Frogs are amphibians, which means they live part of their lives in water and part on land. They have different ways of breathing in each of these unique environments.
All frogs breathe through their skin in a process known as cutaneous respiration. In addition to breathing through the skin, frogs also have well-developed lungs which they use to breathe air.
Most frogs (not all) start their lives as fully aquatic tadpoles. At this stage, they breathe through their gills.
Over time, the tadpoles transform into frogs in a process known as metamorphosis. Once fully transformed, frogs will have two ways of breathing: via the skin, and also via the lungs.
Tadpoles Breathe Through Their Gills
Most frogs start their lives as tiny embryos inside an egg sac. With time, tadpoles will hatch from these eggs.
Tadpoles are very different from adult frogs; this is because they’re adapted for a fully aquatic life, while adult frogs are adapted for a semi-aquatic, or mostly terrestrial life (depending on the species).
Frog tadpoles have gills on either side of their head, and a flat paddle-like tail fin to help them swim in the water.
At this stage, frog tadpoles breathe in the water through their gills, just like fish.
The gills have thin membranes and a network of blood vessels. These membranes absorb dissolved oxygen from the water and move it into the bloodstream.
At the same time, carbon dioxide from the bloodstream passes through the membranes into the water.
Apart from their gills, tadpoles also breathe through their skin.
Dissolved oxygen in the water that comes in contact with their skin is diffused into their bloodstream, and carbon dioxide is released from the bloodstream into the water.
Tadpoles Grow Into Adult Frogs
After a few months, to as long as three years in some species, the tadpoles undergo a process known as metamorphosis and transform into adults.
During metamorphosis, the tadpoles lose the features that help them live in the water and develop features more suitable for living on land:
- They lose their gills, and their lungs enlarge and develop further
- The tail shortens and is eventually absorbed into the body
- They develop strong legs for moving on land
Once metamorphosis is complete, the tadpoles are young frogs that will leave the water to live on land. These young frogs will grow and mature into adults over time.
Since most frogs are semi-aquatic, they will still occasionally return to the water to soak, swim, or mate and lay their eggs.
All Adult Frogs Can Breathe Through Their Skin
As adults, all frogs can breathe through their skin and thin membranes in their mouth and throat to get extra oxygen.
The skin is made up of thin membranous tissue that is highly permeable to water and contains a large network of capillaries and other blood vessels close to the surface.
The thin membranous skin allows oxygen from the air to be absorbed into the bloodstream via diffusion. At the same time, carbon dioxide from the bloodstream passes through the skin and membranes and is diffused into the air.
This process of “skin breathing” known as cutaneous respiration is very similar to what happens inside our lungs.
To efficiently breathe, frogs have to keep their skin constantly moist by secreting a mucous coating. They can only absorb oxygen through their skin if the skin stays moist. If it dries up, they can suffocate and die.
In some frog species, cutaneous respiration is so efficient that it is their main way of breathing. For example, the Titicaca water frog is an aquatic frog species with wrinkled skin.
This frog’s excessive amount of skin makes it look rather as if it were wearing an oversized wet suit. But it performs a very important function– it provides a bigger surface area to use for cutaneous respiration.
For this reason, Titicaca water frogs have greatly reduced lungs – less than one-third the size that would be expected of other frog species of the same body size.
Because of their large skin surface area, these frogs mainly breathe through their skin, and only occasionally use their lungs. In fact, they can stay for several days without using their lungs.
Frogs Also Breathe Using Their Lungs
In addition to breathing through the skin, adult frogs also have well-developed lungs which they use to breathe air.
That said, frogs, and other amphibians lungs are quite primitive when compared to our own. They do not have diaphragms to push air into the lungs as we do. Instead, they rely on a process known as “buccal pumping.”
How Buccal Pumping Works
This is basically a process of moving air in and out of the lungs through the use of the mouth and throat muscles.
To breathe in, the frog will lower the floor of its mouth, which causes the throat to expand. The air then rushes into the throat, through the mouth, and through the open nostrils.
The frog will then close its nostrils, and raise the floor of its mouth, which causes the throat to contract, and pushes the air into the lungs.
Once the air is in the lungs, oxygen from the air is then absorbed into the frogs’ bloodstream. At the same time, carbon dioxide from the bloodstream is diffused into the air.
To breathe out, the frog will lower the floor of its mouth, which causes the throat to expand and draws the air from the lungs, and into the mouth. It will then open its nostrils, and raise the floor of its mouth, to push the air out of the nostrils.
The Frog That Breathes Entirely Through Its Skin
The endangered Bornean Flat-Headed frog found in Indonesia is the only currently known species of frog that does not have any lungs at all! It breathes entirely via its skin, just like many lungless salamanders.
This aquatic frog lives in cold, fast-moving rivers that provide a rich supply of oxygen and most likely lower the frog’s metabolic rate. So to the frog, oxygen is in high supply and low demand.
Also, the flattened body and head (which gives this frog its name) give its skin a large surface area for absorbing oxygen from the surrounding water.
Conclusion
Frogs are amphibians, with permeable skin. They use this skin to breathe, and also to absorb water required for their bodily functions.
In addition to breathing through their skin, frogs also breathe using their well-developed lungs.
This dual system of respiration gives frogs all the oxygen they need, weather they are on land, or in the water.
Sources:
Tattersall GJ. Skin Breathing in Amphibians. In: Aird WC, ed. Endothelial Biomedicine. Cambridge University Press; 2007:85-91.