Q.Describe the structure of the heart of frog.

A frog's heart has three chambers — two atria and one ventricle — with accessory chambers (sinus venosus, conus arteriosus) that route mixed but largely separated oxygenated and deoxygenated blood streams to the body and the lungs/skin respectively.

Chambers, in order of blood flow:

1. Sinus venosus: A thin-walled triangular chamber that receives deoxygenated blood from the body via the two precavals and one postcaval vein, and passes it into the right atrium.
2. Right atrium: Receives deoxygenated blood from the sinus venosus.
3. Left atrium: Receives oxygenated blood returning from the lungs and skin (cutaneous + pulmonary circulation) via the pulmonary vein.
4. Ventricle: A single, thick-walled, muscular chamber (undivided) into which both atria open through a common atrioventricular aperture guarded by valves. Although the right (deoxygenated) and left (oxygenated) atrial blood streams enter the same ventricular cavity, the spongy internal musculature and near-simultaneous but slightly offset contraction limit complete mixing.
5. Conus (truncus) arteriosus: Arises from the ventricle and contains a longitudinal spiral valve, which — together with differences in the timing/pressure of blood flow — helps direct more oxygen-poor blood into the pulmocutaneous arch (to the lungs and skin for oxygenation) and more oxygen-rich blood into the systemic (carotid + aortic) arches (to the body). …