Leopard Frog - Rana spp. - Internal Anatomy
The internal anatomy of the adult frog is, on a very basic level, quite similar to that of mammals - they have the standard set of vertebrate organs (a heart, some lungs, a nervous system, a stomach, and some other digestive organs), but once you look closer, you can see how different the frog really is.
For one, their skin absorbs oxygen directly through water, and if they dry out, they suffocate - their lungs aren’t nearly large or strong enough to provide the oxygen for their entire body. The frog has no functional ribs or diaphragm, and they must breathe using buccal pumping - moving the floor of their mouth up and down to inflate and deflate the lungs.
Frogs have a single excretory orifice, like birds and reptiles, called the cloaca - all waste and reproductive excretions go through the same hole. Their paired kidneys actually function fairly similarly to mammalian kidneys, but their endocrine system does not conserve water inside the body, like our kidneys and endocrine system do. Because of this, even frogs with strong lungs would die quickly if they had no access to water, due to dehydration.
The hearts of frogs are also somewhat different from mammals. They have only three true chambers. The oxygenated blood from the lungs, and the deoxygenated blood from the tissues enters the heart through separate atria. When the heart beats, it pumps the blood into a common ventricle, which has a partial septum (dividing tissue), to minimize the mixing of the oxygenated and deoxygenated blood. The blood then passes through a spiral valve to the appropriate vessels - the aorta for oxygenated blood, and the pulmonary artery for deoxygenated blood.
Brehms Tierleben, Allgemeine Kunde des Tierreichs. Dr. Otto zur Strassen, 1913.