All animals need oxygen to survive. Those that are not lucky enough to be able to absorb this element directly through the skin, like jellyfish and earthworms can, must have a way of physically removing it from their surroundings in a usable form (O2). For terrestrial vertebrates this is accomplished using lungs.
Lungs are complex organs, but the process of gas exchange is pretty simple once the infrastructure is in place. As we breathe, air enters the lungs through the bronchi and then follows a path of narrower and narrower passageways until it reaches the tiny air sacs known as alveoli. Here the air meets small blood vessels embedded within the lung tissue called capillaries. Hemoglobin in the capillary blood have collected carbon dioxide from the cells within your body. Hemoglobin, however, more readily attracts oxygen molecules, so when the air sacs and the blood vessels meet through the thin alveoli walls the two gases are exchanged. The now oxygenated blood continues on its path to provide the body with oxygen and the now carbon dioxide-rich air is exhaled.
Animals that live underwater have some problems with respiration that must be overcome. First of all, water is much thicker than air, making both the intake of it, as well as the extraction of dissolved gases from it, more difficult. Not only that, water has roughly twenty times less oxygen (in its free form) than air. Although a water molecule (H2O) is one third oxygen, this cannot be used because it is bound with the hydrogens. Only dissolved O2 can be used.
So aquatic animals have evolved gills and in order to supply the animal with sufficient oxygen to survive, these organs must be much more efficient than our lungs are. Most marine invertebrates such as crustaceans, mollusks and echinoderms have gills, and although they vary structurally somewhat from teleosts, the basic process is the same. So to keep things simple, we’ll be using fish as our example.
Blood vessel rich gills dissected from
a tuna fish.
Fish gills have three main components. 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 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 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).
Coho Salmon back in the stream
it was born in.
What about shark gills? Well, shark gills work exactly like those of other fish with one exception. I mentioned the gill covers above (the operculum). This bony structure is what helps to pressurize and slow down the flow of water through the gills to maximize oxygen absorption. Being cartilaginous fish, sharks, skates and rays lack an operculum. So water must continuously flow over the gills. The multiple gill slits, usually five to a side, help somewhat to regulate the water flow. This leads to the common misconception that sharks must swim constantly or they will drown. This is false in all but a handful of cases. Many sharks rest regularly on the bottom, although they must constantly inhale water to provide the gills with a constant supply. These resting sharks (as well as rays) have structures called spiracles behind their eyes. These are basically holes in the head that lead to the gills, supplementing the water taken in by the mouth.
You can see the spiracles behind
each eye of this skate.
There are a few oceanic sharks, such as great whites, that are unable to do this so must swim constantly with the mouth at least partially open to keep a steady flow of water. This is known as "ram ventilation" as the fish must keep in motion to "ram" water down its throat. This is also why a shark will drown if it is towed backwards by its tail. I think they mention this in the movie "Jaws".
Other diaries in this series can be found here.