
After water enters the fish's mouth it passes through a structure called the gill rakers.

The function of the gill rakers is to act as a filter system by sieving out particles such as food or any other foreign particles that may have entered the mouth from the inflow of water. The filtered water then travel through the gill arches thus passing over the gills. Here each gill consists of two rows of extremely thin membranes called gill filaments. The structure of the gill filaments consist of sequence rows of a thin, disc-like membrane loaded with a capillary network called lamellae. Gill filaments which are projected out into the water flow allows water to flow across the lamellae. This will result in gas exchange where, oxygen and carbon dioxide are exchanged directly across the capillary membrane.As water flows through the gills, the oxygen it contains (dissolved oxygen) passes into blood circulating through the gill structures (filaments and lamellae) while simultaneously, carbon dioxide ( the waste product) in the fish’s bloodstream passes into the water. This deoxygenated water is then carried away and out of the body through the operculum. This process repeats itself continually and this is the process by which fish breathe. This is process is also known as respiration.
Where do fish get their oxygen from?
Where do fish get their oxygen from?
Water is formed from the combination reaction of hydrogen gas (H2) and oxygen gas (O2) to produce liquid water (H2O). In other words, each molecule of water is made up of two hydrogen atoms and one oxygen atom. However the oxygen that fish breath is not the oxygen found in H2O - the molecular formula of water. The oxygen in liquid water is bound to two hydrogen atoms forming water molecules, thus oxygen from is not free to take part in respiration. Therefore the fish cannot breathe the resulting liquid water, and the oxygen gas (O2) that the fish breaths comes from that which is dissolved in the water. Hence, fish "breathe" the dissolved oxygen out of the water using their gills. This also means that if there is not enough dissolved oxygen in the water, then the fish can suffocate, just as humans and other animals will if there is not enough oxygen in the air that we breathe.
Geraldine Lee(10):)