--- title: "Fick's Law and What Sets the Rate of Exchange" description: "For a fixed gas, temperature and barrier material, total net diffusion rate increases with exchange area and concentration difference, and decreases with diffusion distance. The relationship supports " canonical: https://lightmysky.com/learn/science/ficks-law-and-what-sets-the-rate-of-exchange-mt_6JreZAFeb- source: https://lightmysky.com/learn/science/ficks-law-and-what-sets-the-rate-of-exchange-mt_6JreZAFeb-.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Fick's Law and What Sets the Rate of Exchange For a fixed gas, temperature and barrier material, total net diffusion rate increases with exchange area and concentration difference, and decreases with diffusion distance. The relationship supports calculations about exchange surfaces. Subject: Science · Area: Organisms & Life Processes · Ages 16 to 18 Page: https://lightmysky.com/learn/science/ficks-law-and-what-sets-the-rate-of-exchange-mt_6JreZAFeb- ## Ready when they can - Interprets net diffusion and predicts how area, concentration difference, distance and diffusion coefficient affect rate; distinguishes total transfer from transfer per unit area. - Predicts effects of diffusion distance, including doubling and inverse targets, and identifies the controls needed to test distance alone. - Explains how thin alveolar walls shorten diffusion paths and identifies the gas partial-pressure gradient driving air-to-blood exchange. ## Lesson: Three dials that set the speed of crossing Fick's law models total net diffusion rate: rate = D × area × concentration difference / distance. D is the diffusion coefficient. For a fixed substance, temperature and barrier material, take D as constant. With other factors fixed, greater area or concentration difference increases rate; greater distance reduces it. *(drawing: Use the same units and the same diffusion coefficient when comparing changes.)* Alveoli provide a large combined exchange area. Oxygen crosses the thin alveolar lining and capillary wall into blood; carbon dioxide crosses the other way. Thin walls shorten this path. For exchange between air and blood, compare each gas's partial pressure, the pressure due to that gas. Total oxygen carried in blood is not the same measure. A one-cell-thick lining is not the shortest path physically possible. With other factors fixed, divide the original rate by the distance change factor. A rate of 20 units per second becomes 10 when distance doubles. A rate of 15 becomes 5 when distance triples. Halving distance doubles rate. For an inverse target, distance factor = 1 / required rate factor. To make rate five times as large by changing distance alone, reduce distance to one fifth. For several changes, multiply the area and concentration-difference factors, then divide by the distance factor. If area and distance both increase fivefold, the factor is 5 / 5 = 1. If area triples and distance increases sixfold, it is 3 / 6 = 0.5. Keep concentration difference and D fixed in these examples. Multiply the original rate by the final factor. A factor of 0.5 means half. Net rate is the amount transferred overall per second, allowing for movement both ways. It is not the speed of an individual molecule. At equal concentrations, molecules can still cross both ways, but net transfer is zero. If D changes, include its factor too: doubling D doubles rate when other factors stay fixed. For the same substance in the same medium, find concentration difference by subtracting the lower value from the higher value. Net diffusion is towards the lower concentration. Values of 7 and 2 give a difference of 5. Adding 2 to both gives 9 and 4, still a difference of 5. Swapping the two sides reverses net diffusion. Adding identical exchange patches increases total area and total transfer, without changing transfer per unit area. The original patches still work at the same rate. To test distance alone, compare barriers with different distances but equal area, concentration difference and D. If several factors change, a rate change cannot be attributed to distance alone. **Recap.** With other factors fixed, total net rate rises with area and concentration difference and falls with distance. Combine changes using their multiplication and division factors. *Lesson, questions and flashcards written for this dataset. OpenStax Biology 2e gas-exchange material was consulted for facts; its prose was not adapted.* ## Practice 20 questions on this page, each with its working shown. ## Needs first - [Gas Exchange Surfaces Across Taxa](https://lightmysky.com/learn/science/gas-exchange-surfaces-across-taxa-mt_gCcrXUH0aP) ## Opens up - [Ventilation in Insects, Fish and Mammals](https://lightmysky.com/learn/science/ventilation-in-insects-fish-and-mammals-mt_Thd06O06Vr) - [Digestion and Absorption in the Mammalian Gut](https://lightmysky.com/learn/science/digestion-and-absorption-in-the-mammalian-gut-mt_xZ2-3ZClDa)