Video Transcript
Why do the villi that line the
small intestine contain a network of capillaries? (A) To transport unwanted
substances to the lymph glands. (B) To absorb and transport
nutrients into the bloodstream. (C) To provide essential nutrients
to the lumen of the small intestine. Or (D) to release enzymes and bile
into the small intestine to break down food molecules.
To answer this question, let’s
review some key facts about the small intestine, focusing on the structure and
function of the villi. The small intestine is an organ in
the human digestive system that is specially adapted for breaking down several
different large nutrients in the food that we eat and absorbing the products of this
digestion so that they can be transported to the different cells of the body that
require them. The small intestine is a tubelike
organ with a space in its center called the lumen. The lumen does not need to be
provided with nutrients as option (C) suggests, as instead it provides a tract
through which food can travel as it is digested.
The small intestine is made up of
three main parts: the duodenum, the jejunum, and the ileum. The duodenum is the part of the
small intestine in which proteins and polypeptides are broken down into peptides and
amino acids. Large carbohydrates are broken down
into smaller sugars like glucose. And lipids are broken down into
fatty acids and glycerol. This occurs with the help of
enzymes that are secreted into the duodenum from the pancreas. And bile secreted from the
gallbladder also helps in the digestion of lipids. As we know that the pancreas and
gallbladder secrete enzymes and bile, respectively, to help break down food
molecules, this is not a role of the capillaries. So option (D) is incorrect.
The small intestine is especially
long, which provides it with a large surface area for the absorption of the products
of digestion. The majority of this absorption
occurs in the jejunum and the ileum. And these two parts of the small
intestine have further adaptations to increase the surface area across which
nutrients can be absorbed and therefore the rate of absorption. One such adaptation are the villi,
one of which is called a singular villus. Villi are fingerlike projections of
the wall of the small intestine that increase its surface area, lined by a layer of
epithelial cells. This layer is just one cell thick
to decrease the diffusion distance for nutrients and, as a result, increase the rate
of nutrient absorption.
As you can see, these epithelial
cells also have microvilli, which are tiny projections of each cell’s cell membrane,
which further increase the surface area for nutrient absorption. The villi move back and forth in a
swaying motion to help food moving through the small intestine mix with digestive
enzymes. Inside the villi, there is a rich
network of capillaries and lacteals that are responsible for transporting away
absorbed nutrients to maintain a steep concentration gradient for absorption. Water-soluble nutrients like amino
acids and small sugars enter the capillaries to be transported through the
bloodstream to the various cells of the body.
Digested lipids are not water
soluble, and they are also too large to enter the capillaries directly. So instead, they are absorbed into
the lacteals to be transported via the lymphatic system to empty into the
bloodstream later. Although the lacteals are involved
in the lymphatic system, they do not transport unwanted substances to the lymph
glands. And this is definitely not the role
of the capillaries that are also found in the villi which the question concerns. So option (A) is incorrect.
Now that we have looked at some key
facts, we have enough information to answer the question. The function of the network of
capillaries within the villi that line the small intestine is (B): to absorb and
transports nutrients into the bloodstream.
