after injury, and certain of the
changes are only an accentuation of those which take place in the
usual life; but others, such as the formation of the exudate, are
unusual; not only is the outpouring of fluid greatly increased, but
its character is changed. In the normal transudation[2] the substances
on which the coagulation of the blood depends pass through the vessel
wall to a very slight extent, but the exudate may contain the
coagulable material in such amounts that it easily clots. The
interchange between the fluid outside the vessels and the blood fluid
takes place by means of filtration and osmosis. There is a greater
pressure in the vessels than in the fluid outside of them, and the
fluid filters through the wall as fluid filters through a thin
membrane outside of the body. Osmosis takes place when two fluids of
different osmotic pressure are separated by animal membrane.
Difference in osmotic pressure is due to differences in molecular
concentration, the greater the number of molecules the greater is the
pressure, and the greater rapidity of flow is from the fluid of less
pressure to the fluid of greater pressure. The molecular concentration
of tissue and blood fluid is constantly being equalized by the process
of osmosis. In the injured tissue the conditions are more favorable
for the fluid of the blood to pass from the vessels: by filtration,
because owing to the dilatation of the arteries there is increased
amount of blood and greater pressure within the vessels, and the
filtering membrane is also thinner because the same amount of membrane
(here the wall of the vessel) must cover the larger surface produced
by the dilatation. It is, moreover, very generally believed that there
are minute openings in the walls of the capillaries, and these would
become larger in the dilated vessel just as openings in a sheet of
rubber become larger when this is stretched. Osmosis towards the
tissue is favored because, owing to destructive processes the
molecular pressure in the injured area is increased; an injured tissue
has been shown to take up fluid more readily outside of the body than
a corresponding uninjured tissue. The slowing of the blood stream, in
spite of the dilatation of the vessels, is due to the greater friction
of the suspended corpuscles on the walls of the vessels. This is due
to the loss from the blood of the outstreaming fluid and the relative
increase in the number of corpuscles, added to by the uneve