Fish and Counter Current Flow and that. (:

I have a feeling I've missed a little from what we studied, so I'll find my old notebook later and get it from there, but alas, fish: :P

Counter-current flow = Constant concentration gradient.

The water, and the blood in the blood vessels are flowing in opposite directions. Oxygen diffueses from the water into the blood, which causes the oxygen concentration in the water to fall. The concentration of oxygen in the blood, subsequently, rises. However, concentration of oxygen in water is always higher then in blood, therefore, a constant concentration gradient is maintained.

In the gills, the flow of water is in the opposite direction to the flow of blood, therefore, the concentration gradient is maintained. This means the blood is continually meeting fresh water with a higher percentage concentration of oxygen.
The concentration gradient is maintained across the gill lamellae and oxygen continues to diffuse into the blood. The same would not happen if they flowed in the same direction, as diffusion would only occur until equilibrium is reached, which is less effcient.
When in counter current flow, 90% of the oxygen the water contains if difused, whereas if it was parallel flow, only 70% would.


Thoughts?
-Nin.