Blood starts in a frog's heart where it gets sent off to either the lungs or the skin to become oxygenated, then it gets sent back to heart to be mixed with less oxygenated blood before being pumped to all organs and extremities through blood vessels. Once the blood has been used by organs and muscles, the newly deoxygenated blood is transported back to the heart.
The circulation of blood through a frog is dictated by the three-chamber process of a frog's heart. A frog's heart consists of two atriums that carry either oxygenated or deoxygenated blood, and a single large ventricle that mixes the two types of blood. Deoxygenated blood enters the left atrium of a frog's heart, where most of it is transported to both the lungs and the skin. Frogs are able to oxygenate blood at both of these locations, because they are able to partially breath through their skin. Once the blood has been oxygenated, it is sent back to the right atrium of the heart.
From here, the oxygenated blood in the right atrium spills into the ventricle where it mixes with a little bit of the deoxygenated blood from the left ventricle. The blood mix is then pumped up to the brain and then to all of the frog's limbs and organs.