The Respiratory System of a Goldfish

Goldfish obtain oxygen from the water surrounding them.
i goldfish image by martin schmid from

The goldfish is a common household pet and a favorite among children because it is an easy-to-care-for hardy freshwater fish. Of course, because goldfish live under water, they have a very different respiratory physiology than mammals have. For example, like most other aquatic fish, goldfish are adapted to breathe water instead of air, using a one-way-flow respiratory system.

One-Way Flow

A one-way-flow respiratory system means that water is pumped in one direction across the fish’s breathing organs, the gills. The water flows into the mouth, over the filaments in the gills, and out through the gill slits in the sides of the fish’s head. Thus, the water flows in one continuous stream.

Oxygen Exchange

Goldfish need oxygen just like humans, and they obtain oxygen from the water. The fish need to transfer the oxygen from the water into their bloodstream. This transfer of oxygen occurs over a large surface area of capillary membranes contained in the feathery filaments of the gills. According to Dr. Raylman at West Virginia University, only 3 percent of the amount of oxygen in the air is found in the same volume of water, so fish have had to evolve to efficiently push the dense volume of water over their gills to get the oxygen that they need. In fact, fish are able to extract 85 to 90 percent of the oxygen from the water that flows over their gills, whereas humans extract only about 25 percent from the air.

Factors Affecting Respiration

The respiration of a goldfish will change depending on certain factors. First, if the oxygen concentration in the water is high, their breathing will decrease so that they do not take in toxic amounts of oxygen. If there the concentration of oxygen in the water is low, their breathing will increase to obtain the necessary amounts. A goldfish’s breathing is also affected by water temperature. At lower temperatures the surface area for gas exchange increases, thus the breathing rate decreases. When the water temperature increases, the respiration increases to compensate. Finally, respiration is affected by pH change in the water. Fish thrive on pH levels close to neutral, around 7. If the water is too acidic or alkaline, oxygen intake is reduced.

Aquarium Considerations

If you decide to have a goldfish as a pet, make sure you consider all the factors affecting respiration for the water in your aquarium. Keep in mind that overcrowding your aquarium with too many goldfish can lead to polluted water and insufficient oxygen content, forcing your fishes' respiratory system to work harder. Appropriate, clean water leads to happy, healthy goldfish.

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