Have a nice day.
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oozing on the muggy shore of the gulf coast
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Greetings patriots and knights of the CS art,
CAPILLARY, con`t
The capillaries are so numerous that they penetrate into all parts of
every tissue; no cell is far removed from at least one capillary. It is
estimated that in muscle tissue there are as many as 60,000 capillaries
per square centimeter of cross section. This ensures that all portions
of the tissues will be supplied, but also that a very great capillary
surface area will be available for the exchange process. It has been
estimated that every cubic centimeter of blood contacts nearly one
square meter of capillary surface each time it passes through a
capillary bed.
Exchange of materials between the blood in the capillaries and the
tissue fluids outside the capillaries can occur in at least three ways:
(1) The materials may move entirely by diffusion through the membrane of
an endothelial cell in the wall of the capillary, across the cytoplasm
of the cell to the other side,and out through the cell membrane on that
side.
(2) Electron microscope (EMscope)studies of the endothelial cells of
capillaries have revealed large numbers of vesicles that apparently pick
up materials by endocytosis on one side of the cell, move across the
cell, and expell the material by exocytosis on the other side.
(3) EMscope studies have also shown that in most capillaries of the body
there are clefts between adjacent endothelial cells wide enough to
permit filtration of water and most dissolved ions and molecules, but
NOT proteins. The fact that the 2nd and 3rd mechanisms do not require
movement through cell membranes explaines why the rates at which
dissolved substances move between blood and tissue fluid often bear
little relation to the rates predicted on the basis of their individual
diffusion characteristics.
Lets examine the 3rd mechanism in more detail as it operates in humans.
At the arteriole end of a representative capillary bed, the hydrostatic
blood pressure is, on average, about 36 mm Hg highter than the
hydrostatic pressure of the tissue fluid outside the capillaries. The
pressure differential has fallen to about 15 by the time the blood
reaches the venule end of the capillary bed. The hydrostatic blood
pressure tends to force materials out of the capillaries into the
surrounding tissue fluid. If this were the only force involved, there
would be a steady loss from the blood by filtration of both water and
those dissolved substances that can readily be carried by the water
through the clefts in the capillary walls. It has been demonstrated that
normally there is relatively little net loss of water from the blood in
the capillaries. Clearly some other force must be at work in opposition
to the hydrostatic force.
This other force derives from the difference in OSMOTIC CONCENTRATION
between the blood and the tissue fluid. Our blood contains a relatively
high concentration of proteins, and these large molecules cannot easily
pass through the capillary walls. The same kinds of proteins occur in
the tissue fluids, but in much lower concentration. Because of the
difference in protein concentration on the two sides of the capillary
wall, the blood and tissues will have different osmotic pressures.
Normally, the osmotic pressure of the blood is about 25mm Hg higher that
that of the tissue fluid, with the result that water tends to move into
the capillaries from the tissue fluid by osmosis.
The net movement of water will be determined by the relative magnatudes
of these two opposing forces. Notice that at the arteriole end of our
representative capillary bed the hydrstatic pressure differential is 36
and the osmotic pressue is 25. We find that there is a net pressure of
11 tending to force water out of the capillaries. Now, at the venule end
of the capillary bed, the hydroststic pressue has fallen to 15 ,while
the osmotic pressure has remained about 25. There is now a net pressure
of at least 10 tending to force water into the capillaries. The balance
between hydrostatic blood pressure and osmotic pressure is such that
water is forced out at the arteriole end and forced in at the venule end
of the bed. The net effect is that nearly all of the (about 99%) water
filtered out of the capillaries is reabsorbed. Since the water carries
with it ions and molecules of many disolved substances, we say that the
blood in the capillaries first unloads materials for the tissues at the
arteriole end of the capillary bed and then picks up materials for
transport at the venule end of the capillary bed. There is nornally a
slight net loss of water from the blood which becomes part of the lymph
and eventually returns to the blood via the lymphatic system.
Bless you Bob Lee
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oozing on the muggy shore of the gulf coast
[email protected]
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