The respiratory system of fish is the gills. Breath condition is in the water. Fish in the water, each gill piece, gill small pieces are completely open, so that the gill and water contact area expansion, increase intake of water by the dissolved oxygen opportunities. there is a small gill capillaries, where the skin is thin, when the blood flows through here, on the completion of the gas exchange: carbon dioxide through the gills will bring a small piece of thin-walled, sent to the water; the same time, drain the water of oxygen, oxygen With the blood circulation and transported to all parts of the body to go. As the mouth and gill cover, alternate opening and closing can make the water continuously from the mouth into the oral cavity and pharynx to reach through the gill cavity, and gill filaments approach, and from the gill holes routed outside the respiration of fish is in the process of completed. The water of oxygen in the process, was to absorb the blood vessels within the gills of fish, the fish would not have oxygen. Therefore, we say that the fish is to use gills to breathe, but the gills can not directly absorb oxygen from the air, the fish out of water, a result of later die of suffocation.
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