Biomedical Engineering Reference
In-Depth Information
(salts) and waste metabolites generated through ordinary digestive and metabolic
processes. As a result, the concentrations of such metabolites will increase to toxic
levels unless a device is introduced to help remove them. The major source of waste
metabolites is the liver since this is where most of the energy conversion processes
in the body take place. The end products of carbohydrate and fat metabolism tend
to be water and carbon dioxide, both of which are removed from the body through
respiration. The end products of protein metabolism, however, are generally elimi-
nated through the kidneys. Urea is the largest mass of waste metabolite produced in
the liver from protein metabolism.
Dialysis is based upon diffusion of a substance such as urea from a region of
high concentration (blood) to a region of low concentration (dialysate). Two differ-
ent models are developed to understand the clearance characteristics: (1) describing
the dialysis treatment based on the production and clearance of urea to understand
the patient characteristics, and (2) evaluating the performance characteristics of
the dialyzers. To understand the urea kinetics within the body, a two-compartment
model for generation and elimination of urea in the body (Figure 10.7) is an ap-
proach. Urea is generated in the liver and passed into the bloodstream (plasma
compartment). The bloodstream is considered a single “compartment” in the body
that acts as a reservoir for urea. Urea also distributes intracellularly in some cell
types, which is grouped into the tissue compartment. Urea can be removed from
the body by the kidney, eliminated with urine. If the kidneys are not functioning,
urea is eliminated by dialysis and is lumped as one parameter of urea clearance.
Differential equations are written similar to the multicompartmental model de-
scribed in Section 10.2.4 and production and clearance rates are approximated.
For evaluating the performance of the dialyzers, modeling is also focused on
evaluating the clearance rate of urea (or other components such as creatinine, vi-
tamin B
12
, and
-2-microglobulin). During hemodialysis, blood is perfused from
the body and passed through a dialyzer before returning to the body (discussed in
Chapter 1). Hollow fiber dialyzers consist of thousands of hollow fibers contained
in a shell. Unlike the shell, which is made of impermeable material, the hollow fib-
ers are permeable to the passage of low molecular weight molecules across the fib-
er. Blood flows through the center (lumen side) of the fibers. Dialysate fluid flows
on the exterior of the fibers (shell side). Dialysate is a fluid that contains electrolyte
concentrations similar to that found in a healthy person's blood. The difference in
β
Figure 10.7
Two-compartment system for urea elimination.
