Treading Water – The Science of Gills

Did you just get 90's goosebumps? I know I did.

Did you just get 90’s goosebumps? I know I did.

Sometimes, science lets me down in a very devastating, stood up at prom, “Santa Claus isn’t real” kind of way. That may be a surprising statement coming from yours truly but even I like to daydream occasionally without science butting in.

Here’s what happened…

I had the afternoon off. I’m sitting in my easy chair with the torn, pleather upholstery eating fried rice, watching Water World on Netflix (a B-movie classic), and generally loving life with a capital L. There’s this scene where Kevin Costner’s character is scavenging the ocean floor for trinkets to sell back at the surface of a post apocalyptic Earth and I had this rush of childhood nostalgia. Ever since I was a kid I would think “How bad-ass would it be if humans had gills?!” It just seemed like such an unfortunate detour for human evolution to move us out of the water, a medium that 71% of the Earth’s surface is covered in. For the remainder of the movie I just played “what if” scenarios out in my head. Eventually, science (specifically human physiology) stepped in to yank my head out of the fluffy clouds and bring my senses back to the boring-ass ground.

I realized that while gills are a marvel of natural selection, gills alone are simply not enough to allow human beings to breathe under water. Let me explain…

# 1 – We make lousy fish

Try not to be too negative. It’s not that we are poorly adapted to living under water but that we are perfectly adapted for living on land. We breath atmospheric oxygen (which makes up 21% of the atmosphere). Fish breath dissolved oxygen in water (which makes up a fraction of a percent). In case you didn’t know, oxygen is amazing and because we get ours in abundance our metabolism can burn hotter and longer. Since we’re mammals that typically means that a considerable amount energy goes into maintaining a constant core body temperature. Sure fish are pretty active as cold-blooded critters go; their sleek and slender bodies allow them to use less energy moving through the water, and being cold-blooded (with lower metabolic demand) their overall oxygen demand is already quite low. Humans, however, need that extra oxygen to power an engine that fish don’t have, a highly developed brain.

You could argue (I encourage that) that our dependence on oxygen is the price we pay to evolution for the gift of conscious thought. We are not just mindless clusters of cells responding to stimuli, governed by instinct, swimming in schools like minnows. We make decisions, hesitate, regret, learn, and even celebrate when we get it right. Yay humanity!

#2 Gills are not a good look for us…

The secret to gills is surface area. In case you didn’t know, anytime you have a mechanism that can fit a lot of material into a relatively small space without disrupting its functionality, scientists go “ooh, look at that surface area!” For example, within our small intestine is a vast area of protruding microvillae. These protrusions have their own unique surface and that diversity of surfaces aids in absorption.

The accordion-like folding of gills are able to effectively extract the oxygen from water by cramming in countless, minute rows of cells and vasculature. In fish, gills are the site of gas exchange and it doesn’t get anymore direct. They literally just have arteries from the heart branching into arterioles at the site of the gills. The oxygen poor blood from the heart gets juiced-up with fresh O2 from the gills that goes back into circulation. It’s creepily similar to the way our lungs work; our heart pumping O2 poor/CO2 rich blood to the lungs in exchange for the good stuff every time we inhale and exhale. Our lungs however are inside our bodies. A fish’s breathing apparatus is completely fly-open and exposed to the surrounding water for diffusion to take place and I don’t know if you’ve noticed but gills take up a lot of space on a fish. The more active the fish, the larger its gills are and the their size increases exponentially with body size. If humans had gills we’d need more than a turtle neck to cover them up when we’re hitting up the mall.

Sure sharks, dolphins and whales seem to breath just fine. Well whales and dolphins have to surface to breath through their blow holes. Sharks have gills but they have to pump gallons of water through their mouths constantly and some sharks have to move constantly to increase the flow water over their gills. I don’t know if I’d call that winning.

Oh future technology...I wish you were real!

Oh future technology…I wish you were real!

#3 Water is the problem…

The bottom line is that there just isn’t enough oxygen available in sea water to support our metabolism. Even if we could design artificial gills that did a fine job of extracting oxygen from the water (and there are many impressive prototypes out there) it wouldn’t get us very far. We can’t turn the dial down on our metabolism. Our bodies will still use energy to maintain our core temp as the colder water draws heat away.

Even if we could manage just a gasp of precious air our muscles would demand their share, rapidly yanking O2 from hemoglobin in the circulating blood as we struggle to swim, demanding even deeper breaths. Eventually our muscles would fail, burning with lactic acid from the oxygen debt.  Carbon dioxide accumulates in our body. Our gag reflex gets overwhelmed. With nowhere to escape, water drains into our stomachs and…I’m not even going to mention the resulting brain death. Did I mention the brain death?

Hey listen, don’t be so dark. We live on land! The surface is where it’s at. We have sunsets, flowers, and sooo many colors. Trust me, blue water would get really old really fast. Not to mention, we don’t have to eat soggy cheeseburgers. So close your eyes, take a deep breath (because we can) and take a moment to appreciate life on land.

Stay curious, stay classy, and never stop learning my friends 🙂