Capillaries, having no smooth muscle, are not generally thought to actively participate in blood flow control. However, in a series of studies in the 1980s and 1990s, Tyml and others showed that capillaries in skeletal muscle are capable of responding to vasoactive stimuli such as norepinephrine by producing vasoconstriction elsewhere in the network and hence decreasing capillary flow. The implication was that these signals were axially propagated along the vessel wall. In the 1990s, Sarelius and coworkers showed that capillaries could sense contraction of underlying skeletal muscle fibers and send a dilator signal to upstream arterioles, thus increasing flow through the "active" capillary network. They showed that this signal was specific to capillaries associated with actively contracting muscle fibers. This mechanisms explains how capillary recruitment can be matched closely with metabolic requirements, despite the fact that the controllers of these vessels are located upstream of the capillaries that they control, and often remote from the contracting fibers.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.