Fish Gills and Respiration: How Fish Breathe

Gills do far more than breathe: counter-current exchange pulls oxygen from water while the same organ excretes ammonia and balances salts. Here is how it works.

Gills are the defining organ of fish, and they do far more than absorb oxygen. They are simultaneously the lungs, a major excretory organ and a salt-balancing organ. Because they sit in direct contact with the water, they are also the first thing harmed by poor water quality, which is why understanding them is so useful for fishkeepers.

Structure

A fish's gills are built in a hierarchy. Bony fish typically have five pairs of gills supported by gill arches; each arch bears thread-like gill filaments, and the filaments carry fine folds called lamellae packed with capillaries. This arrangement provides a very large surface area for exchanging oxygen and carbon dioxide across a thin epithelium. To move water across this surface, fish pull oxygen-rich water through the mouth and pump it over the gills, and in bony fish the operculum lets them do this without relying on constant swimming.

Counter-current exchange

The reason gills are so efficient is counter-current exchange: in the lamellae, capillary blood flows in the opposite direction to the water. Because the two flow against each other, a concentration gradient is maintained along the whole length of the contact, so oxygen keeps diffusing from water into blood the entire way rather than reaching equilibrium halfway. The maximum transfer achievable is higher with counter-current than with parallel flow, and in practice this lets gills extract over 80% of the oxygen available in the water.

More than breathing

The gill filaments also handle the exchange of ions, water, acids and ammonia. Most of a fish's nitrogenous waste leaves not through the kidney but across the gills as ammonia. The gills also run osmoregulation: marine fish excrete excess salt through specialized cells, while freshwater fish use their gills to take up the ions they need to maintain blood osmolarity.

Air-breathing accessories

Some fish supplement gill breathing with accessory organs. Anabantoids such as bettas and gouramis have a labyrinth organ, a much-folded suprabranchial structure that lets them take in oxygen directly from the air at the surface and survive in low-oxygen water. Fittingly, fish from low-oxygen waters tend to have larger, more complex labyrinth organs than those from fast-flowing, oxygen-rich waters.