Particles tend to move from an area of high concentration to an area of low concentration in a process known as

This is a process which is called as:

"Diffusion"

In the process of diffusion, molecules move from regions of higher concentration to regions of low erconcentration, just by moving at random. In one example, there is more oxygen in the lungs than there is in the blood, so by diffusion oxygen molecules will move into the blood.