Research Background*

     How can a fish, which is underwater, breathe if there is no air? When we go under water, we have to bring air with us to survive. Whales and dolphins have lungs that store air from the surface. Fish do not have lungs, and they rarely ever venture into the air, so how do they survive? We all know it has something to do with gills, but what exactly?

     The water surrounding a fish contains a small percentage of dissolved oxygen. In the surface waters there can be about five ppm of oxygen in the water. This is much less than the 210,000 ppm of oxygen in the air, so the fish must use a special system for concentrating the oxygen in the water to meet their physiological needs. Here it comes again, a counter current exchange system, similar to the one found in the fish's swim bladder and in the tuna's muscles.

     The circulation of blood in fish is simple. The heart only has two chambers, in contrast to our heart, which has four. This is because the fish heart only pumps blood in one direction. The blood enters the heart through a vein and exits through an artery on its way to the gills. In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body. The oxygen is used in the body and goes back to the heart. This is a very simple closed-circle circulatory system.

     The gills: The gills are composed of a gill arch (which gives the gill rigid support), gill filaments (always paired), and secondary lamellae (where gas exchange takes place).

     The blood flows through the gill filaments and secondary lamellae in the opposite direction from the water passing the gills. This is a couter current system and is very important for getting all of the available oxygen out of the water and into the blood.

     If the blood flowed in the same direction as the water passing it, then the blood would only be able to get half of the available oxygen from the water. The blood and water would reach equilibrium in oxygen content and diffusion would no longer take place.

     By having the blood flow in the opposite direction, the gradient is always such that the water has more available oxygen than the blood, and oxygen diffusion continues to the place after the blood has acquired more than 50% of the water's oxygen content. The counter current exchange system gives fish 80-90% efficiency in acquiring oxygen.

     How do fish ventilate (or move water through) their gills? Fish must pass new water over their gills continuously to keep a supply of oxygenated water available for diffusion. Fishes use two different methods for keeping a continuous supply of new water available, one is very simple and the other complex.

     --Ram Ventilation: Swim through the water and open your mouth (such as a shark would). Very simple, but the fish must swim continuously in order to breathe, not so simple.

     --Normal Ventilation: Occurs by the fish taking in water through the mouth. The mouth closes, forcing the water back over the gill filaments and out through the gill slits.

How do fish gills work?

Fish gills are a pretty complex structure, and are very well adapted to getting oxygen out of water. Gills are made up of filaments (the feathery red things) attached to a rigid gill arch. The arches are hollow and have arteries inside them that contain blood low in oxygen. These arteries branch into smaller arterioles that run inside the filaments. Each flat filament has many tiny folds on it (called lamellae) to increase surface area. In fast moving fish, the surface area of the gills may be ten times that of the actual animal. Tiny capillaries branch off of the arterioles and carry the blood close to the inner surface of the lamellae. Because the oxygen concentration is less in the blood than in the water flowing over the gills, the oxygen from the water naturally diffuses into the blood.

There is an adaptation that fish have to maximize the flow of oxygen into the blood called countercurrent exchange. This is when the water flowing over the lamellae is in the opposite direction as the blood flowing through the capillaries. In this way, the concentration of oxygen in the blood as it moves through the capillaries is always lower than the water, and oxygen will diffuse over the whole length of the lamellae.

Once the blood is fully oxygenated from it's trip through the gills, it is pumped back into the body and used by the fish for energy, filling the float bladder, and for nearly all of the metabolic processes in the fishes' body.

Breathing Underwater

Fish extract oxygen from the water using their gills in a manner somewhat similar to how land animals extract oxygen from the air using lungs. In both cases, oxygen diffuses into the blood through a thin, permeable membrane. Of course, there are some major design differences between gill and lungs - it would take a comparative physiology class to cover everything, but basically, the lungs of mammals are like a balloon with a single opening. Air goes in, and then goes out again through the same opening (bi-directional). Oxygen is transferred from the air to the blood across a thin membrane in the lungs, in the tiny, surface area enhancing pockets called alveoli . Gills, on the other had, can have water flowing past them constantly in one direction (unidirectional). Most fish take advantage of this by having a "counter current" blood system where the blood in the gills travels in the opposite direction to the water flow. This allows for up to up to 80% efficiency in getting the oxygen from the water to the blood, much better than what lungs can accomplish. This efficiency is also helped by the very fine structure of the gills, which greatly increases their surface area.

Fish have very these very efficient gills because of the fact that water contains about one-thirtieth as much oxygen per volume as the atmosphere above it. To put that in perspective, if we were somehow able to "breath" water, we would need to take about 450 "breaths" per minute just to get enough oxygen into our lungs!

People have been thinking about how humans could "breath" underwater for quite some time (just think of the military implications!) For a review of this topic, see:
Kylstra J. 1982, Liquid breathing and artificial gills, in P. Bennett and D. Elliott, The Physiology and Medicine of Diving, Bailliere and Tindall, London.

For an interesting experiment in which dogs did just fine breathing a liquid other than water, see:
Model, J., C. Hood, E. Kuck and B. Ruiz, 1971. Oxygenation by ventilation with flourocarbon liquid FX-80.l Anesthesiology 34:312-320.

* - Sections in Research background have been taken from http://www.geocities.com/aquarium_fish/how_fish_breathe.htm and http://oceanlink.island.net/ask/fishy.html#8457450