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Created By: Travis Lascalza
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Fish gills have three main components. {1}The gill arches, which provide structural support for the other gill parts, the gill filaments, which provide deoxygenated blood flow to the gill surface, and the lamellae. The lamellae are the most important parts of the gills since this is where gas exchange actually takes place. Fish have two main ways of maximizing the amount of oxygen that is diffused through the gill. The first is the enormous surface area of these lamellae, which are cell-thick and sheet-like membranes that are actually extensions of the filaments (the same blood flows through both).

The {2}second is the use of a countercurrent system of blood flow within the gills. This means that the blood flowing through the filaments and lamellae is in the opposite direction as the water entering the gills. This is accomplished by the fish having a unidirectional blood flow (rather than the bidirectional flow found in mammals). The heart of a fish only has two chambers, one to receive blood and the other to send it out to the rest of the body. Notice that our blood leaves the lungs and enters the heart, while a fish’s blood leaves the heart and enters the gills.

Why is this unidirectional blood flow important? Remember, compared to air, there is relatively little oxygen in the same volume of water. After entering the fish’s mouth, the {3}water is pressurized and forced into the gill cavity, and then leaves through the gill slits. Although by closing the operculum (the bony gill cover) the fish can increase the amount of time the water and blood are in contact, the gills must still extract as much oxygen as possible with each gulp. If you think back to high school chemistry you’ll remember a process called diffusion. Diffusion is a passive process (as opposed to active transport processes like that caused by pumps) where particles, such as oxygen and carbon dioxide, gradually flow from a high concentration area to a low one, resulting in both areas being equal.

In the gill’s case the blood (high CO2, low O2) comes into contact with the water (low CO2, high O2) through the microscopically thin membrane of the lamellae. Carbon dioxide seeps into the water and oxygen seeps into the blood vessels. Having a unidirectional and countercurrent blood flow the gill is able to exchange more gases because equilibrium between the two fluids is not reached (which would stop the gas transfer periodically). This essentially doubles the amount of gases the gills are able to exchange. Think of the lung as a balloon (air goes in and out the same opening) and the gill like a system of pipes (blood flows in one end and out the other).

Ok, this diary needs to be perked up a bit, so let’s take a look at some interesting gill factoids. For example, why does a fish die when it’s removed from the water?

Fish gills, being so much more efficient at extracting oxygen, can actually utilize oxygen in the air. The problem is the gill structures have evolved in the relative weightlessness of their aquatic world. The reason they die is because the lamellae, with their enormous surface area, collapse when taken out of the water, preventing gas exchange from occurring. Fast swimming open water fish, which need more oxygen in their blood to support their high energy lifestyles, have finer filaments and lamellae than slow moving coastal species. While a coastal fish like a minnow or carp may survive quite some time out of water, any fisherman can tell you how quickly a tuna or other powerful pelagic swimmer dies when landed.

Of course, some fish have adapted to near terrestrial living and they’ve accomplished this by strengthening their lamellae to prevent it from collapsing. An example is the walking catfish.

How about the question of why fresh water fish die in salt water and vice-versa? Here again the culprit is the gills, but this time it’s not oxygen that is the problem, but salts in the blood. This has to do with osmosis, which is related to diffusion discussed above (osmosis is the movement of liquids from high to low concentration, while diffusion is the movement of elements within the liquid). Fresh water fish have a blood salt concentration higher than the surrounding water so are constantly losing water through the gills. They correct this by drinking nearly constantly. In salt water this doesn’t work as it serves to increase the salt concentration within the body and the fish basically dessicates internally.

Saltwater fish have the opposite problem. Their blood has a lower concentration of salts than the surrounding water and must constantly excrete moisture. When placed in fresh water the fish cannot remove moisture fast enough and bloats, destroying internal organs and the fragile blood vessels in the gills.

Some fish, such as salmon can survive in both fresh and salt water. These species are known as osmoregulators and are able to control the salt concentration in their bodies to adapt to any salinity using both the gills (to remove excess water) and the kidneys (to absorb excess salts).
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