Amphibians have a specialized method of breathing through their skin called cutaneous respiration.
This lesson will walk us through its definition, relation to lung breathing, function, and adaptive nature in amphibian groups.
Why Cutaneous Respiration?
While anatomy teachers may joke that anything having to do with the word cutaneous is just darn cute, in reality, the term has nothing to do with cuteness and it has everything to do with the skin. Cutis-, Latin for ‘skin,’ is where we get words like the cuticles, in reference to our fingernails, and cutaneous respiration, which is breathing through the skin.Amphibians, you may recall, are vertebrate animals (those having a backbone) that complete a portion of their life cycle in water and another portion on land. There are also some completely aquatic, or water-living, amphibians.
Because of their dependence on water, many amphibians have retained the traits of their ancestors, including reduced or even absent lungs. Many rely on breathing through the skin, cutaneous respiration, for some or all of their oxygen intake, and especially for carbon dioxide output.The trait of cutaneous respiration is present in all three groups of amphibians: Caudata (salamanders and newts), Anura (frogs and toads), and Caecilians (uniquely legless amphibians).
How Does Cutaneous Respiration Work?
Cutaneous respiration occurs by way of countercurrent exchange, where blood circulating through the skin flows in the opposite direction of the absorbing oxygen or surrounding water.
Think of countercurrent exchange as two pipes pressed parallel together, with water flowing through at the same speed. Water in one tube is hot, and flows one direction, and water in the other tube is cold, flowing in the opposite (counter) direction. If the tubes are made of conductive material, like metal, and contact one another, heat will pass across one tube to the other.
This is similar to crosscurrent exchange, which bird lungs use to pick up oxygen. Blood capillaries branch across an air capillary at right angles, almost as if they were wrapped around one another. In contrast, our own lungs also pass carbon dioxide and oxygen between blood capillaries and in the tiny spaces in our lungs, but this transfer is across a gradient.
To understand how cutaneous respiration works, we have to take a look at how the first organisms started to breathe air out of water and make the first transitions onto land. Cutaneous respiration can take place in water or air, although it is much more efficient to take in oxygen through the air. Nevertheless, a moist barrier, or liquid surfactant, is necessary for efficient gas exchange, even in air. The water-to-air barrier reduces surface tension for oxygen uptake and carbon dioxide release. Our own lungs have surfactant liquid within them.It might sound like leaving the water in the first place was an evolutionary gamble.
Why did organisms adapt to live on land? And how did organisms such as amphibians keep both adaptations simultaneously?Lungs allow a direct gas exchange with a greatly increased surface area, where oxygen and carbon dioxide are exchanged into and out of the bloodstream across grape-like pockets known as alveoli, or larger pockets called faveoli in amphibians. This adaptation allowed vertebrates to fully take on the transition to land, to develop thicker and drier skins, and to colonize areas far from water sources. Despite this, cutaneous respiration still persists in amphibians. Now let’s look at some of the advantages and disadvantages of skin breathing, across the evolutionary history of vertebrates.
Evolutionary History of Cutaneous Respiration
Embryonic vertebrate animals already exchange respiratory gases through the thin, skin-like membrane that surrounds them.
But by the time they mature and are born, organisms like reptiles, birds, and mammals carry out relatively little gas exchange across the skin. This is not so in amphibians.The transition to land found early tetrapods, or four-limbed vertebrates, adapting to a mixture of air gulping and skin breathing. Populations of animals had to find efficient ways to take in oxygen, or they would perish. Lungs were one essential way to take in oxygen and expel carbon dioxide, and they persisted in many vertebrates. Even though lungs and gills continued as primary breathing organs for many organisms, skin remained an important barrier with a sizable surface area for the same gas exchange.In amphibians, cutaneous respiration was a throwback, an evolutionary holdover from the more aquatic days of their ancestors, the first fishes to gulp air.
Think of lungfish today, which still retain skin breathing and lung breathing. This throwback helps amphibians take advantage of the most efficient form of breathing, whether they are in dormant hibernation or simply have higher metabolic needs, such as during mating seasons.
Let’s look at a few amphibians and other vertebrates that have differentiated, or developed specialized adaptations to make the most of cutaneous respiration.
Some frogs, like the high altitude Titicaca water frog, have big folds and pockets of skin. These increase gas exchange in environments like the thin air of the Andes Mountains. The hairy frog produces hair-like projections for defense and supplementary respiration during breeding seasons.Even reptiles can exchange gases across the membrane of their cloaca, or all-purpose anus-like opening. Humans, too, respire very minutely through the skin, and bats use their wing membranes to eliminate up to 12% of carbon dioxide waste through their highly vascularized (full of blood vessels) skin on the wings.One family of salamanders, the Plethodontidae, lacks lungs and gills entirely in the adult form. These salamanders rely entirely on cutaneous respiration to maintain their metabolism.
This incredibly risky adaptation somehow applies to the largest living group of salamanders.Despite widespread cutaneous respiration in many animals, there are notable drawbacks. Amphibians are highly susceptible to toxins that are absorbed through the moist skin barrier, and drying out can damage or kill amphibians that rely in large part on cutaneous respiration.
Cutaneous respiration serves to supplement oxygen exchange in amphibians, and is sometimes the sole or major breathing apparatus for certain amphibian groups.
There is a long and varied evolutionary history that ties cutaneous breathing to our earliest vertebrate ancestors, and to the ancestors of amphibians.Respiration through the skin has its benefits and drawbacks, but it has maintained a presence in vertebrate groups like amphibians for its metabolic advantages. It has manifested itself in a variety of interesting physiological ways, such as skin folds, skin projections, and lack of lungs.
Even we humans carry out just a little cutaneous respiration. Breathe a sigh of relief that we are not covered in slimy skin surfactant!