Scientific Principles Used in Dialysis

Tuesday, October 19, 2010

Scientific Principles Used in Dialysis

Article by a Renal Nurse

Sections of Scientific Principles Used in Dialysis

SOLUTIONS

• A solution is a mixture of a solvent and a solute.
• The solvent is a fluid.
• The solute is any substance that can be dissolved into the solvent.
• Example: Salt water - water is the solvent and salt is the solute.
• Dialysate is the solution that is used during dialysis.
• Water is the solvent.
• The solutes are electrolytes (e.g., potassium, calcium, sodium, magnesium, and chloride ions) and glucose (sugar)
• Electrolytes levels in the dialysate closely match the levels in human blood.
• This reduces the loss of these electrolytes out of the blood and into the dialysate during dialysis.

SEMIPERMEABLE MEMBRANE
• A semipermeable membrane is a type of thin, flexible filter - a barrier that allows only particles smaller than a certain size to pass through it.
• In dialysis, the semipermeable membrane's holes allow small molecules, such as water and urea, to pass through easily.
• The small size of the pores keeps larger molecules and blood cells from passing through the membrane.

DIFFUSION
• Diffusion is the process by which atoms, molecules, and/or other particles move from an area where they are in high concentration to an area where they are in concentration.
• Diffusion can occur in solids, gases, or liquids, such as blood.
• In dialysis, diffusion occurs across an artificial semipermeable membrane

Source: advancedrenaleducation.com

Factors Affecting Diffusion: the Nature of the Solution

1. Concentration Gradient: How concentrated is the fluid on each side of the membrane?
     • Solutes can move through a membrane in either direction, but always toward the area of lesser concentration..
     • As the concentration gradient increases, solute movement increases too.
     • Diffusion stops when the concentrations on both sides of the membrane are equal.
     • Concentration gradient allow dialysate to remove wastes from a patient's blood and to balance electrolytes in the blood with electrolytes in the dialysate.

2. Molecular weight of the solutes: How large are the dissolved particles?
     • Smaller particles (urea and salts) diffuse more easily and quickly than larger ones (such as RBcs, WBCs, albumin, platelets, viruses and bacteria.

3. Temperature: How warm is the fluid?
     • Molecules move faster at higher temperatures, so warmer fluids allow faster diffusion.
     • Dialysis temperature is controlled during dialysis forpatient safety, comfort and faster diffusion.

Factors Affecting Diffusion: the Nature of the Membrane
1. Membrane permeability: How plentiful and large are the pores?
     • More pores allows faster diffusion
     • Larger pores allow larger molecules to pass through

2. Surface area of the membrane: How big is the membrane?
     • Surface area is the amount of membrane in direct contact with the blood and dialysate
     • Larger surface areas allow more diffusion

3. Flow geometry: How do the fluids flow?

• In dialysis, blood flows one way while dialysate flows the opposite way.
• This countercurrent flow of blood to dialysate speeds up diffusion, because with this arrangement, a high concentration gradient between the blood and dialysate cm be maintained throughout the length of the dialyzer

A concurrent flow -would occur if blood and dialysate moved in the same direction

Only molecules smaller than the pores will
pass through. Source: http://www.fmqai.com/library/attachment-library/4scientificprinciples.pdf

OSMOSIS
• Osmosis is movement of a sol vent through a semipermeable membrane from an area of lower solute concentration toward an area of higher solute concentration.
• The difference in concentration is called an osmotic pressure gradient.
• Osmotic pressure can be overcome by hydraulic pressure using a pump, gravity, or other means.
• Hydraulic pressure is pressure created naturally (such as from gravity) or artificially (such as a pump).
• Hydraulic pressure affects the amount of water thai is removed from the patient during dialysis.

Source: advancedrenaleducation.com

FILTRATION AND ULTRAFILTRATION
• Filtration - is the movement of fluid through a filter as the result of hydraulic pressure.
• Ultrafiltration-water removal from blood due to a pressure, gradient across a membrane.

Source: toltecint.com

CONVECTION
• The transfer of heat and solutes by physical circulation or movement of the parts of a liquid or gas.
• Also called 'solute drag'

FLUID DYNAMICS

• A fluid is a liquid or gas that changes shape at a steady rate when acted upon by a force.
• The field of "dynamics" addresses the motion and equilibrium of systems.
• Fluid dynamics applies to dialysis, because it describes how two fluids (blood and dialysate) are pumped through tubing.
• Several factors affect the movement, or flow, of fluid through tubing:
     • Flow rate - the amount of fluid that flows through the tubing in a given period of time (e.g., 10ml/min)
     •  Flow velocity -is the speed at which the fluid moves through a given length of tubing.
     • Velocity is based on the rate of flow and the area of a cross section of the tube.
• If the flow rate is held constant but the cross section of the tube is reduced by half, the flow velocity will double.