From Wikipedia,
the free encyclopedia.
In
biology, tissue fluid
is a solution which bathes and
surrounds the cells of
multicellular animals and is also
called the
intercellular fluid.
Composition of tissue fluid
The composition of tissue fluid
depends upon the exchanges between
the cells in the
tissue and the
blood. This means that tissue
fluid has a different composition
in different tissues and in
different areas of the body.
Not all of the contents of the
blood passes into the tissue,
which means that tissue fluid and
blood are not the same.
Red blood cells,
platelets and
plasma
proteins cannot pass through
the walls of the
capillaries. The resulting
mixture that does pass through is
essentially blood plasma without
the plasma proteins. Tissue fluid
also contains some types of
white blood cell, which help
combat infection.
Sugars, mineral ions and waste
Formation of tissue fluid
In the capillaries, there are
two forces acting on the movement
of water and other
aqueous substances;
hydrostatic pressure and
water potential.
The hydrostatic pressure is
generated by the pumping force of
the
heart. It pushes water out of
the capillaries.
The water potential is created
due to the inability of large
solutes to pass through the
capillary walls. This buildup of
solutes induces
osmosis. The water passes from
a high concentration (of water) to
a low concentration in an attempt
to reach an
equilibrium. This draws water
back into the vessels. Because the
blood in the capillaries is
constantly flowing, equilibrium is
never reached.
The balance between the two
forces is different at different
points in the capillaries. At the
atrial end of the vessel, the
hydrostatic pressure is greater
than the water potential, so the
net movement (see
net flux) favors water and
other solutes being passed into
the tissue fluid. At the
venous end, the water
potential is greater, so the net
movement favors substances being
passed back into the capillary.
This difference is created by the
direction of the flow of blood,
and the imbalance in solutes
created by the net movement of
water favoring the tissue fluid.
Removal of tissue fluid
To prevent a buildup of tissue
fluid surrounding the cells in the
tissue, the
lymphatic system plays a part
in the transport of tissue fluid.
Tissue fluid can pass into the
surrounding
lymph vessels, and eventually
ends up rejoining the blood.
Sometimes the removal of tissue
fluid does not function correctly,
and there is a buildup. This
causes swelling, and can often be
seen around the
feet and
ankles. The position of
swelling is due to the effects of
gravity.
