bio

Swim bladder

An internal gas sac in bony fishes, used to hold neutral buoyancy without swimming continuously. The machinery that fills it is remarkable: a dense counterflow weave of capillaries called the rete mirabile runs alongside a gland that secretes acid into the blood; the acid makes haemoglobin release its oxygen, and the freed oxygen accumulates in the weave until its pressure exceeds the bladder's and flows in. That is active gas secretion against a gradient, achieved with nothing but plumbing and pH. The inherent weakness is Boyle's law: descend and the sac compresses so the fish sinks faster, ascend and it expands so the rise accelerates, which means constant active correction.