how do fish respire?

In fish respiration occurs through gills. Gills are composed of several comb like filaments that are filled with blood capillaries. When water flows over the gills oxygen dissolved in water enters inside the blood capillaries and gets dissolved with blood. Blood then carries the dissolved oxygen to the heart from where it travels in all parts of the fishs body. At the same time the carbon dioxide that is produced in the body gets released in water from the blood capillaries of the gills.