Homo aquaticus: The Science of an Underwater Gill
In 1962, underwater explorer Jacques Cousteau predicted the arrival of Homo aquaticus: people surgically equipped with gills who could live and breathe in any depth for any amount of time without harm. Lately, there has been a lot of buzz about Triton, a conceptual gill (http://goo.gl/pWkd5k) that supposedly could allow humans to breathe underwater. There are many reasons why this device is still in the realm of science fiction. But first, it’s helpful to understand how some animals breathe air underwater.
Breathe, Breathe in the Air: Like us, insects breathe oxygen from air, using a system of canals connected to the outside by breathing holes or spiracles. So how do aquatic insects survive submerged underwater, often for their entire lives? Mosquito larvae develop tiny snorkeling tubes, called siphons, that poke out of the water for regular refills. Others, like mayflies and damselflies, develop biological gills that extend into the water to extract oxygen by diffusion. The champion for ingenuity, however, is the diving beetle which carries a bubble of water tucked under its body, seen as a silvery sheath in the photograph. The air bubble is a short term supply of oxygen, that is replenished from the surrounding water based on a few simple physical principles that are fun to consider!
Love is like Oxygen: Water contains dissolved oxygen, reaching up to 5% in volume in icy-cold streams, but much less than the 20% found in the atmosphere. As oxygen is consumed by the insect, it creates a partial pressure difference inside the air bubble. This is “corrected” by dissolved oxygen that diffuses in from the water. There is a lot of unused nitrogen in the air bubble, 80% by volume, which is free to diffuse out , also creating a similar partial pressure deficit. Because there is very little dissolved nitrogen present in water (it has lower water solubility than oxygen), some of the nitrogen’s partial pressure deficit is “corrected” by oxygen diffusing in, enriching the insect’s air supply. So as long as the rate of oxygen diffusing in keeps up with the rate at which it is consumed by the insect, all is well. Unfortunately, the surrounding pressure of the water can shrink the size of the bubble over time, reducing the surface to volume ratio and hampering gas exchange. That’s why some insects make the occasional trip to the water surface, to refill their air bubbles. For those insects that don’t have this option, a plastron is the answer.
What the Fakir?: A plastron is a special array of rigid, closely-spaced hydrophobic hairs (setae) that create a fixed “airspace” next to the body. Air trapped within a plastron operates as a physical gill (just like air in a bubble) but this airspace cannot shrink in volume because a double layered fortress of setae prevents encroachment of surrounding water. Think of the analogy of a fakir lying on a bed of nails: while one nail can puncture through his skin, lying on many nails effectively distributes his body weight so that the skin, like the surface of water (inset images below), is not broken. Also, the setae do a good job of repelling water using the lotus effect covered in an old post (http://goo.gl/yW7QpC).
Triton or not Triton?: Back to the beginning, will a physical gill work for humans? Humans need a lot more oxygen than beetles, so enormous surface areas will be needed to extract oxygen from water. Too much or too little oxygen in the air we breathe can be toxic. Still, a terrier named Muggins survived a 3 hour dip in the Mississippi river using articificial gills. Check out the story (http://goo.gl/xdJeQd) and tell me if you think Homo aquaticus will soon be in a pool near you!