Biomedical Engineering Reference
In-Depth Information
Interstitium or interstitial volume
1
that is separated from the plasma
2
by the capillary
walls
Plasma that is separated from the blood
Variables tracked in compartmental analysis are typically quantity or concentration of a
solute, temperature, and pressure. Substances of interest are exogenous, such as a drug or
tracer, or endogenous, such as glucose or an enzyme or hormone like insulin. Radioactive
and stable isotopes
3
are used to track the dispersion in a compartmental system and are
easily measured. Often a tracer dose of a radioactive isotope is used so the radiation emitted
is small and does not interfere with the system. Typically, a tracer dose is less than 1 percent of
the total amount of solute in the compartment. High-performance liquid chromatography
(HPLC) is used to measure proteins and other macromolecules. Radioimmunoassay is used
to measure hormones or proteins, a method based on the immune response of the body to
an antigen, which is then bound to an antibody. Other modes of tracking involve injecting a
dye (e.g., Evans blue) at one site in the cardiovascular system and measuring the concentration
at one or more sites as a function of time.
Fluid in the body is separated into intracellular and extracellular fluid. A typical 70 kg
adult contains 42 L of fluid, which is approximately 60 percent of the total body weight.
Fluid in the body is tightly regulated so a relatively constant fluid volume is maintained.
Intracellular fluid consists of the fluid inside the approximately 75 trillion cells in the body,
totaling about 28 L and 40 percent of total body weight. Extracellular fluid consists of two
major compartments, the interstitial fluid compartment and the plasma, and two minor
compartments, the transcellular fluid and the lymph. The interstitial fluid is 11 L and the
plasma is 3 L. The transcellular fluid includes fluids from the synovial, peritoneal, peri-
cardial, intraocular spaces, and the cerebrospinal fluid. These compartments contain
approximately 1 to 2 L of fluid. Lymph is the fluid that originates in the interstitial fluid that
diffuses into the lymphatic system through lymph capillaries. It returns to the venous
plasma after passing through the lymph nodes and has a volume of approximately 1 L.
The blood in the circulatory system is a mixture of intracellular and extracellular fluid
totaling 5 L of fluid and is 7 percent of total body weight. It consists of the plasma (3 L)
and the fluid in the red blood cells (2 L). Table 7.1 summarizes various volumes in the body.
Typically, we work with the plasma compartment rather than the blood compartment,
except when dealing with the cardiovascular system. Fluids in the body continually flow
from one compartment to another without much change in fluid volume. As will be
described more thoroughly in Section 7.3.5, fluid moves from the plasma to the interstitial
fluid through the arterial side of the capillary bed and returns from the interstitial fluid
to the plasma from the venous side of the capillary bed. Approximately 10 percent of the
interstitial fluid does not immediately return to the plasma but moves into the lymphatic
system through lymph capillaries by diffusion. The lymphatic system acts like a second
parallel circulatory system, with the lymph returning to the plasma after traveling through
1
Interstitial volume is the fluid that bathes the cells and is separated from the blood by capillaries.
2
Plasma is the noncellular part of the blood. The plasma volume bathes the blood cells.
3
Isotopes of an atom have the same atomic number but differ in mass. Radioactive isotopes disintegrate with
time and emit ionizing radiation.
