What is the process by which gas moves from an area of higher concentration to an area of lower concentration?

Diffusion is the process described in the question where gas moves from an area of higher concentration to an area of lower concentration. This movement occurs as the molecules of the gas spontaneously spread out and mix due to their random thermal motion, resulting in an even distribution over time when the system reaches equilibrium.

In biological contexts, diffusion plays a critical role, particularly in the exchange of gases in the lungs where oxygen moves from the alveoli, where its concentration is high, into the blood, where its concentration is lower. Conversely, carbon dioxide moves from the blood, where it is in higher concentration, to the alveoli, facilitating gas exchange essential for respiration.

Osmosis, while similarly a form of movement, specifically involves the diffusion of water across a semipermeable membrane and does not apply to gases in the same way diffusion does. Filtration involves the movement of water and solutes through a membrane due to pressure differences, while active transport refers to the movement of substances against their concentration gradient, which requires energy input, unlike diffusion that occurs naturally without energy expenditure.