Question:
I am having a lot of trouble understanding
the counter current principle regarding oxygen uptake in fish. Please explain.
Replies:
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.
PF
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---->
150----120----90---60----30---
< ----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.
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