Diffusion is the primary mechanism for
the exchange of gases (e.g., oxygen)
and lipid-soluble substances across the
capillary barrier. The rate of diffusion
is directly related to the concentration
difference of the molecule across the
capillary wall.
Exchange of water and electrolytes
across capillaries (and postcapillary
venules) occurs primarily by bulk flow
through intercellular clefts ("pores”)
between endothelial cells. Bulk flow
is governed by the same factors that
determine the blood flow through
Movement of fluid across a capillary is
determined by hydrostatic and osmotic
driving forces, the permeability of the
capillary to fluid movement, and the
surface area for exchange of fluid.
The net driving force that determines
fluid movement is the net hydrostatic
pressure difference across the capillary
wall minus the opposing effective
oncotic pressure difference across the
capillary wall.
transcapillary fluid exchange, is
determined by arterial and venous
pressures, and precapillary and
postcapillary resistances.
Changes in venous pressure have a
much greater quantitative influence
on capillary pressure than do similar
changes in arterial pressure.
Filtration occurs when the net driving
force is greater than zero, which
generally occurs at the arteriolar end
of the capillary. Reabsorption occurs
when the net driving force is less than
zero, which generally occurs at the
venular end of the capillary where
capillary hydrostatic pressure is lower.
An increase in tissue fluid volume
(edema) occurs when the rate of fluid
filtration exceeds the sum of the rate of
fluid reabsorption and lymphatic flow.
Edema can be caused by increased
capillary hydrostatic pressure,
increased capillary permeability,
decreased plasma oncotic pressure, or
lymphatic blockage.
Capillary hydrostatic pressure, which
plays a major role in regulating
For each question, choose the one best
Which of the following mechanisms is
most important quantitatively for the
exchange of electrolytes across capillaries?
a. Bulk flow
b. Diffusion
c. Osmosis
d. Vesicular transport
Which of the following can increase the
rate of oxygen diffusion from blood to
Arteriolar vasodilation
h. Decreased arteriolar P02
c. Increased tissue P02
d. Decreased number of flowing capillaries
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