Dialysis: Passive diffusion through a membrane
Introduction to Dialysis
The goal of this lab session is to understand dialysis and the factors effecting performance. Hemodialysis is the most frequently prescribed type of dialysis treatment in the United States for severe kidney disease and involves circulating the patient's blood outside of the body through a dialysis machine.
Dialysis treatment replaces the function of the kidneys, which normally serve as the body's filtration system. Through the use of a blood filter and a dialysate solution, the treatment removes waste products and excess fluids from the bloodstream, while maintaining the proper chemical balance of the blood. The blood is filtered and cleansed inside the hemodialyzer and returned to the body.
The basic component of the kidney is the nephron (see Figure 1). There are about one million nephrons per kidney. Dialysis becomes necessary if more than 90% of the nephrons are not functioning.
The nephron is divided in 4 sections:
The glomerulus is composed of a network of capillaries which sit over the Bowman's capsule. When blood come from the afferent arteriole, 10% of the circulating plasma and ions are absorbed. Considering that cardiac output is 5 liters per minute, and that 20 to 25% of the blood circulates through the kidneys, nearly 180 liters are absorbed per day by the Bowman's capsule. Each of the nephron sections is responsible for absorption, secretion, and reabsorption of plasma and ions (see Figure 2).
The mass transfer in the nephron is complex: it includes
filtration, osmosis, diffusion convection and active transport (see
Urine is composed of water, end products of protein metabolism and other waste, and excess ions and other components of metabolic processes. By controlling ion reabsorption, the kidneys have an important role in acid-base regulation ( H+ and COH-) and also play a role in vasoconstriction (through Na+ concentration regulation).
The Artificial Kidney
When in contact with a porous membrane, the smallest solutes pass from the blood compartment to the other compartment. Thus, making blood flow between layers of this membrane or inside hollow fibers, with dialysate flowing on the outside, the blood can be cleaned of some solutes. This effect is enhanced using a counter current system where the blood flows in one direction and the dialysate flows in the other. The average concentration difference across the membrane is larger this way.
The majority of hemodialysis treatments in the United States use hollow fiber dialyzers. A hollow fiber dialyzer such as you will use in this lab is composed of thousands of tube-like hollow fiber strands encased in a clear plastic cylinder several inches in diameter. There are two compartments within the dialyzer: the blood compartment and the dialysate compartment. The membrane that acts as a barrier that separates these two compartments is semipermeable: in general smaller molecules pass through it but larger molecules do not. Several factors such as hydrostatic pressures, solute size and concentration, and type of membrane play an important role in regulating exchange between the compartments. See Figure 6.
As blood is pushed through the blood compartment in one direction, suction or vacuum pressure pulls the dialysate through the dialysate compartment in a countercurrent, or opposite direction. These opposing pressures drain excess fluids out of the bloodstream and into the dialysate, a process called ultrafiltration.
Diffusion moves waste products in the blood across the membrane into the dialysate compartment. Electrolytes and other chemicals in the dialysate solution can cross the membrane into the blood compartment.
Large surfaces, such as in this hollow fiber dialyzer,
are needed to clean a person's blood.
Mass transfer and Clearance
Figure 7: Hemodialysis system
In order to circulate the patient's blood outside of the body, two needles are inserted into the patient's vein, or access site, and are attached to tubing connected to the dialyzer and dialysis machine that monitors and maintains blood flow and administers dialysate. For patients with temporary treatment needs, access to the bloodstream is gained by inserting a catheter into the subclavian vein near the patient's collarbone. Patients in long-term dialysis require fistulas or grafts for stronger and more durable access.
An alternative to hemodialysis is peritoneal dialysis where the patient's peritoneum acts as a blood filter. A catheter is surgically inserted into the patient's abdomen. During treatment, the catheter is used to fill the abdominal cavity with dialysate. Waste products and excess fluids move from the patient's bloodstream into the dialysate solution. After a waiting period, the waste-filled dialysate is drained from the abdomen, and replaced with clean dialysate.
Conductimeter - Wheatstone bridge
A Wheatstone bridge is basically two voltage dividers connected in
parallel, where the output signal comes from the difference in their
individual outputs. The Wheatstone bridge has good sensitivity and helps
to get useful values when the part of the signal changing due to the input
change is a small percentage of the output signal.
The Early Development of Dialysis and Transplantation
History of Dialysis