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How the Human Digestive System
Processes Food
OVERVIEW
The human body requires nutrients to sustain life. These nutrients are supplied through the food we eat and
drink, however when food enters the body in its normal form (that is its form before digestion begins) the
body cannot use it as nourishment. Through the process of digestion, the body breaks down food and drink
into smaller molecules that are used to nourish the body and provide energy. This process takes place in the
series of tube-like organs that make up the gastrointestinal tract, also known as the GI tract. The GI tract is
divided into two major sections, the upper GI tract and the lower GI tract.
UPPER GI
“The upper GI tract consists of the oral cavity, esophagus and
stomach, as well as associated valves and accessory organs”
(Windelspecht, 2004). Even before food enters the body, the
process of digestion begins when the body smells, anticipates,
or simply thinks about food, causing the secretion of saliva
through the salivary glands located in the oral cavity (See
Figure 1).
When we place food in our mouth, it enters the oral cavity (also known as the buccal cavity). It is here that our food
begins its journey through the digestive system. The teeth,
located within the oral cavity, are used to break the food into
smaller pieces so it can easily travel through the esophagus
to the stomach. During this process of chewing, the food is
expanded allowing it to easily mix with saliva, moistening the
food making it easier to swallow. The tongue moves the food
to the back of the oral cavity, or pharynx, forming a ball like
mass called a bolus. This action creates pressure on the pharynx that triggers receptors to send a message to the medulla
oblongata (the section of the brain which controls swallowing), which in turn sends a message to the respiratory center
of the brain to shut down temporarily to allow the bolus to
enter the digestive system and not the respiratory system on
accident.
Several other functions occur to ensure the bolus is directed
to the digestive system. The tongue moves to the roof of the
mouth to avoid food reentering the oral cavity. The uvula (the
“U” shaped flap that hangs from the rear roof of the mouth)
moves up to keep food from entering the nasal cavity. The
vocal cords close over the entry to the glottis (or windpipe) to
keep food from inadvertently entering here. (See Figure 2).
Figure 1: From the Oral Cavity
to the Esophagus to Stomach
Figure 2: The Oral Cavity
(Close Up)
After all these functions are preformed and the bolus enters
the esophagus, it causes a flap called the epiglottis to move
down adding extra coverage to the glottis and again prohibiting food from entering the respiratory system accidentally.
Figure 3: From the Esophagus
to Stomach to Small Intestine
Once the bolus has entered the esophagus, a hollow organ that
has a pipe like structure, it travels downward to the stomach.
It is moved through the esophagus through a series of wave
like motions, or muscle contractions, that slowly push the food
to the stomach. These muscle contractions are known as peristalsis or peristaltic contractions. The movement is involuntary and only takes a few seconds. When the bolus reaches the
end of the esophagus, it meets the gastreosophageal sphincter, or valve like apparatus, that works to let the bolus enter the
stomach from the esophagus and prevents anything else from
reentering the esophagus (See Figure 3).
The bolus next enters the stomach, which is elastic, J-shaped,
and both stores and breaks down the bolus. This breakdown
is performed by the same peristaltic muscle contractions as
before, that causes the bolus to mix with gastric juices that
are excreted from the stomach walls. The stomach functions
like an electric mixer creating chyme. The formation of this
semi-solid mixture called chyme is to create a consistency that
can enter the duodenum, or first part of the small intestines,
through the pyloric sphincter.
LOWER GI
The lower GI tract consists of the small intestine and large
intestine (See Figure 4).
Small Intestine
The small intestine, is so named because it has a diameter of
one inch, but it is actually ten feet in length when stretched
out. The small intestine is composed of three parts; the first
part is the duodenum, followed by the jejunum, and lastly the
ileum (See Figure 5 on the next page).
After the chyme enters the duodenum, the liver excretes
bile through the bile duct, which connects the liver and the
duodenum. The bile aids in the continued breakdown of the
chyme, moving the process of digestion forward. Two other
organs, the gallbladder and pancreas, aid the process of
digestion within the duodenum. The gallbladder also stores a
more concentrated form of bile, which it excretes into the duodenum through a sphincter valve called the Sphincter of Oddi.
Figure 4: Gastrointestinal Tract
(Upper and Lower)
The pancreas produces and later excretes pancreatic juice
through the same bile duct as before, emptying into the duodenum. The enzymes in this pancreatic juice are used to process nutrients and because of their alkaline nature they disable
acid from the stomach that remains in the chyme.
Figure 5: From the Small Intestine
to the Large Intestine
Chyme moves through the sections of the small intestine
through peristalsis contractions (like those in the esophagus
and stomach) as well as by segmentation contractions. It is
primarily the segmentation contractions that move chyme
through the small intestine in a series of circular contractions.
During this process, chyme mixes with the aforementioned
bile and pancreatic juice allowing nutrients to be absorbed
(See Figure 5).
Large Intestine
The large intestine, so named because of its diameter of three
to four inches, stretched out it would measure five feet long.
Now that the small intestine has absorbed most of the nutrients, waste remains. The large intestines, has three main sections the cecum, colon, and rectum, which are all connected
in a tube-like series (See Figure 6).
The cecum forces the chyme into the colon (which makes up
the majority of the large intestine). The colon absorbs left over
water and minerals that the small intestine didn’t remove from
the chyme. Through this process the chyme becomes more
and more like a solid as it passes through the large intestine.
Contractions force the chyme down and eventually out of the
large intestines meanwhile, the chyme mixes with bacteria
and mucus creating feces. When the feces reach the
rectum, or end of the large intestine, it is stored
here until it is expelled through the anus,
due to contractions of the anal sphincter.
Figure 6: From the Large Intestine
to the Anus