Bulk Flow
A second mechanism for exchange is bulk
flow. This mechanism is important for the
movement of water and small lipid-insoluble
substances across capillaries.
Bulk flow of fluid
and electrolytes, and of small molecules, occurs
through intercellular clefts between endothelial
(see Fig. 8.1). These extracellular path-
ways are sometimes referred to as “pores.”
The physical structure of capillaries varies
considerably among organs; these differences
greatly affect exchange by bulk flow. Some cap-
illaries (e.g., skeletal muscle, skin, lung, and
brain) have a very “tight” endothelium and
continuous basement membrane (termed con-
tinuous capillaries), which reduces bulk flow
across the capillary wall. In contrast, some vas-
cular beds have fenestrated capillaries (e.g.,
in exocrine glands, renal glomeruli, and intes-
tinal mucosa), which have perforations (fenes-
trae) in the endothelium, resulting in relatively
high permeability and bulk flow. Discontinu-
ous capillaries (found in the liver, spleen, and
bone marrow) have large intercellular gaps, as
well as gaps in the basement membrane, and
therefore have the highest permeability.
Bulk flow follows Poiseuille’s equation for
hydrodynamic flow (see Chapter 5, Equa-
tion 5-7). Changes in pressure gradients (either
hydrostatic or colloid osmotic) across a capil-
lary alter fluid movement across the capillary.
In addition, changes in the size and number of
“pores” or intercellular clefts alter exchange.
Pore size and path length are analogous to
vessel radius and length in Poiseuille’s equa-
tion; they are major factors in the resistance to
bulk flow across capillaries. In some organs, the
number of perfused capillaries can be regulated.
As described in Chapter 7, the number of per-
fused capillaries in contracting skeletal muscle,
for example, is greater than at rest. An increase
in perfused capillaries increases the surface area
available for fluid exchange and the net move-
ment of fluid across capillaries by bulk flow.
Vesicular and Active Transport
Vesicular transport is a third mechanism by
which exchange occurs between blood and tis-
sue. This mechanism is particularly important
(e.g. proteins) across capillary endothelium.
Compared to diffusion and bulk flow, vesicu-
lar transport plays a relatively minor role in
transcapillary exchange (except for macromol-
ecules). Evidence exists, however, that vesicles
can sometimes fuse together, creating a chan-
nel through a capillary endothelial cell, thereby
permitting bulk flow to occur across the cell.
Active transport is a fourth mechanism of
exchange. Some molecules (e.g., ions, glucose,
amino acids) are actively transported across
capillary endothelial cells; however, this is
not normally thought of as a mechanism for
exchange between plasma and interstitium, but
rather as a mechanism for exchange between
an individual cell and its surrounding milieu.
Oxygen Diffusion
Oxygen diffuses from the blood to tissue cells
to support mitochondrial respiration.
lipid solubility of oxygen enables it to readily
diffuse through tissues; however, the distance
that oxygen is able to diffuse within a tissue is
limited by cellular utilization of oxygen. For
example, as oxygen diffuses out of a capillary
into surrounding skeletal muscle cells, oxy-
gen is consumed by the mitochondria. Con-
sequently, little oxygen diffuses all the way
through one cell to reach another. Therefore,
in tissues having a high demand for oxygen, it
is essential that the capillary density is great
enough to provide short diffusion distances.
Large amounts of oxygen diffuse across
the capillaries not only because of their thin
walls and high diffusion constant for oxy-
gen, but more importantly, because of their
large surface area available for diffusion. It
has been observed that significant amounts
of arterioles.
Some of this oxygen diffuses through arteri-
olar walls into the surrounding cells, and in
some cases, it diffuses from arterioles into
the venules that often are found adjacent to
arterioles. Normally, systemic arterial blood
is fully saturated with oxygen and has a PO2
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