Video Transcript
All of the following is not usually
found in the urine of a healthy person except blank. (A) White blood cells, (B) albumin,
(C) glucose, or (D) sodium.
Let’s describe the steps of urine
formation to find out which molecules and cells are found in the urine and which are
not. The production of urine is done by
the nephron, which is a structure within the kidney, and occurs in three steps:
glomerular filtration, selective reabsorption, and urine secretion.
The first step is the filtration
step. Blood from the renal arteries
enters a ball of capillaries called the glomerulus. The capillaries of the glomerulus
are especially porous. And almost everything is filtered
out of the blood except for cells and large proteins like albumin. As cells and albumin are not
filtered out of the blood, we can already exclude answer options (A) and (B). The molecules which are filtered
out are, for example, water, ions, nutrients like, for example, glucose, and waste
products like, for example, urea. This mixture is called the
filtrate.
The second step of urine formation
is called selective reabsorption. The long, twisted renal tubule of
the nephron is surrounded by more capillaries. These capillaries are responsible
for reabsorbing materials that the body needs. This is because much of the
filtrate is actually still useful and necessary to the body. During selective reabsorption,
things like ions and water are reabsorbed from the filtrate and transported back
into the bloodstream. As nutrients like glucose are very
useful for the body, glucose is reabsorbed as well. Glucose is therefore not usually
found in the urine of a healthy person. With this information, we can
exclude answer option (C).
This leaves us with the answer
option (D), sodium. Let’s see if sodium is usually
found in the urine of a healthy person by discussing the last step of urine
formation, urine secretion. Whatever remains after the
selective reabsorption in the tubule is officially considered urine. Urine typically contains urea, as
mentioned. It also contains some inorganic
salts like, for example, sodium, some other ions, and water. So, indeed, the substance that is
usually found in the urine of a healthy person is sodium.
