Biomedical Engineering Reference
In-Depth Information
FIGURE 4.8 The relationship between ani-
mal mass and heart rate. As the mass of the
animal increases, there is a general decrease in
heart rate. The relationship between these two
measurements can be correlated to many dif-
ferent properties of the animal, as described in
the text.
Canary
1000
Rat
Mouse
Turkey
Large
dog
Duck
100
Small
dog
Human
Horse
10
10
100
1000
10,000
100,000 1,000,000
Animal mass (gram)
relationship, some of which account for if the animal is warm-blooded or cold-blooded,
what the daytime activity of the animal is, and how the animal developed from an evolu-
tionary standpoint. For this textbook, it is important to keep in mind that, in general, ani-
mals with a lower mass will have a higher heart rate.
4.4 HEART MOTION
During cardiac myocyte contraction, the heart as a whole moves in three-dimensional
space. This movement affects the blood flow within the heart and also the blood flow
within the coronary blood vessels. Before discussing the motion of the heart, one must first
understand how to model muscle contraction. In 1939, A.V. Hill modeled the active con-
traction of heart muscle using a three-component model ( Figure 4.9 ). The three compo-
nents of this model are a parallel element, a contractile element, and a series element. The
parallel element models the connective tissue of the heart. This component is typically
modeled as a purely elastic material, and the empirical values that describe its responses
to a load can be calculated from standard length-tension curves of explanted muscle. The
contractile element models the actin-myosin mechanism that actively causes muscle con-
traction. This component of the system is modeled as a temporally active element that can
generate tension and/or displacements. Known values for cross-bridge formation, includ-
ing forces and displacements, can depict this element. The series element models the
FIGURE 4.9 A.V. Hill's model for heart mus-
cle contraction, which couples the active contrac-
tion component, a viscoelastic relaxation, and a
parallel resistance to motion. This is perhaps the
most often used model of
Parallel element
Myosin
F
F
Actin
cardiac muscle
contraction.
Contractile element
Series element
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