed

AAA aRT ln

Chemical potential (μ)• Chemical potential is also known as the molar Gibbs energy

• Dialysis operates by keeping the system from reaching equilibrium by adding new buffer solution, so the chemical potentials cannot equilibrate

Osmosis and Osmotic Pressure• Osmosis is the flow of water across a semi-permeable membrane from an area of

low solute concentration to an area of higher concentration

• There is a certain pressure required to stop this flow; the osmotic pressure given by the van’t Hoff equation:

π= [c]RT• Dialysis tubes are permeable to water, so the osmotic gradient as the chemical potential gradient of the system affects the efficiency of the process

Entropy• The affinity of the diffusion reaction is the thermodynamic force• The extent of reaction with respect to time is the thermodynamic flow•These two quantities can be related to the entropy production of the system by the equation:

• Therefore, as the diffusion reaction progresses and the affinity decreases, entropy production eventually stops at a point (equilibrium)• In this state, entropy of the system is maximized and the Gibbs free energy is minimized

Entropy

Demonstration: Diffusion Through Dialysis Tubing