O2, CO2,
Ions, small
small molecules
■ FIGURE 8.1 Mechanisms of exchange across the capillary endothelium. Lipid-soluble substances like
oxygen and carbon dioxide readily exchange across capillary endothelial cells by diffusion. W ater and
electrolytes move across the endothelium prim arily by bulk flow through intercellular clefts (“pores”).
Vesicular transport mechanisms move large molecules across the endothelium. Active transport mecha-
nisms move ions and other small molecules across the endothelium.
are primarily transported by one mechanism,
whereas other substances are able to use more
than one mechanism. This is determined by
the physical and chemical characteristics of
the substance as well as the type of capillary
endothelium, which differs among organs.
Diffusion is the movement of a molecule from a high
concentration to a low concentration
. This mecha-
nism of exchange is particularly important for
gases (O2 and CO2) and other lipid-soluble sub-
stances (e.g., steroid hormones, anesthetics).
Fluid and electrolytes also are exchanged across
the endothelium, in part, by diffusion.
The movem ent of a substance by diffusion
is described by Fick’s first law of diffusion
(Equation 8-1), in w hich the movem ent of
a m olecule per unit time (flux JS; m oles/s)
equals the
constant (D)
of the
barrier (e.g., capillary wall) multiplied by
the surface area (A) available for diffusion
which is the concentration difference across
the barrier (AC) divided by the diffusion
distance (AX).
Eq. 8-1
= DA —
The diffusion constant is a value that represents
the ease with which a specific substance can
cross the capillary wall (or other barrier) by dif-
. The higher the diffusion constant for a
specific substance, the greater its flux across
the barrier at a given concentration gradient.
The diffusion constant is determined by the
physical and chemical structure of the barrier
as well as the physical and chemical charac-
teristics (e.g., size, electrical charge) of the
diffusing molecule. For example, the diffu-
sion constant for oxygen (small and highly
lipophilic) across cell membranes (which are
lipid bilayers) is very high compared to glu-
cose (large and hydrophilic).
Equation 8-1 indicates that
the rate of dif-
fusion is directly related to the concentration
difference, the diffusion constant, and the area
available for diffusion, and it is inversely related
to the diffusion distance
. The diffusion distance
(AX) in Equation 8-1 is sometimes combined
with the diffusion constant (D) and called the
permeability coefficient (P). This simplifies
Equation 8-1 to JS = PS(AC) in which S is the
surface area available for exchange. The com-
bined value of the permeability coefficient
times the surface area has been calculated
for different substances in many organs and
tissues; it is called the PS product.
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