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The respiratory system is a system of organs of humans and other animals, which serves for the gas exchange of the body with the environment (ensures the supply of oxygen and the removal of carbon dioxide). Oxygen organisms can receive from the air (air breathing), or consume oxygen dissolved in water (water breathing). Respiratory organs are present only in aerobic organisms, in anaerobic organisms they are absent. In humans, other mammals and birds, the anatomical features of the respiratory system include the respiratory tract, lungs, and special muscles. In some animals (in particular, amphibians, fish, and a number of crustaceans), skin respiration plays a vital role in gas exchange when oxygen enters through the surface of the body. Intestinal respiration is often referred to as skin respiration when the function of gas exchange is performed by the intestinal lining (in the intestinal cavity). In fish and other aquatic animals, the main respiratory organ is the gills — the outgrowths covered with blood vessels. Insects have a very simple respiratory system – the trachea (thin pneumatic tubes). Plants also have a respiratory system, but the direction of gas exchange is opposite to that of animals. In protozoa and lower multicellular organisms (protozoa, sponges, intestinal cavities, many worms) respiratory organs are absent, and gas exchange is carried out only by diffuse respiration (through the surface of the body).
Respiratory organs of animals were formed in connection with an increase in the area of the respiratory surface: protrusion or sticking of the skin. Most primarywater animals have protrusions of the outer integuments that perform the respiratory function: the gills of fish and crustaceans, the ctenidia of mollusks, the gillbook of the horseshoe crabs, the skin gills of echinoderms. In some aquatic animals, internal respiratory surfaces were formed: aquatic lungs of sea cucumbers, anal respiratory system in aquatic larvae of dragonflies, plastrons in some water bugs.