4. Gas exchange and the structure of fish gills

Mammals have a double circulatory system, but fish have a single circulatory system in which deoxygenated blood from the fish's body is pumped to the heart, which then pumps it through the gills to absorb oxygen from the water and round through the rest of the body in one continuous loop - just one circuit in operation (unlike the double circulatory system of mammals).

This single circulatory system is fine for cold blooded animals like fish, but not for warm blooded mammals.

Fish have very thin gills covered in protective muscular flaps. Water is continuously through the mouth and forced over the gill surfaces and ejected out through the flaps.

Gills are the gas exchange system in fishes and the structure provides a large surface area for oxygen to be absorbed into the blood stream and waste carbon dioxide passed out.

Water, containing dissolved oxygen, enters the fish through its mouth and passes out through the gills facilitating gas exchange.

In the gills, oxygen diffuses from the water to the blood, simultaneously, carbon dioxide diffuses from the blood into the water.

To make the gas exchange process as efficient as possible, the surface area of the gills is greatly increased by the presence of lots of thin plates called gill filaments - diagram on the right.

The surface area is increased even more by lots of tiny thin tissues called lamellae (plural of lamella).

The lamellae of lots of blood capillaries, increasing the contact area to speed up the diffusion of gases - oxygen or carbon dioxide.

The lamellae also have a thin layer of surface cells to minimise the gas diffusion distance and shorten diffusion times.

The blood flows through the lamellae in one direction and water flows over them in the other direction and this produces a continuous high concentration gradient between the blood and water.

The concentration of oxygen in the water is always higher than its concentration in the blood so maintaining a good supply of oxygen to the blood by diffusion from the water.

I presume the concentration of carbon dioxide is higher in the blood than in the water, so the waste gas is continually diffusing out of the blood?

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