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Name: Cassie M.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 2000-2001

I am having a lot of trouble understanding the counter current principle regarding oxygen uptake in fish. Please explain.

I have never studied the countercurrent exchange in fish, but if it like all the others I have studied it is simply related on diffusion. The principle of diffusion is based on molecules moving from a high concentration to a n area of lower concentration. In the case of the fish I would imagine that as the blood passes through the gills the blood which is depleted of some of its oxygen passes by water which has a higher concentration of oxygen allowing for diffusion.


The structure of the fish gill apparatus is important to the exchange of oxygen from the water into the blood. The stream of water coming in over the gill and the blood flowing within the gill are situated so that they are opposite. This is called counter current flow. In nature, substance always flow from a high concentration to a lower concentration (think of osmosis, warm flows to cold, high pressure to low pressure). As water comes in over the gills, the oxygen concentration is high and the concentration in the blood is lower. If they both ran in the same direction there wouldn't be a concentration gradient.

       Oxygen concentration of water---->
< ----140---110----80---50----20
         oxygen concentration in blood
so the blood comes into the gill at a low concentration but at a lower concentration than the oxygen in the water. Even as it picks up oxygen, the water is losing oxygen so the concentration is always lower in the blood than in the water, so oxygen keeps flowing into the blood from the water. This allows the fish to get the most oxygen out of the water as possible.

Van Hoeck

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