Dialysis is a medical treatment that takes over the work of failing kidneys, which are no longer able to adequately filter the blood. This life-sustaining process requires a specialized liquid known as dialysate, which acts as the medium for cleansing the body. Dialysate is a carefully formulated solution that draws impurities from the bloodstream, effectively simulating the natural filtration and balancing functions of a healthy kidney.
The Essential Components of Dialysate
The foundation of dialysate is highly purified water, which must undergo extensive treatment, often involving filtration and reverse osmosis, to remove harmful contaminants. This purified water serves as the solvent for a precise mixture of solutes designed to mirror the chemical makeup of healthy blood plasma.
The solution contains a balanced profile of electrolytes, including sodium, potassium, calcium, and magnesium. The concentrations of these minerals are meticulously adjusted to prevent the removal of necessary electrolytes from the blood while helping to restore the patient’s overall chemical balance. For instance, the sodium level in the dialysate is often customized to the patient’s needs to manage fluid balance and prevent complications like low blood pressure.
A buffering agent, typically bicarbonate, is another component added to the dialysate. Patients with kidney failure often develop acidosis, a condition where the blood becomes too acidic, and bicarbonate helps to correct this imbalance.
The concentration of bicarbonate in the dialysate is set higher than that in the patient’s blood, allowing the buffer to diffuse into the bloodstream. This transfer of bicarbonate helps to stabilize the blood’s pH level, which is a key function of healthy kidneys.
How Dialysate Cleans the Blood
Dialysate cleanses the blood through three primary physical processes that occur across a semipermeable membrane, which separates the blood from the dialysate. The first mechanism is diffusion, which governs the movement of waste products like urea and creatinine. These molecules are present in high concentrations in the patient’s blood but are nearly absent in the fresh dialysate, creating a steep concentration gradient.
Following this gradient, the waste solutes naturally move from the blood (high concentration) to the dialysate (low concentration). By continuously supplying fresh dialysate, the machine ensures this concentration gradient is maintained throughout the treatment, maximizing the removal of toxins.
The second process is osmosis, which involves the movement of water across the membrane driven by differences in solute concentration. However, the main way excess fluid is removed during dialysis is through ultrafiltration.
Ultrafiltration is achieved by creating a pressure difference across the semipermeable membrane, known as a hydrostatic pressure gradient. The dialysis machine actively lowers the pressure on the dialysate side of the membrane, which physically pulls excess water and dissolved substances from the blood. This pressure-driven removal allows for precise control of the patient’s total fluid volume, correcting the fluid overload that often accompanies kidney failure.
Dialysate Application in Hemodialysis and Peritoneal Dialysis
The method of delivering and managing dialysate differs significantly between the two main types of dialysis treatment.
Hemodialysis (HD)
In hemodialysis (HD), the dialysate is prepared instantaneously by a machine at the time of treatment. The machine mixes purified water with liquid concentrates containing the electrolytes and buffer to create the final solution. This freshly prepared dialysate flows through the dialyzer, or artificial kidney, in a direction opposite to the flow of the patient’s blood, which is known as counter-current flow. The used dialysate, now containing the patient’s removed waste and excess fluid, is a single-pass system and is immediately discarded.
Peritoneal Dialysis (PD)
Peritoneal dialysis (PD) uses the patient’s own abdominal lining, the peritoneum, as the semipermeable membrane, and the dialysate is managed differently. The fluid is pre-mixed and infused into the peritoneal cavity through a catheter, where it remains for a prescribed “dwell” time. During this period, the blood vessels lining the peritoneum exchange waste and fluid with the dialysate inside the cavity.
A key difference in PD dialysate is the inclusion of a high concentration of an osmotic agent, most commonly dextrose (a sugar). This dextrose creates a powerful osmotic gradient, which is the primary force that draws excess water from the patient’s blood across the peritoneal membrane and into the dialysate. After the dwell time, the fluid is drained from the abdomen and replaced with a fresh bag of solution to begin the cycle again.