Biomedical Engineering Reference
In-Depth Information
and dispose of gaseous waste; to eliminate excess fluid and soluble toxins; and
to perform other tasks. These systems—labeled respiratory, circulatory, diges-
tive, neurological, and so on—share several features.
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Physiological systems are spatially distributed. Food has to get from
mouth to anus. Urine is made in the kidneys but has to exit the urethra.
Blood may be pumped by the heart but has to reach the great toe.
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Physiologic systems are generally space-filling and structurally fractal.
Each cell has demand for nutrients; each cell excretes waste. Information
has to travel from the brain throughout the body. While not every physiol-
ogic system is self-similar at all levels of granularity, there is typically a
nested architecture that facilitates function: microvillus to villus to intesti-
nal mucosa; alveolus to alveolar unit to bronchial segment; capillary to ar-
teriole to artery.
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Physiologic systems are functionally integrated. After eating, blood is re-
distributed to the gut and splanchnic circulation. When alveoli become
atelectatic, blood is shunted away from these hypoxic regions. Ingestion
and delivery of excess fluid to the circulation is quickly followed by aug-
mented production of urine.
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Physiologic systems have characteristically variable time signatures that
lose their variability in aging and disease (1). Instantaneous cardiac and
respiratory rates vary from one event (heart beat or breath) to the next.
Many hormones exhibit not only diurnal variation, but also a superim-
posed pattern or irregular pulses. The product of physiologic systems—
such as gait, which combines neural, musculoskeletal, cardiac, and respi-
ratory systems into a semivoluntary activity (one usually does not think
about putting one foot in front of the next)—display characteristically
variable time signatures.
The first three features of physiologic systems have medical consequences.
Both aging and illness can compromise one or more physiologic systems. Man-
agement of such compromise was, until about fifty years ago, directed exclu-
sively toward minimizing the performance demand placed on the system. For
example, in advancing pulmonary insufficiency, patients were progressively
confined to home, to chair, and finally to bed. Each organ system had a critical
level of compromise, and once the compromise exceeded the critical level, the
patient simply died.
Two major advances in the last half-century have changed the clinical tra-
jectory. The first major advance was the development of mechanical supports
for failing mechanical systems. Ventilators (respirators), ventricular assist de-
vices, and renal replacement therapies (dialysis machines) have come into wide-
spread clinical use. These supports have evolved to the point where individual
