Frogs respire on land and in the water by two different methods. In water, skin acts as aquatic respiratory organ (cutaneous respiration). On land, the buccal cavity, skin and lungs act as the respiratory organs.
If mouth and nostrils are forcible kept closed for a few days, frog would die due to absence of oxygen as air from nostrils will not enter into lungs and pulmonary respiration cannot take place.
Frog heart has three chambers, two atria and one ventricle and is covered by a membrane called pericardium. A triangular structure called sinus venous joins the right atrium.
Frogs are generally ureotelic (excreting nitrogenous waste as urea) in their adult life on land. They do not change to uricotelic, which is a characteristic of animals that need to conserve water, like birds and reptiles. It's the tadpole, living in water, that is ammonotelic (excreting ammonia), and it turns ureotelic as it metamorphoses into a terrestrial adult.

──────────────────────────────────────────────────────────────────────────────────────────