As in many other organ systems, skeletal muscle microcirculation consists of, in series, a divergently branching arteriolar network, a system of branching capillaries (the principal exchange bed for respiratory gases and nutrients), and a converging venular system. Until the early 1980s, most people viewed skeletal muscle microcirculation as a simple branching hierarchy, with each branch level being in series with those proximal and distal to it, and with all elements at any specified branch level being functionally equivalent and arranged in parallel with each other. During the 1980s and subsequently, rekindled interest in understanding the organization of skeletal muscle microcirculation, particularly arterioles and capillaries, has led to a better understanding of the organization of this system.
First, it became clear that like many other organs, skeletal muscle has prominent arcading arteriolar systems. These arteriolar arcades presumably serve to equalize the driving pressure for flow through subsequent branches across wide regions of the tissue; each arcade gives rise to several smaller arterioles that each branch two or three times and then give rise to capillaries. In general, the arcading arterioles are larger (greater than about 80 to 100 mm in diameter) than the terminally branching arteriolar systems. These larger vessels are also the arterioles that are most likely to be "paired" with collecting venules: The functional consequences of this anatomical arrangement for arteriolar control will be discussed later.
Second, the terminally branching arteriolar trees (diameter usually less than about 65 mm, down to approximately 10 mm) are not paired anatomically with venules and are the controllers of capillary perfusion (see Section VI). Capillaries arise from the distal ramifications of these arteriolar trees. They are arranged in groups (variously called networks, units, or modules), and capillary recruitment occurs when these groups of capillaries are perfused as a result of dilation of the small arterioles from which they arise.
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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.