Respiration is the process by which oxygen is obtained from the external environment and carbon dioxide is eliminated. There are two phases of breathing; external respiration and internal respiration, and also three main respiratory structures: gills, integument exchange areas and lungs. External respiration occurs in the capillaries of the gills or lungs, although in some vertebrates, such as amphibians, the skin is also used, which constitutes the areas of integumentary exchange. Internal respiration is the cellular utilization of carbohydrates, which then produces carbon dioxide, water and releases energy. This is created by a complex chain of chemical reactions called cellular respiration with the help of respiratory enzymes present in the protoplasm of all cells. The function of the respiratory system is to supply oxygen to the blood and eliminate carbon dioxide and other waste gases. A respiratory organ, in addition to gill respiration, appeared in fish about 400 million years ago. It consisted of a single primitive lung ventilated by a buccal pump. This breathing system was first adopted by ...