Biomedical Engineering Reference
In-Depth Information
2.3
Chapter 2.3
Signals and systems
John Semmlow
2.3.1 Biological systems
Given this systems-based approach to physiology and
medicine, it is not surprising that early bioengineers ap-
plied their engineering tools, especially those designed
for the analysis of systems, to some of these physiological
systems. Early applications in bioengineering research
include the analysis of breathing patterns and the oscil-
latory movements of the iris muscle. Applications of
basic science to medical research date from the eigh-
teenth century. In the late nineteenth century, Einthoven
recorded the electrical activity of the heart, and
throughout that century, electrical stimulation was used
therapeutically (largely to no avail). Although early re-
searchers may not have considered themselves engineers,
they did draw on the engineering tools of their day.
The nervous system, with its apparent similarity to
early computers, was another favorite target of bio-
engineers, as was the cardiovascular system with its ob-
vious links to hydraulics and fluid dynamics. Some of
these early efforts are discussed in the sections on system
and analog models (Sections 2.3.3.2 and 2.3.3.3). As
bioengineering has expanded into areas of molecular bi-
ology, systems on the cellular, or even subcellular levels,
have become of interest.
Regardless of the type of biological system, its scale, or
its function, we must have some way of interacting with
that system. Interaction or communication with a bi-
ological system is done through biosignals. The commu-
nication may only be one-way, such as when we attempt
to infer the state of the system by measuring various
biological or physiological variables to make a medical
diagnosis. From a systems analytic point of view, changes
in physiological variables constitute biosignals. Common
signals measured in diagnostic medicine include elec-
trical activity of the heart, muscles and brain; blood
pressure;
A system is a collection of processes or components
that interact for some common purpose, although that
purpose may only be the invention of human intellect.
Many systems of the human body are based on func-
tion. The cardiovascular system's function is to deliver
oxygen-carrying blood to the peripheral tissues. The
pulmonary system is responsible for the exchange of
gases [primarily oxygen (O
2
) and carbon dioxide
(CO
2
)] between the blood and air, whereas the renal
system regulates water and ion balance and adjusts the
concentration of other types of ions and molecules.
Some systems are organized around mechanism rather
than function. The endocrine system mediates a range
of communication functions using complex molecules
distributed through the blood stream. The nervous
system performs an enormous number of tasks using
neurons and axons to process and transmit information
coded as electrical impulses.
The study of classical physiology and of many medical
specialties is structured around human physiological sys-
tems. (The term
classical physiology
is used here to mean
the study of whole organs or organ systems as opposed to
newer molecular-based approaches.) For example, cardi-
ologists specialize in the cardiovascular system, neurolo-
gists in the nervous system, ophthalmologists in the visual
system, nephrologists in the kidneys, pulmonologists in
the respiratory system, gastroenterologists in the di-
gestive system, and endocrinologists in the endocrine
system. There are medical specialties or subspecialties to
cover most physiological systems. (Another set of medical
specialties is based on common tools or approaches, in-
cluding surgery, radiology, and anesthesiology, whereas
one specialty, pediatrics, is based on the type of patient.)
heart
rate;
blood
gas
concentrations
and
