As blood passes through the blood capillaries, continual
exchange of extracellular fluid also occurs between
the plasma portion of the blood and the interstitial fluid
that fills the intercellular spaces. This process is shown
in Figure 1-2. The walls of the capillaries are permeable
to most molecules in the plasma of the blood, with the
exception of plasma protein molecules, which are too
large to readily pass through the capillaries. Therefore,
large amounts of fluid and its dissolved constituents
diffuse back and forth between the blood and the tissue
spaces, as shown by the arrows. This process of diffusion
is caused by kinetic motion of the molecules in both
the plasma and the interstitial fluid. That is, the fluid and
dissolved molecules are continually moving and bouncing
in all directions within the plasma and the fluid in the
intercellular spaces, as well as through the capillary pores.
5
U n i t I
Few cells are located more than 50 micrometers from a
capillary, which ensures diffusion of almost any substance
from the capillary to the cell within a few seconds. Thus,
the extracellular fluid everywhere in the body—both that
of the plasma and that of the interstitial fluid—is continually
being mixed, thereby maintaining homogeneity of the
extracellular fluid throughout the body.