Video Transcript
The diagram provided shows a
simplified section of a dialyzer. Which of the following best
explains why the blood flows in the opposite direction to the dialysis fluid? (A) The opposite direction of the
flow maintains a steep concentration gradient so waste products continually diffuse
out of the blood. (B) The opposite direction of the
flow has no benefit and is just a coincidence. (C) The opposite direction of the
flow maintains a steep concentration gradient so useful products continually diffuse
out of the blood. Or (D) the opposite direction of
the flow maintains a steep concentration gradient so waste products continually
diffuse into the blood.
Let’s remove the answer options for
now and remind ourselves of what a dialyzer is and how it works.
A dialyzer is a medical device
which filters waste products out of the blood. This process is normally carried
out by the kidneys. So dialysis is only required when a
patient’s kidneys are no longer functioning correctly. As we can see in the diagram,
during dialysis, arterial blood flows through the dialyzer, which also contains
dialysis fluid. Dialysis fluid contains the same
concentrations of useful products as the blood, but very low concentrations of waste
products, such as urea. This means that as the blood flows
through the dialyzer, the waste products diffuse out of the blood into the dialysis
fluid, which is then removed.
In order to understand why the
blood and dialysis fluid flow in opposite directions, let’s imagine what would
happen if they flowed in the same direction. When the blood and dialysis fluid
enter the dialyzer, there is initially a steep concentration gradient between
them. So waste products rapidly diffuse
out of the blood. However, once they reach the other
end of the dialyzer, because so much waste has moved into the dialysis fluid, there
is no longer a steep concentration gradient between it and the blood. This means no more waste products
can diffuse out of the blood into the dialysis fluid, and the blood leaving the
dialyzer will not be as well filtered.
In order to make the removal of
waste products from the blood more efficient, blood and dialysis fluid flow in
opposite directions inside the dialyzer. This allows a steep concentration
gradient between the two fluids to be maintained. So waste products can continually
diffuse out of the blood. This mechanism is known as
countercurrent flow.
We can now answer our question. We have determined that the correct
answer is (A). The opposite direction of the flow
maintains a steep concentration gradient so waste products continually diffuse out
of the blood.
