Cardiovascular Physiology
397
utilization of oxygen and nutrients lowers their tissue concen-
trations, whereas increased production of carbon dioxide and
other end products raises their tissue concentrations. In both
cases, the substance’s transcapillary concentration difference
increases, which also increases the rate of diffusion.
Bulk Flow Across the Capillary Wall:
Distribution of the Extracellular Fluid
At the same time that the diffusional exchange of nutrients,
oxygen, and metabolic end products is occurring across the
capillaries, another, completely distinct process is also taking
place across the capillary—the bulk fl ow of protein-free plasma.
The function of this process is not exchange of nutrients and
metabolic end products, but rather distribution of the extra-
cellular fl uid (
Figure 12–41
). Recall that extracellular fl
uid
comprises the plasma and interstitial fl uid. Normally, there is
almost four times more interstitial fl uid than plasma—11 L
versus 3 L in a 70-kg person. This distribution is not fi xed,
however, and the interstitial fl uid functions as a reservoir that
can supply fl uid to or receive fl uid from the plasma.
As described in the previous section, the capillary wall
is highly permeable to water and to almost all plasma solutes,
except plasma proteins. Therefore, in the presence of a hydro-
static pressure difference across it, the capillary wall behaves
like a porous fi lter, permitting protein-free plasma to move by
bulk fl ow from capillary plasma to interstitial fl uid through
the water-fi lled channels. (This is technically termed
ultrafi l-
tration
but we will refer to it simply as
ltration.
)
The concen-
trations of all the plasma solutes except protein are virtually the
same in the fi
ltering fl
uid as in plasma.
The magnitude of the bulk fl ow is determined, in part,
by the difference between the capillary blood pressure and the
interstitial-fl uid hydrostatic pressure. Normally, the former
is much higher than the latter. Therefore, a considerable
hydrostatic-pressure difference exists to fi
lter protein-free
plasma out of the capillaries into the interstitial fl
uid, with the
protein remaining behind in the plasma.
Why doesn’t all the plasma fi lter out into the interstitial
space? The explanation is that the hydrostatic pressure differ-
ence favoring fi ltration is offset by an osmotic force opposing
fi ltration. To understand this, we must review the principle
of osmosis.
In Chapter 4, we described how a net movement of
water occurs across a semipermeable membrane from a solu-
tion of high water concentration to a solution of low water
concentration—that is, from a region with a low concentra-
tion of solute to which the membrane is impermeable (non-
penetrating solute) to a region of high nonpermeating solute
concentration. Moreover, this osmotic fl ow of water “drags”
along with it solutes that can penetrate the membrane. Thus,
a difference in water concentration secondary to different
concentrations of nonpenetrating solute on the two sides of a
membrane can result in the movement of a solution contain-
ing both water and penetrating solutes in a manner similar
to the bulk fl ow a hydrostatic pressure difference produces.
Units of pressure (mmHg) are used in expressing this osmotic
fl ow across a membrane just as for fl
ow driven by a hydrostatic
pressure difference.
This analysis can now be applied to osmotically induced
fl ow across capillaries. The plasma within the capillary and
the interstitial fl uid outside it contain large quantities of low-
molecular-weight penetrating solutes (also termed
crystal-
loids
); for example, sodium, chloride, and potassium. Because
the capillary is highly permeable to all these crystalloids, their
concentrations in the two solutions are essentially identical.
Thus, the presence of the crystalloids causes no signifi cant dif-
ference in water concentration. In contrast, the plasma pro-
teins (also termed
colloids
), being essentially nonpenetrating,
have a very low concentration in the interstitial fl
uid. The
difference in protein concentration between the plasma and
the interstitial fl uid means that the water concentration of
the plasma is slightly lower (by about 0.5 percent) than that
of interstitial fl
uid, inducing an osmotic fl ow of water from
the interstitial compartment into the capillary. Because the
crystalloids in the interstitial fl uid move along with the water,
osmotic fl ow of fl
uid, like fl ow driven by a hydrostatic pressure
difference, does not alter crystalloid concentrations in either
plasma or interstitial fl
uid.
A key word in this last sentence is
concentrations.
The
amount of water (the volume) and the amount of crystal-
loids in the two locations do change. Thus, an increased fi l-
tration of fl uid from plasma to interstitial fl
uid increases the
volume of the interstitial fl uid and decreases the volume of
the plasma, even though no changes in crystalloid concen-
trations occur.
In summary, opposing forces act to move fl
uid across the
capillary wall (
Figure 12–42a
): (1) the difference between
capillary blood hydrostatic pressure and interstitial fl
uid hydro-
static pressure favors fi ltration out of the capillary; and (2) the
water-concentration difference between plasma and interstitial
fl uid, which results from differences in protein concentration,
favors the
absorption
of interstitial fl uid into the capillary.
Therefore, the
net fi ltration pressure (NFP)
depends directly
upon the algebraic sum of four variables: capillary hydrostatic
pressure,
P
c
(favoring fl uid movement out of the capillary);
interstitial hydrostatic pressure,
P
IF
(favoring fl
uid movement
into the capillary); the osmotic force due to plasma protein con-
centration,
π
c
(favoring fl uid movement into the capillary); and
Extracellular fluid (ECF)
Interstitial fluid
(11 L)
Plasma
(3 L)
Systemic
capillaries
Filtration
Absorption
Figure 12–41
Distribution of the extracellular fl
uid by bulk fl ow.
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