When the kidneys fail, the body loses its natural ability to filter blood, leading to a dangerous buildup of waste products and excess fluid. Dialysis artificially replaces this filtering function using a cleansing solution known as dialysate fluid. This liquid is formulated to mimic the chemical balance of healthy blood plasma, but without the accumulated toxic wastes. By bringing the patient’s blood into contact with the dialysate across a specialized membrane, the treatment safely removes unwanted substances and restores balance.
The Essential Functions of Dialysate
The primary job of dialysate fluid is to facilitate the removal of metabolic waste products that the failing kidneys cannot excrete. Waste molecules, such as urea and creatinine, accumulate in the blood during kidney failure. The dialysate provides a clean, low-concentration environment, which encourages these toxins to move out of the blood and into the solution.
Another function is the restoration of the body’s delicate electrolyte and acid-base balance. Kidney disease frequently causes imbalances in substances like sodium, potassium, and calcium, and a condition called metabolic acidosis, where the blood becomes too acidic. The dialysate’s composition is carefully calibrated to correct these issues by supplying necessary buffers, like bicarbonate, and adjusting electrolyte levels to healthy ranges.
The Standard Composition of Dialysate
Dialysate is a precise mixture of ultra-pure water and various dissolved substances, known as solutes. The water used is extensively treated, often exceeding drinking water standards, to remove contaminants that could be harmful if introduced directly into the blood. This purified water acts as the solvent base for all the other components.
Electrolytes, including sodium, calcium, and magnesium, are added at specific concentrations to encourage the blood to achieve a balanced state. Potassium is typically set at a very low or zero concentration in the dialysate to ensure that excess potassium, which can affect heart rhythm, is pulled out of the blood. Bicarbonate serves as the primary buffer, helping to neutralize excess acid in the patient’s blood and correct the metabolic acidosis common in kidney failure.
Glucose or dextrose is often included, especially in peritoneal dialysis (PD), where its concentration creates an osmotic gradient to pull excess water out of the patient’s body. In hemodialysis (HD), glucose is often included in a lower concentration to prevent the patient’s blood sugar from dropping during the treatment. The final mixture is a sterile solution customized to the patient’s immediate needs.
Principles of Solute Exchange
The physical cleaning of the blood occurs through two main principles: diffusion and ultrafiltration. Diffusion is the process where dissolved substances move from an area of higher concentration to an area of lower concentration. Since the patient’s blood has high concentrations of waste products and the dialysate has none, these toxins passively move across the semipermeable membrane and into the dialysate.
If the blood contains an excess of an electrolyte, like potassium, it diffuses into the dialysate, which is formulated to contain a lower concentration of that substance. Conversely, if the blood is deficient in a substance, such as bicarbonate, the higher concentration in the dialysate causes it to diffuse into the blood, restoring the correct balance. This movement continues until the concentration gradient across the membrane is minimized.
Ultrafiltration is responsible for removing excess fluid from the patient’s body. In hemodialysis, this is achieved by creating a pressure difference between the blood and the dialysate compartments, physically pushing the water across the membrane. In peritoneal dialysis, the high concentration of dextrose in the dialysate creates an osmotic pull, drawing water out of the blood and into the dialysate solution.
Variations Based on Dialysis Type
The way dialysate is prepared and delivered differs significantly between hemodialysis (HD) and peritoneal dialysis (PD). In HD, the dialysate is typically mixed on-site by a machine just before use. Ultra-pure water is combined with concentrated solutions of electrolytes and buffers, flowing rapidly past the patient’s blood inside the dialyzer.
For PD, the fluid is pre-mixed in sterile bags and instilled directly into the patient’s abdominal cavity. The lining of the abdomen, called the peritoneum, acts as the semipermeable membrane. The notable variation is the use of high concentrations of dextrose or other osmotic agents in PD dialysate to drive the fluid removal process. This is a batch-style process where the fluid “dwells” for several hours before being drained and replaced.