Module 3: Principles Of Dialysis

Principles Of Dialysi

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Dialysis replaces 3 main kidney functions
Removing wastes from the blood Removes excess fluid from the blood Keeping electrolytes in balance
a mixture of a solvent and a solute
any substance that can be dissolved into the solvent
the solution that is used during dialysis Solvent: Water Solute: Electrolytes (potassium, calcium, sodium, magnesium, and chloride ions) and glucose
Electrolyte Levels in Dialysate
closely match the levels in the human blood. This reduced the loss of these electrolytes out of the blood and into the dialysate during dialysis.
Controlling pts blood electrolyte levels
can be controlled by changing the dialysate. Adding an electrolyte to the dialysate at a level higher than in the blood will allow the electrolyte to enter the pts blood during a treatment.
Semipermeable Membrane
a type of thin, flexible filter- a barrier that allows only particles smaller than a certain size to pass through it.
Dialysis semipermeable membrane
In dialysis, the semipermeable membrane's holes allow small molecules, such as water and urea, to pass through easily. Middle molecules can also pass through, but more slowly. The small...
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 low concentration.
Energy for Diffusion
Energy for the movement comes from the molecules themselves, and does not depend on outside forces.
Factors affecting diffusion: The nature of the solution
Concentration Gradients Molecular weight of the solutes Temperature
Concentration Gradients
Solutes move through a semipermeable membrane from an area of greater concentration to an area of lesser concentration. Solutes can move through a membrane in either direction, but...
Molecular weight of the solution
Smaller molecules diffuse more easily and quickly than larger ones. Large blood components such as RBC, WBC, albumin, and platelets diffuse more slowly because they are bigger. Small...
Molecules move faster at higher temperatures, so warmer fluids allow faster diffusion. Dialysate temperature is controlled during dialysis for pt safety, comfort and faster diffusion.
Factors Affecting Diffusion: The Nature of the Membrane
Membrane permeability Surface area of the membrane Flow geometry
Membrane permeability
A membrane with more pores allows faster diffusion. Larger pores allow larger molecules to pass through. The membrane's thickness and design also affect the diffusion rate.
Surface area of the membrane
Surface area is the amount of membrane in direct contact with the blood and dialysate. Larger surface areas allow more diffusion.
Flow geometry
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...
In diffusion, solutes move. In osmosis, the solvent moves across the membrane. Osmosis is the movement of a solvent through a semipermeable membrane from an area of lower solute concentration...
Osmotic Pressure Gradient
The difference in concentration
Hydraulic pressure
Osmotic pressure can be overcome by hydraulic pressure using a pump, gravity or other means. The total pressure on a fluid will include both osmotic and mechanical forces.
Reverse osmosis water treatment devices
Hydraulic pressure can reduce or overcome osmotic pressure. This principle is the basis for reverse osmosis water treatment devices.
the movement of fluid through a filter as the result of hydraulic pressure. Fluid will always move from a higher pressure to a lower pressure. The filter traps any matter that is too...
Ultrafiltration (UF)
Water removal from blood due to a pressure gradient across a membrane. In dialysis, UF is used to remove excess water that has built up. The filter used in UF is semipermeable membrane.
transfer of heat and solutes by physical circulation or movement of the parts of a liquid or gas. In dialysis, convective transport leads to solvent drag.
Solvent drag
as a solvent crosses a semipermeable membrane, it drags along smaller solutes
a liquid or gas that changes shape at a steady rate when acted upon by a force
addreses the motion and equilibrium of systems
Fluid dynamics
describes how two fluids-blood and dialysate-are pumped through tubing. Several forces affect the movement of fluid through the tubing: Flow rate Flow velocity
Flow rate
amount of fluid that flows through the tubing in a given period of time
Flow velocity
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.
Pressure in any fluid system
the pressure in any fluid system is always related to the flow and the resistance; the greater the flow and the greater the resistance, the greater the pressure.
Fluid compartments
The human body is made mostly of fluids. Fluids are found in 3 compartments: Intracellular (inside the cells) Interstitial (between the cells) Intravascular (inside the blood vessels)
Pressure gradients in fluid compartments
Differences in the level of sodium and electrolytes between compartments (gradients) cause water to move.
Dialysis and body compartments
During dialysis, only fluid from the intravascular compartment can be removed
Extracorporeal Circuit
Outside the body Together, the tubing and dialyzer are called the extracorporeal circuit
Fluid intake
Food Beverages Intravenous fluids
Fluid Output
Respiration Stool Perspiration adn skin water loss Dialysate loss Residual urine Vomiting
Fluid dynamics in dialysis
blood pump speeds the flow of blood from the pt. Blood passes through the needle-the first restriction in the circuit. Because the pump is pulling blood through this restriction, the...
Resistance in the blood pump
The tubing The tiny hollow fibers in the dialyzer The small opening of the venous blood return needle or catheter
Resistance in the blood pump creates:
positive pressure inside the lines and dialyzer fibers. As blood passes through these resistances, te pressures change. The high positive pressure is measured in the arterial header,...
Venous pressure
The pressure measured after blood leaves the dialyzer (venous pressure) is the lowest positve pressure in the blood path.
Positive hydraulic pressure
The average pressure of blood entering and leaving the dialyzer fibers is the true amount of force (positive hydraulic pressure) that aids UF of water out of the blood, through the...
Countercurrent flow.
dialysate flows through the dialyzer and around the hollow fibers in one direction. Blood flows in the opposite direction for countercurrent flow.
Transmembrane pressure (TMP)
The machine can control the pressure differential between the blood and dialysate compartment as needed to reach the desired fluid removal. This pressure difference across the dialyzer...
Diffusion in dialysis
The hollow fibers in the dialyzer are the semipermeable membrane. Blood passes through the insides of these tiny fibers (capillaries), dialysate surrounds them on the outside. Molecules...
Electrolyte balance in dialysis
is also maintained with diffusion. It is vital to pts health to keep the right level of electrolytes in the blood. To control the balance, electrolytes can be added to the dialysate....
Diffusion occurs continuously in the pts body
as clenased blood is returned to the pt, it slowly dilutes the rest of the blood. The drop in the concetration of solutes in the blood creates a gradient between the blood plasma and...
UF in dialysis
UF requires pressure to force fluid through the membrane. The dialysis machine can create a hydraulic pressure difference, with higher pressure in the blood compartment than in the...
Convection in dialysis
As water (a solvent) moves from the blood compartment to the dialysate compartment, molecules of dissolved solute are dragged along to (solvent drag). The ease with which the solute...
Osmosis in dialysis
the pressure of UF pushes fluid out of the blood nad into the dialysate. But osmotic forces decide which way water will move from one body compartment to another. In hemodialysis, diffusion...
Example of FIltration and UF
Soak a sponge in water. Hold the sponge over a bucket of water and watch the sponge filter the water. If you apply positive pressure to the sponge by squeezing it, much more water can...

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