Krogh was a pioneer in comparative physiology. He wrote his thesis on the respiration through the skin and lungs in frogs: "Respiratory Exchange of Animals", 1915. Later Krogh took on studies of water and electrolyte homeostasis of aquatic animals and he published the books: "Osmotic Regulation" (1939) and "Comparative Physiology of Respiratory Mechanisms" (1941). He contributed more than 200 research articles in international journals. He was a constructor of scientific instruments of which several had considerable practical importance, such as the spirometer and the apparatus for measuring basal metabolic rate.