The essay delves into counter-current flow systems, pivotal in animal survival, elucidating their mechanisms and significance. It defines this system as the exchange of substances or heat between fluids moving in opposite directions, contrasting it with concurrent flow. Animals exploit this by maxi...
What is a counter-current flow system? How have animals
exploited the properties of counter-current systems?
Counter-current flow systems play an important role in animal life, serving a
wide variety of functions, from allowing fish to breathe to enabling desert
mammals to survive on very little water. But what is a counter-current flow
system? Firstly we will need to define this phenomenon. Secondly, we can
demonstrate the importance of counter-current flow systems by looking at the
ways animals have exploited their properties. Such exploitations include
maximising uptake of substances from the environment, minimising the loss of
heat to the environment, and allowing the creation and maintenance of large
concentration gradients.
Counter-current flow can be described as the exchange of a substance
or heat between two fluids flowing in opposite directions to each other. This is
different to concurrent flow, in which the two fluids flow in the same direction
(figure 1). The disadvantage of concurrent flow is that complete transfer of a
substance from one fluid to another is not possible; at best the two fluids can
have equal concentrations by the end of the exchange. However, a counter-
current flow system maintains a constant gradient between the two flows over
the entire length of contact, therefore enabling more complete transfer.
Combined with slow flow rate and long tubes, which allow more time for
exchange to occur, this has become a mechanism of huge significance to
animals.
One major way animals have exploited counter-current exchange
systems is in allowing maximal uptake of a substance from an environment
where the substance is in low concentration. For example fish, which being
water breathers must extract oxygen from a concentration of just 7ml oxygen
per litre of water, pump blood through the gill lamellae capillaries in the
opposite direction to the flow of water. This means that blood leaving the
lamellae, which has a very high oxygen content, meets water that has not yet
had its oxygen extracted. The blood can therefore take up yet more oxygen,
allowing the oxygen content to reach the highest possible level. As the water
runs further along the lamellae, it meets blood with a lower oxygen content,
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