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.
As blood passes through the blood capillaries, continualexchange of extracellular fluid also occurs betweenthe plasma portion of the blood and the interstitial fluidthat fills the intercellular spaces. This process is shownin Figure 1-2. The walls of the capillaries are permeableto most molecules in the plasma of the blood, with theexception of plasma protein molecules, which are toolarge to readily pass through the capillaries. Therefore,large amounts of fluid and its dissolved constituentsdiffuse back and forth between the blood and the tissuespaces, as shown by the arrows. This process of diffusionis caused by kinetic motion of the molecules in boththe plasma and the interstitial fluid. That is, the fluid anddissolved molecules are continually moving and bouncingin all directions within the plasma and the fluid in theintercellular spaces, as well as through the capillary pores.5U n i t IFew cells are located more than 50 micrometers from acapillary, which ensures diffusion of almost any substancefrom the capillary to the cell within a few seconds. Thus,the extracellular fluid everywhere in the body—both thatof the plasma and that of the interstitial fluid—is continuallybeing mixed, thereby maintaining homogeneity of theextracellular fluid throughout the body.
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