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Capillary Exchange
The ultimate goal of the cardiovascular and respiratory systems is to
deliver oxygen-rich and nutrient-rich blood to the capillary networks.
Although the amount of blood within the capillaries at any given time is
only about 5 percent of the total volume, it is the most important blood
in the body. It is at the level of the capillaries that capillary
exchange occurs. The thin and porous construction of capillary walls
provides an avenue for substances such as oxygen and glucose to move
out of the blood plasma and into the interstitial fluids surrounding the
tissue cells. The reverse process, uptake of substances such as carbon
dioxide and wastes from the interstitial fluids into the plasma, occurs
simultaneously. In addition to the back-and-forth exchange of solutes,
water also moves between the bloodstream and interstitial fluid in both
directions within the capillary networks. There are three mechanisms by
which substances enter and leave capillaries:diffusion, transcytosis,
and bulk flow.
Diffusion
Diffusion, the movement of a solute from an area of high
concentration to an area of low concentration, is a passive process.
Diffusion is the driving force for the exchange of solutes at the capillary
tissue interface. Thus, the oxygen and glucose, whose concentrations are
high in the blood, will move out of the vessels into the interstitial fluids.
Carbon dioxide levels are high in the interstitial fluid due to diffusion
from the tissue cells. Carbon dioxide will move across the capillary walls
into the blood plasma, where the concentration is lower. The physical
characteristics of each solute determine the route by which that solute
will diffuse. Small water-soluble molecules, such as glucose and amino
acids, diffuse primarily through the intercellular clefts between the
endothelial cells in continuous capillaries or through the fenestrations of
fenestrated capillaries. Lipid-soluble molecules, such as oxygen and
steroid hormones, can diffuse through the plasma membranes of the
endothelial cells. Larger, lipid-insoluble substances must move down
their concentration gradients via transcytosis.
Transcytosis
Transcytosis is a combination of endocytosis and exocytosis that is
necessary when the substance to be transported cannot be exposed to
the cytoplasm of an intervening cell. Transcytosis is important in
moving large lipid-insoluble molecules such as proteins across the
capillary endothelial cells. A protein, for instance albumin or insulin,
which is high in concentration in the blood plasma, will be enclosed in
tiny vesicles formed from the endothelial plasma membrane. This
process is considered pinocytosis because the molecules dissolved in the
blood plasma are taken up randomly along with the fluid. These
molecules are transported within the vesicles across the endothelial cell.
At the opposite plasma membrane surface, the vesicle merges with the
membrane, and the molecules are released into the interstitial fluid. In
the developing fetus, certain plasma proteins and antibodies are moved
from maternal to fetal circulation via transcytosis.
Bulk Flow
Bulk flow is the movement of fluids due to pressure gradients. Fluids
will move en masse from an area of high pressure to an area of low
pressure. While diffusion is most important in the transfer of larger
solutes, bulk flow controls the movement of fluids plus the ions and
other small particles that are dissolved within the fluid. The movement
of fluids from the capillaries into the interstitial fluid at the arterial end
of the capillary bed is called filtration. The movement of fluids from
the interstitial fluid into the capillaries at the venous end of the capillary
bed is called reabsorption.