Salty Penguins Filter Salt Out Their Nose

The Venture Brothers season 1, episode 5, via [adult swim]

Why yes, penguins do have an organ that converts sea water into fresh water! Except it's not an organ, it's a gland. And it doesn't directly convert sea water to fresh water, it filters salt from the blood.

Hm, maybe I should start from the beginning.

First of all, this organ/gland/whatever that Dean is talking about is called the supraorbital gland, and it's something all marine birds have. Basically any mammal or bird that is going to have to drink sea water to quench thirst is going to need this gland.

Normally, salt that we ingest is absorbed into the blood stream, filtered out by the kidneys, and secreted in urine. However, the penguin's small kidneys can only filter out enough salt to create urine that's about 1/3 the concentration of sea water. If the blood is still too salty, then water must be taken from other tissues to dilute it, and this quickly leads to dehydration.

Penguins have a very high salt load because they drink sea water to quench thirst and eat a lot of salty foods like crustaceans. Located above the nose, in between the eyes, the supraorbital gland lends a helping hand. Both the kidney and supraorbital gland filter salt from the blood in a process called counter-current exchange.

The blood flowing along the gland and the fluid within the gland flow counter, or in opposite directions to one another. One of the principles of osmosis is that molecules will move from fluids with high concentrations of solutes (in this case, salt) to fluids with lower concentrations. I.e. they move down their concentration gradient. So salt leaves blood and goes to the relatively less salty fluid in the duct. Since the blood and the duct fluid are moving in opposite directions, the blood will always remain saltier than the duct fluid, therefore a concentration gradient will be maintained, and the salt will always flow out the blood to the fluid in the duct.

Still confused? There's a pretty great diagram here. For the more verbally inclined, here's a metaphor: Imagine the blood stream and the fluid stream are two trains moving parallel to each other in opposite directions (I promise this won't involve any math). Every car in the "blood" train is full of (rather salty) people, and every car in the fluid train is empty. As the trains meet, the first two cars of each train will be facing each other, and the people from the blood train (with great nimbleness) will jump from the blood train to the fluid train, until the first car in the fluid train is full. Then as the the trains continue to pass, people will continue to jump from the blood train and fill all the cars in the fluid train. People will not jump back onto the blood train from the fluid train because remember, they're going in opposite directions. So as the people that jumped on the first car of the fluid train keep going, they're passing the full cars of the blood train going in the other direction.

What would happen if the blood and the fluid were running in the same direction? That would be called concurrent exchange. The salt would still move from the blood, but quickly the concentrations of salt within the blood and the fluid would become the same and there wouldn't be any net gain or loss of salt in either the blood or the fluid. Pretend those trains are now running in the same direction, and people jump onto the fluid train, but then they see open space in the blood train and jump back. Since there will always be space in both the fluid and the blood trains, and they keep running along next to each other, passengers will keep jumping back and forth.

This diagram illustrates the difference between concurrent (top) and counter-current (bottom) exchange. Blue, in this case, would indicate low salinity, and red would indicate high salinity. As for the percentages, there is never "near 100%" absorption of salt from the bloodstream, as that would be as deadly as having too much salt. So take the percentages with a grain of salt.

From Wikimedia commons

The result is a fluid that is actually saltier than sea water. It flows from the gland and is excreted through the nasal passages. Penguins will often look like they have runny noses, but it's really this salty substance coming out their noses. You could, supposedly, say that they pee out their nose, but that would take away some of the mystique from the magestic emperor penguin, wouldn't it?

Immature jokes aside, this gland allows penguins to consume massive amounts of salt, and still be healthy. Considering how high sodium intake contributes to heart disease, maybe penguins could unlock the biotechnology of the future to allow us to have our salt cake and eat it, too!

Reference:

John Sparks & Tony Soper. (1987). Penguins. Facts on File, Inc. 460 Park Avenue South, New York, NY.