Microvascular Beds - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 175

6.1 MICROCIRCULATION PHYSIOLOGY

Throughout the first two sections of this textbook we have restricted our discussion of
fluid mechanics to flows that remain within their particular container (e.g., blood vessel).
This means that mass continuity only includes inflows, outflows, changes in the container
volume, or changes in the fluid density. However, in the microcirculation, there is trans-
port of fluid and molecules out of the blood vessel into the interstitial space and vice
versa. This transport occurs across the blood vessel wall. To model this phenomenon, there
is no discrete inflow or outflow along the vessel wall; permeability concepts will need to
be included. This transport across the vascular wall is arguably the most crucial function
that the entire cardiovascular system performs. Without this function, few cells within the
human body would be able to obtain oxygen or glucose, and they would not be able to
remove cellular wastes from their local environment. Microvascular beds are composed of
capillaries, which are small thin blood vessels. In fact, the vascular wall of capillaries is
composed of a single layer of endothelial cells without any substantial connective tissue or
smooth muscle cell layer. These cells are highly permeable and partially regulate the trans-
port of molecules across the vascular wall. It has been estimated that there are on the
order of 35 billion capillaries within the systemic circulation, with a total surface area of
approximately 2000 square meters, all of which are available for molecular transport.
Nearly all cells are within 50

m of a capillary, which is within the free diffusion distance
for all essential nutrient transportation (this will be discussed more in Chapter 7). Cells
outside of this range would require some form of facilitated (and possibly active) transport
to obtain nutrients.
At the point where the arterial circulation has branched into arterioles that are on the
order of 10 to 15

μ

m in diameter, the regulation of the blood flow through these vessels
becomes critically important to the capillary beds. Vessels within this diameter range are
typically called metarterioles (or terminal arterioles). These are the last blood vessels that
have a smooth muscle cell layer prior to blood entering the capillary bed (most capillaries
have an internal diameter ranging between 5 to 8

μ

m). At the point where a metarteriole
diverges into a capillary, there is one last smooth muscle cell that encircles the blood ves-
sel. This smooth muscle cell is termed the precapillary sphincter. Like all sphincters, it
closes the lumen of the particular vessel (or channel) when it constricts and opens the
lumen when it dilates. Therefore, in the case of precapillary sphincter constriction, the
resistance to flow through the particular downstream capillary network increases signifi-
cantly. In the microcirculation, it is most likely that when a precapillary sphincter con-
stricts, blood is shunted from the downstream capillary bed to a different microvascular
bed because the resistance to flow through the constricted sphincter is too large. Once
blood has passed through a capillary, blood vessels begin to converge into venules, mak-
ing its way back to the heart via the inferior and superior vena cava. This leads to an
important definition. A true capillary is defined by divergent flow at its inlet and conver-
gent flow at its outlet ( Figure 6.1 ).
It is important to note here (although we will discuss this more fully in Section 6.3 ) that
the metarterioles (and hence the precapillary sphincters) are in a close proximity to the
capillaries themselves. This allows for the local changes in nutrient concentrations

μ