1. Water enters the mouth: Trout open their mouths and take in water.

2. Water passes over the gills: The water flows over their gills, which are located in the pharynx (throat) area.

3. Oxygen diffuses into the blood: The gills are covered in thin, filamentous structures called lamellae. These lamellae have a large surface area and are rich in blood vessels. As water flows over the lamellae, oxygen dissolved in the water diffuses across the thin membranes into the trout's bloodstream.

4. Carbon dioxide diffuses out: Simultaneously, carbon dioxide from the blood diffuses out of the capillaries and into the water.

5. Water exits the gills: The water, now depleted of oxygen and enriched with carbon dioxide, exits the gills through openings called gill slits.

Important note: Trout are cold-blooded, and their metabolic rate (and thus oxygen demand) is lower in cold water. This allows them to survive in water with low oxygen levels, as long as the water temperature is cold enough.