zwemblaas

Zwemblaas, in English known as the swim bladder, is an internal gas-filled organ found in most bony fishes. It lies within the body cavity and serves mainly to regulate buoyancy, allowing the fish to maintain a chosen depth with minimal energy expenditure. Gas volume within the bladder is adjusted through physiological mechanisms that exchange gas with the bloodstream or by venting gas.

There are two main types of buoyancy regulation. Physostomous fishes have a pneumatic duct connecting the swim bladder to the esophagus, so they can gulp air from the surface to fill the bladder or release air to deflate it. Physoclistous fishes lack this duct; gas exchange occurs via a gas gland and a resorptive region called the ovale, enabling buoyancy control without gulping air.

The gas inside the swim bladder is mainly oxygen and nitrogen, with regulation driven by specialized vascular structures such as the gas gland and the rete mirabile. The bladder wall is highly vascularized and its activity can be influenced by environmental factors and depth. In addition to buoyancy, in many species the swim bladder also acts as a resonating chamber for sounds produced by sonic muscles, contributing to communication and courtship in some fish.

Not all fishes possess a swim bladder. Cartilaginous fishes (sharks and rays) rely on other buoyancy strategies, such as oily livers. Humans have historically used dried swim bladders, or isinglass, for clarification of wines and beers and other applications.