A frog's heart rate can be anywhere from 40 to 50 beats per minute. Frog's are tailless amphibians. Like lizards, their hearts have three chambers, not four.
When a frog breathes, the oxygenated blood and the deoxygenated blood flow through separate atria, or chambers of the heart. Both bloodstreams then enter a common ventricle, the third chamber of their heart. Narrow cavities divide up this ventricle, however, so as to prevent the blood types from mixing profusely.
The two different types of blood then pump into different cardiovascular pathways. The oxygenated blood enters the aorta and the deoxygenated blood flows into the pulmonary artery. This cardiovascular system allows frogs to have a higher metabolic rate than would be possible otherwise. It also allows them to be more active.