Our bodies are constantly working to keep everything in balance; when we exercise, not only do we breathe harder to get more oxygen to our muscles, but also to get rid of the painful lactic acid `burn'. All air breathing land dwellers deal with acid – an excess of protons – by reacting protons with bicarbonate ions to create water and CO2, which is breathed out. To deal with too much base, breathing slows down, keeping CO2 and therefore protons in the body. Water breathing fish take a different approach, relying on metabolic processes at their gills and kidneys to restore a normal blood pH. But how will animals that can breathe in both air and water deal with pH changes? `Lungfish are poised between air breathing and water breathing, and are a beautiful model to study this,' says Katie Gilmour of the University of Ottawa, who investigated with her colleagues if lungfish cope with pH changes like air breathers or water breathers(p. 1944).

Lungfish bridge the divide between land and water, dependent on their watery environment for food but dying if they can't breathe air with their lungs. They also have gills, which they use to water breathe and get rid of some CO2. Once the team's fish had arrived in the lab from their African home, they trained them to sit in water-filled tube respirometers and pop their heads above the water's surface when they needed a gulp of air.

To find out how the fish would deal with an excess of acid or base, they delicately operated on them to insert a cannula into the dorsal aorta, so that they could change blood pH by injecting acid or base into the blood stream. To find out how the fish dealt with an acid injection, they counted the number of air and water breaths, finding that they took twice as many of both and breathed out more CO2, much like a land dweller. To see if the gills or kidneys were also getting rid of excess acid, the team measured acid excretion into the water in the respirometer. They didn't see a rise in excretion from normal levels, suggesting that lungfish don't rely on their gills and kidneys much to get rid of excess protons.

Injecting base and raising blood pH caused the fish to halve their water breathing rate; they also breathed out much less CO2 into the water. Their air breathing was unaffected, however, probably because they wouldn't get enough oxygen otherwise. The team measured that the fish were excreting much higher levels of base into the water, so to separate out how much the gills and kidneys were each contributing, they collected the fishes'urine via a catheter. They found that the kidneys excreted around 20% of the total base into the urine, and the gills around 80% into the water, much like fish, showing that the lungfish rely more on the water breathers' strategy to get rid of excess base and rebalance pH.

`The lungfish have the best of both worlds,' says Gilmour. Like land dwellers, they rely more on air breathing to redress a more acidic blood pH,but their gills and kidneys deal with an excess of base to return blood pH back to normal, just like water breathing fish.

Gilmour, K. M., Euverman, R. M., Esbaugh, A. J., Kenney, L.,Chew, S. F., Ip, Y. K. and Perry, S. F. (
). Mechanisms of acid–base regulation in the African lungfish Protopterus annectens.
J. Exp. Biol.