Fish use a slightly different mechanism, utilizing the unidirectional passive movement of water through external gills. There are a number of mechanisms that fish can utilize to extract oxygen from the water but the most common one involves the gills and this article will concentrate on them. Due to the low concentration of oxygen in water, the gills must be as efficient as possible in order to extract oxygen. The gills consist of bony or cartilaginous arches which hold pairs of gill filaments. Each gill filament consists of an upper and lower surface covered with minute ridges known as lamellae.
Water flows across the gill filaments and oxygen is removed and passes into the blood by diffusion.
The actual uptake of oxygen depends on a number of factors, the surface area of the lamellae, the thickness of the gill epithelia across which the oxygen moves and the oxygen tension across the membranes. As a result fast moving, highly active fish tend to have larger gill surface areas and very thin gill epithelia. Since the rate of oxygen diffusion depends on an appreciable gradient existing, any decrease in oxygen tension that occurs will affect the rate of oxygen uptake. As the blood absorbs more oxygen from the water in the gill chamber, the oxygen content of the water decreases and the rate of diffusion decreases as well, therefore, fresh, oxygen rich water just be introduced. When the branchial cavity finally contracts, expelling the last of the water, the cycle repeats itself.