Definition
A principle of gas diffusion stating that a gas will move across a permeable membrane from an area of higher concentration (or partial pressure) to an area of lower concentration, and that the rate of movement is proportional to the difference in concentration on either side of the membrane.
Plain English
Gases naturally move from where there is more of them to where there is less of them, and the bigger the difference, the faster they move across.
Context Anchor
Seen in aviation physiology discussions about breathing, oxygen transfer, altitude, and hypoxia.
Derivation
Named after Adolf Fick, a 19th-century German physiologist who described how gases and liquids diffuse across membranes. Knowing the law is named after the person who first measured it helps signal that this is an observed natural rule, not an aviation-specific invention.
Why Pilots Care
Explains why the pressure gradient for oxygen drops at altitude, making supplemental oxygen necessary to maintain adequate diffusion into the bloodstream.
Grounding Statement
At sea level, there is plenty of oxygen pressure in the lungs, so oxygen moves easily into the blood. At 18,000 feet, that pressure is roughly halved, so far less oxygen crosses over — the same membrane, the same blood, but a smaller pressure difference doing the work.
Intuition Check
Fick's Law is not a legal rule or flight procedure. It is a science rule that describes how gases move across a surface.
Example Sentence 1
The ground school instructor used Fick's Law to explain why oxygen transfer in the lungs becomes less effective as altitude increases.
Example Sentence 2
During high-altitude training the instructor referenced Fick's Law to show why thinner air reduces the rate gases cross the alveolar membrane.