Dialysis is a life-sustaining medical process that takes over the function of failing kidneys. The procedure works by circulating the patient’s blood, or utilizing a membrane within the body, against a special fluid called the dialysis solution (dialysate). The dialysate facilitates the removal of harmful waste products, toxins, and excess fluid from the bloodstream. Formulated to mimic the body’s healthy fluid composition, the solution is fundamental to restoring the patient’s internal chemical balance.
How Dialysis Solutions Clean the Blood
The cleansing action of the dialysis solution relies on three physical principles that govern the movement of substances across a semipermeable membrane.
Diffusion
Diffusion describes the passive movement of solutes, such as urea and creatinine, from an area of higher concentration to an area of lower concentration. Since the blood is saturated with waste products and the dialysis solution contains very few, the toxins naturally migrate out of the blood and into the solution.
Osmosis
Osmosis is the movement of water across the membrane based on the concentration of solutes. A concentration gradient is established to pull excess fluid from the blood into the solution. This mechanism helps manage the patient’s fluid volume, preventing the buildup of water that the kidneys would normally excrete.
Ultrafiltration
Ultrafiltration is the removal of fluid volume driven by a pressure gradient. A hydraulic pressure difference is created between the blood and the solution compartments. This pressure difference pushes excess water and dissolved substances out of the blood, ensuring waste is cleared and fluid balance is restored.
Essential Chemical Components
The dialysis solution’s precise chemical makeup is designed to normalize the patient’s electrolyte levels and correct metabolic imbalances. The solution primarily consists of highly purified water, which must be treated rigorously to remove contaminants, bacteria, and trace elements. This purified water acts as the solvent for all dissolved components.
The solution includes specific concentrations of electrolytes such as sodium, potassium, calcium, and magnesium. Sodium maintains the fluid’s tonicity, preventing rapid shifts in fluid balance that could cause a drop in blood pressure. Concentrations of ions like potassium are set lower than those in the patient’s blood to promote their removal and prevent life-threatening accumulation.
A buffering agent, most commonly bicarbonate, is incorporated to address metabolic acidosis, a common complication of kidney failure. Bicarbonate diffuses into the blood, helping to neutralize excess acid and maintain the blood’s pH balance. Sometimes, a precursor like lactate or acetate is used, which the body converts into bicarbonate during treatment.
Variations for Hemodialysis and Peritoneal Dialysis
The composition and delivery of the dialysis solution differ significantly depending on the method of treatment: hemodialysis or peritoneal dialysis.
Hemodialysis Solution
For hemodialysis, the dialysate is typically mixed on-site by a machine that combines purified water with concentrated solutions. This allows for rapid and precise adjustment of electrolyte concentrations, such as potassium and calcium, to suit the patient’s immediate needs during the session. The machine continuously monitors the mixture before it is pumped to the dialyzer, where it flows countercurrent to the patient’s blood.
Peritoneal Dialysis Solution
Peritoneal dialysis utilizes the patient’s own peritoneal membrane, the lining of the abdominal cavity, as the semipermeable filter. The solution is delivered in sterile, pre-packaged bags and relies on an osmotic agent to drive ultrafiltration, making it fundamentally different from hemodialysis. Instead of relying on a machine-driven pressure gradient, the solution contains high concentrations of an osmotic agent, such as dextrose (glucose) or icodextrin.
The high concentration of these agents creates a strong osmotic gradient that pulls excess water from the blood vessels lining the peritoneum into the abdominal cavity. After the prescribed “dwell time,” the fluid, now laden with waste and excess water, is drained and replaced with a fresh solution. This method allows for continuous, gentle filtration, often performed at home.