Frog's heart has one ventricle, while humans have 2, hence deoxygenated blood from body mixes with the oxygenated blood coming from lungs. Source: Socratic
The frog heart has 3 chambers: two atria and a single ventricle (humans have 2). The atrium receives deoxygenated blood from the blood vessels (veins) that drain the various organs of the body. The left atrium receives oxygenated blood from the lungs and skin (which also serves as a gas exchange organ in most amphibians).
