[MUSIC PLAYING] The human digestive
system is very complex and has evolved over
millions of years. It basically consists
of the rectum, the large intestine, the
small intestine, the pancreas, the stomach, also called gaster
and ventriculus, and the liver with the gallbladder. The esophagus is also
part of this system, as well as various salivary
glands near the mouth. First the food is broken up
in the mouth by the teeth, and then mixed with
saliva with the help of the salivary glands. Saliva contains a
digestive enzyme called amylase that already
begins to digest carbohydrates in the mouth. It splits carbohydrates
into smaller units. The ball like mixture
of food with saliva, also known as bolus, is pushed
into the throat by the tongue and finally into the
esophagus which propels the bolus to the stomach. The esophageal lumen,
that is the opening inside the esophagus,
is very flexible, which allows boluses
of different sizes to be transported. The esophagus consists
of several layers. These layers occur throughout
the interdigestive tract. The two outer muscle layers are
responsible for peristalsis. Through these two
muscles the bolus can be transported from
the mouth to the stomach, even if the person is
standing on his head. The stomach is often
divided into six areas. The stomach is composed
of a similar structure to the esophagus. It has a longitudinal
muscle layer on the outside. Underneath we can find
circular muscle fibers. In addition to this, there
is an oblique muscle layer overlaying the mucosa. On the inside there
are rugae that allow the stomach to enlarge
when food is consumed. The stomach wall
contains gastric glands. They produce mucus, which is
able to protect the stomach wall from the
secreted gastric acid. Gastric acid is produced by
simply smelling or seeing food, but also spices, and the
stretching of the stomach causes the secretion. That is, the release
of gastric acid. About one to two
liters of gastric juice are produced per day. Since the esophagus does not
have a protective mucus layer, like the stomach,
stomach and esophagus are separated by a sphincter. It relaxes when a bolus is
pushed from the esophagus into the stomach,
and then contracts to prevent acid and
food from going back up. Gastric juice consists,
among other things, of hydrochloric acid, the enzyme
pepsin, the intrinsic factor, and lipase for the
digestion of fats. In addition to
nutrients, food also contains bacteria that
can damage the body. The components of
hydrochloric acid are able to destroy
harmful bacteria. In addition, hydrochloric
acid converts pepsinogen also released by
the gastric glands into pepsin. Pepsin is able to break down
proteins in the stomach. For a vitamin B12 absorption
in the small intestine, the intrinsic factor
is needed, which is produced by the gastric glands. The vitamin must combine
with intrinsic factor, then it can be absorbed
later by the small intestine. Vitamin B12 helps keep the
body's nerve and blood cells healthy, and helps make DNA. It also contains gastric
lipase, an acid resistant enzyme for fat digestion. In the stomach, gastric
lipase splits a triglyceride into a free fatty
acid, and a diglyceride whereby only the free fatty acid
can be absorbed by the body. More effective fat
digestion takes place in the small intestine. Through gastric juice
and stomach movements, which take place approximately
every 20 seconds, the individual boluses
are mixed to a semi fluid mass of partly digested
food the so-called chyme. The chyme cannot enter the
duodenum at first because there is a sphincter at
the stomach exit. The pyloric sphincter resembles
the esophageal sphincter. The pyloric sphincter opens
only a few millimeters, so that larger pieces
remain inside the stomach. In the first section
of the small intestine, the duodenum bile, and
pancreatic secretions are mixed with the chyme
via the ampulla of vater. Pancreatic juice contains
numerous digestive proenzymes and enzymes. In order for these
to do their job, a higher pH value than that
in the stomach is necessary. For this reason
pancreatic juice contains sodium hydrogencarbonate. Hydrogencarbonate is
able to neutralize the acid in the chyme
and thus produce the optimum pH value of 7 or 8. Pancreatic juice also
contains proenzymes. It is only through enterokinase
released by the duodenum wall that the proenzyme
trypsinogen and becomes tripsin, which
can split proteins and activate other tripsinogens. We also find alpha amalyse,
which we had already found in the mouth. It now does the rest
regarding the splitting of carbohydrates
which it converts into maltose and isomaltose. Furthermore pancreatic lipase
is able to split triglycerides into two free fatty acids. The gastric lipase, as
we have seen before, can produce only
one free fatty acid. The pancreatic lipase can
cleave triglycerides excellently because the bile breaks the
fats down into tiny droplets. This is called emulsification. Numerous other enzymes are
part of pancreatic juice, but these will not be
explained in detail here. Bile is produced
by the liver cells and transported to
the gallbladder. The bile is stored
in the gallbladder, and finally added to
the food in the duodenum via the ampulla of Vater. Of the pancreas also releases
juice via the ampulla of Vater. The small intestine
consists of three sections, duodenum, jejunum, and ileum. The ileum continues
into the large intestine in the right lower abdomen. The duodenum and the
jejunum have circular folds to increase the contact
surface with the food. These circular folds
extend about centimeter into the lumen of
the small intestine. These folds are covered
with small finger like projections called villi. Villi increase the
surface considerably. Villi I are about
one millimeter long. Each villus contains
blood capillaries and a lymphatic
capillary called lacteal, which we will see later. The nutrients,
marked green here, are absorbed by the
villus and transferred to the blood capillaries. Some nutrients, such as glucose,
do not require a carrier. They are transported
freely in the bloodstream. Other nutrients, such as iron,
require transport proteins transferrin. Fats are transported
by chylomicrons, which are lipoproteins. The triglycerides
to be transported are virtually enclosed
in the lipoprotein. Chylomicrons and
triglycerides are then transported through the
lacteal of the villus. Each villus is covered by
even smaller microvilli. They multiply the intestinal
surface considerably. The microvilli absorb
nutrients and transport them to the inside. The last part of the small
intestine is the ileum. It does not have, unlike
the duodenum and jejunum, circular folds. The ileum absorbs electrolytes,
such as calcium for building bones, hair, and teeth. trace elements, such as
zinc for sperm production, and the immune system, vitamins
such as B12 for the formation and maturation of red blood
cells, and remaining bile acid, which is transported back to
the liver via the bloodstream. As with the esophagus the food
is transported by peristalsis. In contrast,
segmentation contractions served to mix the chyme,
which is shown here in yellow and red,
to make it easier to see the mixing process. The large intestine is thicker
than the small intestine. It is about one meter long and
surrounds the small intestine. The small intestine is
connected to the large intestine via the Bauhin's valve. It opens when chyme is to
pass from the small intestine to the large intestine. The large intestine
does not have any villi like the
small intestine, because most digestible
substances have already been absorbed in
the small intestine. However the large intestine
has an estimated 100 billion bacteria inside. They are important
for many other tasks, such as the production
of vitamins, and the decomposition of
fiber for the body's energy production. Many of these bacteria
are an important part of the immune system by
killing harmful germs. Through peristalsis,
the chyme is transported from the ascending colon
to the transverse colon to the descending colon. On its way through the
large intestine water is removed from the chyme. Furthermore, mucus is added
for proper excretion of waste. Substances that cannot be
absorbed through the small intestine or the large intestine
remain in the rectum and are finally excreted
through the anus.
