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Frogs
Frogs have some of the strangest hearts in the animal kingdom. While humans have a four-chambered heart with one atrium and one ventricle on either side, frogs have three chambers: two atria and one ventricle. This means their oxygen-depleted blood is not completely separated from the oxygen-rich blood, creating a less efficient circulation system. This is why frogs need to take frequent breaks while hopping around, as their hearts need to work extra hard to pump blood around their bodies. Interestingly, frog hearts can also stop beating while they are still alive.
