Video Transcript
The following table describes the
structures of the main types of blood vessels in the human body. Which of the options correctly
replace X, Y, and Z in the table? (A) X: artery, Y: vein, Z:
capillary. (B) X: vein, Y: capillary, Z:
artery. (C) X: capillary, Y: artery, Z:
vein. Or (D) X: capillary, Y: vein, Z:
artery.
This question asks us to complete a
table describing different blood vessels in the human body. You might remember that there are
three main types of blood vessels, and these are given in the various answer
options. So let’s have a review of the
structure and function of arteries, veins, and capillaries.
Arteries are blood vessels that
carry blood away from the heart. We can remember this because the
words artery and away both start with A. Due to the constant pumping of the
heart, blood traveling through arteries is under high pressure. To withstand this high pressure,
arteries have thick walls and a relatively small lumen, or inner space.
The blood pressure within arteries
is also constantly changing as the heart contracts and relaxes. To deal with this variation, a
layer of elastic fibers in the wall of arteries allows them to stretch and
recoil. Damage to an artery carrying
high-pressure blood can lead to rapid blood loss and can even be fatal. Because of this, arteries are
located deeper in the body than other blood vessels to protect them from injury.
After being carried away from the
heart, blood will pass through the smallest vessels in the body, the
capillaries. Capillaries connect the arteries to
the veins through networks called capillary beds, which surround all the tissues in
the body. They have the critical role of
allowing the exchange of gases and other materials with other tissues in the
body.
As an adaptation for this function,
capillary walls are permeable and extremely thin. In fact, if we zoom in on a
capillary wall, we can see that it is made of a single layer of epithelial
cells. Capillaries also have a very small
lumen, which forces blood cells to flow through them in a single file line in order
to exchange nutrients, gases, ions, and hormones.
After the blood has delivered
oxygen and picked up carbon dioxide waste from the other cells of the body, it is
transported back to the heart by the veins. Veins are under much less pressure
than arteries. So they have thinner walls that
contain less smooth muscle and fewer elastic fibers. They also have a relatively large
lumen.
Veins tend to be located closer to
the surface of the skin than arteries. Take a moment to see if you can
figure out why. Hopefully, you concluded that
damage to a vein carrying low-pressure blood will not cause as much blood loss as
damage to an artery carrying high-pressure blood. So it is safer for veins to be
closer to the surface than arteries.
A unique feature of veins is that
they have flaps of tissue called valves that prevent the low-pressure blood from
flowing backward. We can remember which of the three
vessel types has valves because the words vein and valve both start with the letter
V.
After discussing the three types of
vessels in the body, we can now fill in our table correctly. The first column in our table
describes the vessels that are only one-cell thick with a small lumen. We just learned that these vessels
are capillaries. The second column describes vessels
with relatively thin walls, a large lumen, and valves, which we now know are the
veins. Finally, the vessels in the last
column have thick elastic walls with no valves. We know that these are the
arteries.
So the correct answer to our
question is (D). In the table, X should be replaced
with capillary, Y should be replaced with vein, and Z should be replaced with
artery.
