The frog's heart has 3 chambers with 2 artia and a single ventricle. The atrium receives deoxygenated blood from the veins. The left atrium receives deoxygenated blood from the lungs and the skin and both the atria empty into a single ventricle. The ventricle is divided into narrow chambers that reduce the mixing of blood and so the oxygenated and deoxygenated bloods separate. This is why it seems that forg heart has incomplete blood circulation because it has got single ventricle. When the ventricle contracts the oxygenated blood from the left atrium is sent to the carotid arteries taking blood to the head and brain. Deoxygenated blood from the right atrium is sent to the pulmocutaneous arteries taking blood to the skin and lungs.  Only the blood passing into the aortic arches is mixed but even it contains enough oxygen to supply blood to the rest of the body.