72
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
elastic modulus (“stiffness”) of the tissue.
The elastic modulus of a tissue is related to
the ability of a tissue to resist deformation;
therefore, the higher the elastic modulus,
the “stiffer” the tissue. When the muscle is
stimulated at the increased preload, there
will be a larger increase in active tension
(curve b) than had occurred at the lower
preload. If the preload is again increased,
there will be a further increase in active
tension (curve c). Therefore,
increases in
preload lead to an increase in active tension.
Not only is the magnitude of active tension
increased, but also the rate of active tension
development (i.e., the maximal slope with
respect to time of the tension curve during
contraction). The duration of contraction
and the time-to-peak tension, however, are
not changed.
If the results shown in Figure 4.6 are plot-
ted as tension versus initial length (preload),
a
length-tension
diagram
is
generated
(Fig. 4.7). In the top panel, the passive ten-
sion curve is the tension that is generated
as the muscle is stretched prior to contrac-
tion. Points
a, b,
and
c
on the passive curve
correspond to the passive tensions and ini-
tial preload lengths for curves
a, b,
and
c
in
Figure 4.6 prior to contraction. The total
tension curve represents the maximal tension
that occurs during contraction at different
initial preloads. The total tension curve is the
sum of the passive tension and the additional
tension generated during contraction (active
tension). The active tension, therefore, is
the difference between the total and passive
tension curves; it is plotted separately in the
bottom panel of Figure 4-7. The active ten-
sion diagram demonstrates that as preload
increases,
there is
an
increase in active
tension up to a maximal limit. The maxi-
mal active tension in cardiac muscle corre-
sponds to a sarcomere length of about 2.2
pm. Because of the passive mechanical prop-
erties of cardiac myocytes, their length sel-
dom exceeds 2.2 pm at maximal ventricular
EDVs.
This discussion described how changes in
preload affect the force generated by cardiac
muscle fibers during isometric contractions
Length
■ FIGURE 4.7 Length-tension relationship for
cardiac muscle undergoing isom etric contraction.
The to p panel shows that increasing the preload
length from points
a
to
c
increases the passive
tension. Furthermore, increasing the preload
increases the total tension during contraction as
shown by
arrows a, b,
and
c,
which correspond to
active tension changes depicted by curves a, b,
and c in Figure 4.6. The length of the arrow is the
active tension, which is the difference between
the total and passive tensions. The bottom panel
shows that the active tension increases to a
maximum value as preload increases.
(i.e., with no change in length).
Cardiac
muscle
fibers,
however,
normally shorten
when they contract (i.e., undergo isotonic
contractions). If a strip of cardiac muscle in
vitro is set at a given preload length and stim-
ulated to contract, it will shorten and then
return to its resting preload length (Fig. 4.8).
If the initial preload is increased and the mus-
cle stimulated again, it will ordinarily shorten
to the same minimal length, albeit at a higher
velocity of shortening.
The length-tension relationship, although
usually used to describe the contraction of
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