Applying Scientific Principles to Dialysis

Fluid dynamics, diffusion, ultrafiltration, and osmosis are applied to every treatment

Is simply a solution in which water is the solvent and electrolytes, glucose, and many other substances are the solutes. Blood also has many suspended particles: rbc, wbc

Expose the pts blood (a solution) to the dialysate (another solution) with a semipermeable membrane between them.

Determines the variations in pressure that occur as blood is pumped out of the pts body and through tubing and the dialyzer, which together are called the extracorporeal (outside the body) circuit.

When the dialysis machine is switched on and treatment begins, the blood pump speeds the flow fo the blood from the pt. The needle that the blood passes through is the first restriction in the circuit.

A pressure less than zero. In dialysis, since the blood pump is pulling rather than pushing, blood through this restriction, the pressure is negative pressure. The amount of restriction determine the neg. pressure just as with pos. pressure. As the flow or restriction increases, neg. pressure will increase.

It is now being pushed through the system against the resistance of: the tubing, the tiny hollow fibers in the dialyzer and the small opening of the venous blood return needle (or catheter) This resistance creates a pos. pressure inside the lines and dialyzer fibers.

The pressures changes correspondingly (pressure=volume(Qb)x resistance) The highest pos. pressure would be measured where blood enters the dialyzer fibers. As blood moves through the fibers and the resistance lessens, the pressure drops.

Pressures measured after blood exits the dialyzer It will reflect the lowest positive pressure in the blood path. The average pressure of the blood entering and leaving the dialyzer fibers is the true amount of force (or pos. hydraulic pressure) that contributes to the ultrafiltration of water from the blood through the membrane into the dialysate

The dialysate flows through the dialyzer and around the hollow fibers in the opposite direction of blood flow (countercurrent flow) The machine can control the pressure differential between the blood compartment and the dialysate compartment as needed to achieve the desired fluid removal.

The pressure difference across the dialyzer membrane is called TMP (transmembrane pressure)

The hollow fibers in the dialyzer are the semipermeable membrane. Blood passes through the inside of these tiny fibers (capillaries), which are surrounded on the outside by dialysate. Molecules of a particular size range pass back and forth between the blood and the dialysate, always moving from an area of higher concentration to an area of lower concentration.

Across the membrane and into the dialysate. The used dialysate is disposed of and continuously replaced with fresh dialysate. Using fresh dialysate maintains a high concentration gradient, which allows the system to remove as much waste from the blood as possible during each dialysis treatment.

To control the electrolyte concentration, electrolytes are added to the dialysate solution. Electrolytes will move based on specific concetration differences until the concetration is equal on both sides of the membrane. Keeping a constant low level of most electrolytes in the dialysate ensures removal of excess levels without totally depleting the blood.

Occurs continuously. Once dialysis has changed the chemical concetrations of the blood, that blood is returned to the pts body, where it slowly dilutes the rest of the blood. The drop in concentration gradient between the plasma water and the fluid in the cells and tissues. Because these cells have their own membranes, solutes (waste products and electrolytes) pass out of the pts cells and into the vascular system. From there they eventually end up in the blood, which in turn can be dialyzed. This allows some of the fluid and waste products from other body compartments to eventually be cleared from the body by dialysis.