Frogs and humans need to breathe for the same reason: to bring oxygen into the body and to expel carbon dioxide. We both have lungs for these tasks, but that's where most of the similarities end. Frogs live in a different environment than humans, and differences in their respiratory systems reflect that.
Humans breathe exclusively through their lungs, but frogs use their lungs for only part of their respiration. Frog lungs have thinner walls and are almost like balloons. They often fill their lungs to help them stay buoyant when swimming. Both species have bronchial tubes leading to the lungs, but human systems are more complicated, with many branching bronchiole. The lungs of frogs and humans have alveoli, tiny vessels that make the actual gas exchange. But the alveoli in humans are more densely packed because we breathe only through our lungs.
As humans, we use our diaphragm muscle to push up on our lungs and help us breathe out. Contracting the diaphragm, which sits under the lungs and separates our thoracic cavity from the abdominal cavity, pushes air out. Releasing the diaphragm lets the lungs stretch back out, making room for air you're breathing in. Frogs don't have a diaphragm. Instead, they use muscles in their throat sacs to help draw in air and push it back into the lungs.
Frogs have another organ they use to breathe: their skin. Frogs can exchange oxygen for carbon dioxide through their skin, but it needs to be moist for the process to work correctly. That's why you typically see frogs near water or burrowed in damp soil. Frogs' skin is an efficient breathing machine, exchanging carbon dioxide 2 1/2 time faster than their lungs can. In comparison, humans eliminate less than 1 percent of the necessary carbon dioxide through the skin.
If you've never seen a frog yawn, it's because they don't. They don't breathe through their mouths, only through their noses. We, on the other hand, have a choice of breathing through our noses or our mouths, or both. Frogs use muscles beneath their jaws to help move air, but their mouths stay closed while they breathe.
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