About 180 liters of plasma are
filtered by the kidney every day. However, the average urine excreted
per day is about 1.5 liters only. Which of the following is
responsible for that?
This question is pointing out that
the amount of plasma filtered by the kidneys is much greater than the amount of
urine that we excrete every day. And it’s asking us to correctly
identify the part of the urine production process which is responsible for this
notable difference. Well, let’s start by recalling that
the kidney filters waste from our blood, which is then removed from our body in the
form of urine. The nephron is the functional unit
of the kidney. It’s a microscopic structure
responsible for urine production. And this process takes place in
three steps. The first step is filtration, in
which almost all of the small molecules are filtered out of the bloodstream and into
the tubule. The filtrate includes water, ions,
nutrients, and waste products such as urea.
The second step in this process is
called selective reabsorption. During this step, the nutrients,
most of the water, and any of the ions that your body needs are returned to the
blood stream from the filtrate. In the last step, anything that’s
left in the tubule is considered to be urine. And several nephric tubules will
empty into one collecting duct. The collecting ducts join into
larger and larger vessels until they converge in the renal pelvis, where the urine
leaves the kidney through the ureter. So let’s return to our answer
choices and figure out which of these is responsible for the volume of filtrate
being much larger than the volume of urine.
Choice (A) is, the filtration
process taking place in the nephric tubule. Well, the filtration process is
actually responsible for generating the filtrate. And it occurs in a structure called
the glomerulus and not the nephric tubule. Our next choice is, the
reabsorption process taking place in the nephric tubule. Well, the reabsorption step is
where most of the water leaves the filtrate and reenters the bloodstream. And this does occur in the nephric
tubule, so this seems correct, but let’s keep going.
The filtration process taking place
in the collecting duct. Well, filtration occurs in the
glomerulus, not in the collecting duct. And our last choice, the
reabsorption process taking place in the collecting duct. Well, reabsorption occurs in the
nephric tubule. The collecting duct just collects
urine. So our correct answer is, the
reabsorption process taking place in the nephric tubule.