Diffusion
All molecules and ions in the body fluids, including water
molecules and dissolved substances, are in constant
motion, each particle moving its own separate way. Motion
of these particles is what physicists call “heat”—the greater
the motion, the higher the temperature—and the motion
never ceases under any condition except at absolute zero
temperature. When a moving molecule, A, approaches a
stationary molecule, B, the electrostatic and other nuclear
forces of molecule A repel molecule B, transferring some
of the energy of motion of molecule A to molecule B.
Consequently, molecule B gains kinetic energy of motion,
while molecule A slows down, losing some of its kinetic
energy. Thus, as shown in Figure 4-3, a single molecule