■ FIGURE 7.13 Microvascular organization in skeletal muscle. Long, parallel muscle fibers are each sur-
rounded by multiple, parallel capillaries that arise from arterioles. As shown in the cross section, there are
typically 2 to 3 capillaries per muscle fiber
(C /F ra tio),
although that varies depending on the muscle type.
Arrows represent direction of flow.
rise to capillaries that generally run parallel
to the muscle fibers. Because a given region of
muscle may be served by multiple arterioles,
the direction of flow in some capillaries may
be opposite to the direction of flow in nearby
capillaries. Each muscle fiber, which can be
20 to 40 pm in diameter, is surrounded by
three to four capillaries. Because more than
one muscle fiber may share an adjacent capil-
lary, the overall capillary-to-fiber ratio is 2 to
3, depending on the type of muscle. Muscle
fibers that have a high oxidative capacity gen-
erally have a higher capillary-to-fiber ratio
than fibers that have a low oxidative capac-
ity, but a high anaerobic (glycolytic) capacity.
Muscles with a higher oxidative capacity and
a greater number of capillaries generally have
a higher maximal flow capacity.
W hen the muscle is not contracting, rela-
tively little oxygen is required and only about
one-fourth of the capillaries are perfused. In
contrast, during muscle contraction and active
hyperemia, all the anatomical capillaries may
be perfused, which increases the number of
flowing capillaries around each muscle fiber
(termed capillary recruitment). This anatom-
ical arrangement of capillaries and the abil-
ity to recruit capillaries decreases diffusion
distances, leading to an efficient exchange of
gases and m olecules between the blood and
the myocytes, particularly under conditions
of high oxygen demand.
In resting humans, almost 20% of cardiac out-
put is delivered to skeletal muscle. This large
cardiac output to muscle occurs not because
blood flow is exceptionally high in resting
muscle, but because skeletal muscle makes up
about 40% of the body mass. In the resting,
noncontracting state, muscle blood flow is about
3 mL/min per 100 g. This resting flow is much
less than that found in organs such as the brain
and kidneys, in which “resting” flows are about
55 and 400 mL/min per 100 g, respectively.
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