Biomedical Engineering Reference
In-Depth Information
generation and the presentation of a spastic stretch reflex in the clinical examination. This
girl uses her hip musculature, right more than left, to a much greater degree than her ankle
plantar flexors to propel herself forward during gait.
This cursory case examination illustrates the process whereby differences from normal
gait are recognized and the associated biomechanical etiology is explored. Some of the
effects on gait of neuromuscular pathology in the sagittal plane have been considered in
this discussion. Clinical gait analysis can also document and elucidate gait abnormalities
associated with static bony rotational deformities. It also is useful in areas of clinical research
by documenting treatment efficacy associated with bracing, surgery, and so forth. It should
be noted, however, that although engineers and applied physicists have been involved in this
work for well over a hundred years, there remains significant opportunity for improvement
in the biomechanical protocols and analytical tools used in clinical gait analysis; in other
words, there remains much to learn.
4.7 CARDIOVASCULAR DYNAMICS
One major organ system benefiting from the application of mechanics principles is car-
diovascular system dynamics, or
, the study of the motion of blood. From a
functional point of view, the cardiovascular system is driven by a complex pump, the heart,
that generates pressure resulting in the flow of a complex fluid, blood, through a complex
network of complex pipes, the blood vessels. Cardiovascular dynamics focuses on the
measurement and analysis of blood pressure, volume, and flow within the cardiovascular
system. The complexity of this elegant system is such that mechanical models, typically
formulated as mathematical equations, are relied on to understand and integrate experi-
mental data, to isolate and identify physiological mechanisms, and to lead ultimately to
new clinical measures of heart performance and health and guide clinical therapies.
As described in Chapter 3, the heart is a four-chambered pump connected to two main col-
lections of blood vessels: the systemic and pulmonary circulations. This pump is electrically
triggered and under neural and hormonal control. One-way valves control blood flow. Total
human blood volume is approximately 5.2 liters. The left ventricle, the strongest chamber,
pumps 5 liters per minute at rest, almost the body's entire blood volume. With each heart-
beat, the left ventricle pumps 70 ml, with an average of 72 beats per minute. During exercise,
left ventricular output may increase sixfold, and heart rate more than doubles. The total com-
bined length of the circulatory system vessels is estimated at 100,000 km, a distance two and
one half times around the earth. The left ventricle generates approximately 1.7 watts of
mechanical power at rest, increasing threefold during heavy exercise. One curious constant
is the total number of heartbeats in a lifetime, around one billion in mammals [31]. Larger ani-
mals have slower heart rates and live longer lives, and vice versa for small animals.
hemodynamics
4.7.1 Blood Rheology
Blood is composed of fluid, called plasma, and suspended cells, including erythrocytes (red
blood cells), leukocytes (white cells), and platelets. From a mechanical point of view, a fluid is
distinguished from a solid as follows. Figure 4.34 shows a two-dimensional block of solid
material (left panel) subjected to two opposite, parallel, transverse external forces, depicted
by the solid arrows at the top and bottom surfaces. This applied shear force is resisted by
