A Change in Hemoglobin Shape Affects the Buoyancy of Fish

Despite their heavy skeletons, bony fish are still buoyant because they possess a swim bladder. This gas-filled sac allows the fish to regulate their buoyant density and remain suspended at any depth in the water. By pumping gas into or sucking gas out of the swim bladder, fish are able to rise or sink in water. The gas flow is regulated by lactic acid, the acidity of which causes hemoglobin in the blood to release oxygen gas. Recently, researchers have shed some light on the mechanism by which the acidity level affects hemoglobin. It turns out that the lower pH alters the hemoglobin's shape so that it is less able to bind oxygen.

Source: "Root cause of fish buoyancy" by Rory Howlett, Nature, March 21, 1996.

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