78
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
■ FIGURE 4.13 Cardiac muscle isotonic contractions. The le ft panel shows how muscle length and tension
are measured in vitro. The lower end of the muscle is attached to a w eight (load) that is lifted up from an
immovable platform as the muscle develops tension and shortens (DL). A bar attached to the top of the
muscle can be moved to adjust initial muscle length (preload). The rig ht panel shows changes in tension
and length during contraction. The periods from
a
to
b
and from
c
to
d
represent periods of isom etric con-
traction and relaxation, respectively. Muscle shortening (DL) occurs between
b
and c, which occurs when
the developed tension
(DT)
exceeds the load.
the velocity will be slower. Higher weights
further reduce the velocity until the weight
can no longer be lifted and the contraction
of the biceps muscle becomes isometric. The
x-intercept in the force-velocity diagram (see
Fig. 4.15) is the point at which the afterload is
so great that the muscle fiber cannot shorten.
The x-intercept therefore represents the maxi-
mal isometric force. The y-intercept represents
an extrapolated value for the maximal velocity
(V
) that would be achieved if there was no
max
afterload. The value is extrapolated because it
cannot be measured experimentally (a muscle
will not contract in the absence of any load).
■ FIGURE 4.14 Effects of afterload on m yocyte
shortening. Increased afterload (curves
a
to c)
decreases the degree of muscle shortening and
maximal velocity of shortening at a given preload,
which is measured as the change in length over
tim e shortly after muscle begins to shorten.
V
represents the intrinsic capability of the
max
muscle fiber to generate force independent of
load, and therefore changes when inotropy is
altered, as discussed later in this chapter.
It is important to note that a cardiac mus-
cle fiber does not operate on a single force-
velocity
curve
(Fig.
4.16).
As
previously
Afterload (Force)
■ FIGURE 4.15 Force-velocity relationship.
Increased afterload (which requires increased force
generation) decreases velocity of shortening by
the muscle fiber. The x-intercept represents the
maximal isometric force that occurs when the load
exceeds the muscle's force-generating capacity,
thus preventing muscle shortening; the y-intercept
represents the maximal velocity of shortening
(Vmax) extrapolated to zero load. Points
a, b,
and
c
represent the maximal shortening velocity gener-
ated in Figure 4.14 for three increasing afterloads.
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