Microvascular Beds - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 176

FIGURE 6.1 Typical structure of a capillary
bed, which is fed by an arteriole. Metarterioles are
the last vessels on the arterial side that have a
smooth muscle layer. These vessels highly regulate
the flow through the microvascular beds. The last
endothelial cell associated with a smooth muscle
cell is termed the precapillary sphincter, and it is
this cell that determines blood flow through
capillaries. True capillaries are defined with diver-
gent flow at their inlet and convergent flow at their
outlet. Capillaries feed into venules. Adapted from
Guyton and Hall (2000).

Metarteriole

Tr u e
capillaries

Arteriole

Precapillary
sphincters

Venule

(oxygen, glucose, among others) or waste concentrations (carbon dioxide, metabolites)
within the capillary bed to partially regulate the dilation and constriction of the precapil-
lary sphincter. Due to the closeness of the capillaries and the precapillary sphincters, the
needs of the tissue can rapidly, within a few seconds, be addressed. There is not a large
demand for an overarching “command center” for every capillary bed within the cardio-
vascular system and furthermore, the response time of such a center would potentially be
too large to accommodate tissue needs.
Flow through the capillary beds is typically not continuous. Depending on the tissue
that the capillary bed is feeding, the flow can alternate between on and off every few sec-
onds or every minute, in a process termed vasomotion. As stated above, the changes in
flow are controlled by the dilation and constriction of the precapillary sphincters. Tissue
oxygenation is the most critical factor that affects the local vasomotion. A tissue that is
under hypoxic conditions for no more than a few seconds will rapidly induce the precapil-
lary sphincters to dilate. The most common cause for decreases in oxygen concentration is
an increase in oxygen usage through a cellular respiration process. Under these conditions,
the flow through that particular vascular network will be turned on for the majority of the
time and it would only be turned off for very short periods of time, so that other tissue
locations do not become hypoxic. This condition would persist until the tissue oxygen con-
centration returns back to normal levels. Once the oxygen levels within the capillary bed
return to a normal level, the flow throughout the network returns to its normal on/off
pattern.
As stated above, flow through the capillary is not the only flow that occurs. There is
also a flow of molecules across the endothelial cell wall; nutrients typically leave the blood
vessel, while metabolic wastes typically enter the blood vessel. The capillary wall is com-
posed of a single endothelial cell and has a thickness on the order of 0.5 m. The extravas-

cular side of the endothelial cell is directly connected to the extracellular matrix of the
tissue. In most capillary beds, there are two major methods for molecular transport across the
endothelial cell barrier: the intercellular cleft and the plasmalemmal vesicles ( Figure 6.2 ).
Both of these methods will be discussed in more detail throughout the remainder of this
chapter.
The intercellular cleft is the location where two neighboring endothelial cells adhere to
each other. Endothelial cells do not come directly into contact with each other, leaving a

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