Final answer:
Diffusion is the passive movement of molecules from an area of high to low concentration, occurring in blood vessel capillaries for gas exchange.
Step-by-step explanation:
The process that moves molecules from an area of high concentration to an area of low concentration is called diffusion. This process happens without the input of energy and is a form of passive transport. In the human body, diffusion occurs in the blood vessels, specifically in the capillaries, where oxygen and carbon dioxide are exchanged between the bloodstream and the body's tissues. Oxygen diffuses from the lungs into the bloodstream because there is a higher concentration in the lungs than in the blood. Conversely, carbon dioxide diffuses from the blood, where its concentration is higher, to the lungs, where it is exhaled.