Extracellular fluid is transported through all parts of the body in two stages. The first stage is movement of blood through the body in the blood vessels, and the second is movement of fluid between the blood capillaries and the intercellular spaces between the tissue cells.
Figure 1-1 shows the overall circulation of blood. All the blood in the circulation traverses the entire circulatory circuit an average of once each minute when the body is at rest and as many as six times each minute when a person is extremely active.
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 the plasma and the interstitial fluid. That is, the fluid and diffuse back and forth between the blood and the tissue dissolved molecules are continually moving and bounc-spaces, as shown by the arrows. This process of diffuing in all directions within the plasma and the fluid in the sion is caused by kinetic motion of the molecules in both intercellular spaces, as well as through the capillary pores.