Module 3: Principles of Dialysis

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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
Solutions a mixture of a solvent and a solute
Solvent Fluid
Solute any substance that can be dissolved into the solvent
Dialysate
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 size of the pores keeps larger molecules and blood cells from passing through the membrane.
Diffusion
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 always toward the area of lesser concentration. As the difference in solute concentration increases, solute movement increases to. Diffusion stops when the concentrations on both sides of the membrane are equal.
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, such as urea and salts, diffuse faster. Middle molecules may pass through, but more slowly.
Temperature 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 gradient between the blood and dialysate can be maintained throughout the length of the dialyzer.
Osmosis
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 toward an area of higher solute concentration.
Movement goes on until the concentration of molecules equilibrates on both sides of the membrane
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.
Filtration 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 large to pass through it.
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.
Convection 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
Fluid a liquid or gas that changes shape at a steady rate when acted upon by a force
Dynamics 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 pressure is usally negative: Less than zero. The amount of flow and restriction determmine negative pressure, just as with positive pressure. As the flow or restriction increases, the pressure will decrease.
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, where blood enters the dialyzer fibers. as blood moves through the fibers, resistance drops, too.
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 membrane, and into the dialysate.
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 membrane is called TMP.
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 of a certain size range pass back and forth between the blood and dialysate, always moving from high to low concentration. Wastes in the pts blood diffuse across the membrane and into the dialysate. Used dialysate is sent to a drain and replaced with fresh dialysate, to maintain a high concentration gradient (allowing as much waste as possible to be removed from the blood during each pass through).
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. Electrolytes will remove until the concetration is equal on both sides of the membrane. Keeping a constant low level of an electrolyte in the dialysate ensures that the excess is removed without allowing the levels in the blood to drop too low.
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 the fluid in the cells and tissues. Because these cells have their own membranes, solutes- such as wastes and certain electrolytes- slowly pass out of thep ts cells and into the bloodstream. From there, they are dialyzed. This process allows some of the wastes from other body compartments to be cleared from the body by dialysis. This slow process of diffusion is why dialysis treatments require more than one pass of blood through the dialyzer to clear wastes from the blood.
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 dilaysate compartment. This TMP pushes excess water out of the blood and into the dialysate.
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 is dragged along by the solvent is determined by the size of the solute molecule compared to the size of the membrane pores. Smaller solutes move easily, so the solution can sieve across the membrane without any change in concentration. But larger solutes move more slowly and the rate of convective transport is slower. Thus, the convective transport of a solute depends on how porous the membrane is. This measurement of porosity is known as the sieving coefficient (SC) of the membrane.
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 lowers the oslute concetration in the blood. Higher solute concentration in the tissues and cells then pulls water out of the blood. Rapid drops in blood volume can occur, which causes drops in bp and other symtoms. Often, sodium is added to the dialysate, so it diffuses into the blood. The higher blood sodium draws water from other body compartments into the blood, so it can be removed by UF. The sodium in the dialysate is then lowered towards the end of the dialysis treatment to pull the sodium back out of the bloodstream.
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 be removed or ultrafiltered by applying the pressure