Video Transcript
Which of the following statements
best describes how ADH helps regulate water balance? (A) If present, ADH increases the
permeability of the collecting duct, so more water is excreted in urine. (B) If present, ADH decreases the
permeability of the collecting duct, so more water is excreted in urine. (C) If present, ADH increases the
permeability of the collecting duct, so more water is reabsorbed into the
bloodstream. (D) If present, ADH increases the
permeability of the collecting duct, so there are higher concentrations of glucose
reabsorbed into the blood. Or (E) ADH has no effect on the
regulation of water balance in the body.
ADH is an acronym that stands for
antidiuretic hormone. A diuretic is a substance that
increases urine production, and anti- is a prefix that means not or the opposite
of. So, ADH is a hormone that decreases
urine production in the body. ADH is produced in the brain, and
it’s carried by the bloodstream to the kidneys, which are the organs in our body
responsible for producing urine. And the production of urine is one
of the main ways that our body regulates the amount of water contained in the
bloodstream, also referred to as water balance.
That being said, we can draw some
conclusions about the relationships between ADH, urine, and water excretion. An increase in ADH would lead to a
decrease in urine production. And since less urine is being made,
less water is being removed from the body. In contrast, a decrease in ADH
production would lead to an increase in urine, which would mean that more water is
being excreted from the body.
Well, the kidney makes urine in
millions of tiny structures called nephrons. And what I’ve drawn is an extremely
simplified diagram that we can use as an illustration. So, how the nephron works is that
blood enters from the renal artery and then flows through a special structure called
a glomerulus.
In the glomerulus, all of the small
molecules filter out of the bloodstream, and this includes most of the water. This liquid, now called filtrate,
enters the renal capsule, also called a Bowman’s capsule. The filtrate flows from the capsule
into a long, twisted tubule. And from the tubule, everything
that is not considered waste is passed back into the bloodstream, which includes
most of the water. What’s left in the tubule is now
considered urine. Several nephrons empty into one
collecting duct, which join into larger and larger vessels until they exit the
kidney through the ureter.
So how does ADH change how much
urine is produced? Well, ADH affects urine production
by changing the permeability of the collecting ducts and the tubules. And permeability means the ability
of water to pass through the linings of these vessels. When more water is able to pass
from the collecting duct and back into the bloodstream, there’s less water present
in the urine. So, the volume of the urine
decreases.
So, we can summarize what we’ve
learned in that if ADH is present in the bloodstream, less urine is produced. That’s because ADH increases the
permeability of the ducts and tubules in the nephrons, which allows more water to be
reabsorbed into the bloodstream. Since more water is reabsorbed,
less water is excreted and we have a lower volume of urine.
Now, we know everything we need to
know to choose the correct answer to our question. So, now, we can review our answer
choices again and choose the correct one. I’d like to start with choice (D),
which refers to the concentration of glucose. This choice doesn’t answer the
question of how water balance is regulated. And then choice (E) says that ADH
has no effect on the regulation of water balance in the body. But we know that ADH affects urine
production, which indeed affects water balance.
So that leaves us with the choices
(A), (B), and (C), which each describe relationships between ADH, permeability, and
the amount of water either excreted or reabsorbed. If we examine them carefully, we
find that the correct choice is (C), which states, if present, ADH increases the
permeability of the collecting duct so that more water is reabsorbed into the
bloodstream.
