Gas exchange

Gas exchange is the transfer of oxygen and carbon dioxide between an organism and its environment. It supplies oxygen for cellular respiration and removes carbon dioxide produced by metabolism.

Diffusion: The net movement of particles from regions of higher concentration to regions of lower concentration. Oxygen and carbon dioxide cross most respiratory surfaces down their partial-pressure gradients.
Alveolus: A microscopic air sac in the lungs where oxygen and carbon dioxide diffuse between air and blood. Alveoli provide the extensive, thin-walled exchange surface of mammalian lungs.
Human respiratory system: The organs and airways that ventilate human lungs and exchange gases with the blood. Human lungs use branching airways and alveoli to exchange gases with the atmosphere.
Ventilation: The movement of air or water through a respiratory system to renew the fluid at an exchange surface. Ventilation brings fresh medium to the surface but is not itself gas transfer across it.
Partial pressure: The pressure contributed by one gas in a mixture of gases. Differences in oxygen and carbon dioxide partial pressures drive their exchange across respiratory surfaces.
Gills: Respiratory organs that extract dissolved oxygen from water and release carbon dioxide. Gills perform gas exchange in fish and many aquatic invertebrates.
Fish gill: A respiratory structure in fish that passes water across thin filaments and lamellae containing blood vessels. Its countercurrent arrangement extracts oxygen from flowing water.
Gas transport: The movement of respiratory gases within an organism between exchange surfaces and metabolizing tissues. Transport distributes gases after exchange with the environment has occurred.
Fick's law of diffusion: A relation describing how diffusion rate depends on surface area, concentration gradient, and diffusion distance. It explains why thin, extensive exchange surfaces support rapid gas transfer.
Tracheal system: A network of air-filled tubes that carries gases directly to tissues in insects and some other arthropods. Insects deliver oxygen to tissues without relying on blood as the main carrier.

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