Q.Describe the structure of the frog's heart.

[!TLDR] The frog heart has two atria and one undivided ventricle, with a sinus venosus and truncus arteriosus. …

[!ANSWER] The frog's heart is organised into three chambers, reflecting its double (pulmonary + systemic) circulation while retaining an undivided ventricle.

Deoxygenated blood returning from the body first collects in a thin-walled chamber, the sinus venosus, which then opens into the right atrium. Oxygenated blood, returning from the lungs and the skin (both of which respire in the frog), enters the left atrium directly. Both atria contract and empty their blood into a single, undivided ventricle, where the oxygenated and deoxygenated blood streams mix to some degree. …

• Describing the frog heart as fully four-chambered — its ventricle is single and undivided, unlike the bird/mammal heart. …