Download Digestive System Ingestion Digestion in the Stomach Words to Know

Digestive System
The digestive system is a group of organs responsible for the conversion
of food into nutrients and energy needed by the body. In humans, the digestive
system consists of the mouth, esophagus, stomach, and small and large
intestines. Several glands—salivary glands, liver, gall bladder, and pancreas—
secrete digestive juices containing enzymes that break down foods chemically
into smaller molecules that are more easily absorbed by the body. The digestive
system also separates and disposes of waste products ingested with the food.
The digestive tube made up of these organs is known as the alimentary canal.
Ingestion
Food begins its path through
the digestive system in the mouth,
where the teeth break it into smaller
pieces through the processes of biting
and chewing. The tongue rolls these
pieces into balls. The sight, taste, and
smell of the food cause the salivary
glands to produce more saliva, the
liquid that always coats the mouth. An
enzyme in the saliva called amylase
begins the breakdown of carbohydrates
into simple sugars. A ball of chewed
food mixed with saliva is called a
bolus.
The process of swallowing involves
moving a bolus to the pharynx at the
back of the mouth. In the pharynx,
rings of muscles force the food into the esophagus, the first part of the upper
digestive tube. The esophagus extends from the bottom part of the throat to the
upper part of the stomach.
The esophagus does not take part in digestion. Its job is to move the bolus into
the stomach. Food is moved through the esophagus and other parts of
the alimentary canal by a wavelike muscular motion called peristalsis
(pronounced pear-i-STALL-sis). This motion consists of the alternate contraction
and relaxation of the smooth muscles lining the tract. At the junction of the
esophagus and stomach sits a powerful muscle, the esophageal sphincter,
which acts as a valve to keep food and stomach acids from flowing back into the
esophagus and mouth.
Digestion in the Stomach
Words to Know
Alimentary canal
Tube formed by the
pharynx,
esophagus,
stomach, and
intestines through
which food passes.
Bile
Bitter, greenish
liquid produced in
the liver and stored
in the gall bladder
that dissolves fats.
Carbohydrate
A molecule made up
of carbon, hydrogen,
and oxygen atoms
that functions as
food, as energy
storage, and as
structural molecules
of living organisms.
Enzyme
A molecule
produced by living
cells that begins and
maintains a
biochemical
reaction.
Peristalsis
Wavelike motion of
the digestive
system that moves
food through
the alimentary canal.
Protein
A large molecule
built of amino acids
that is essential to
the structure and
functioning of all
living cells.
Sphincter
A circular muscle
that controls the
opening of a bodily
passage.
Chemical digestion begins in the stomach. The stomach is a large, hollow,
pouch-shaped muscular organ. Food in the stomach is broken down by the action of gastric juice, which
contains hydrochloric acid and pepsin, an enzyme that digests protein.
Gastric juice is secreted from the linings of the stomach walls, along with mucus that helps to protect the stomach
lining from the action of the acid. Three layers of powerful stomach muscles churn food into a thick liquid called
chyme (pronounced KIME). From time to time, chyme is passed through the pyloric sphincter, the opening
between the stomach and the small intestine.
Digestion and Absorption in the Small Intestine
The small intestine is a long, narrow tube running from the stomach to the
large intestine. The small intestine is greatly coiled and twisted. Its full
length is about 20 feet. The small intestine is subdivided into three sections:
the duodenum (pronounced do-o-DEE-num), the jejunum (pronounced jeJOO-num), and the ileum (pronounced ILL-ee-um).
The duodenum is about 10 inches long and connects with the lower portion
of the stomach. When chyme reaches the duodenum, it is further broken
down by intestinal juices and through the action of the pancreas and gall
bladder. The pancreas is a large gland located below the stomach that
secretes pancreatic juice into the duodenum through the pancreatic duct.
There are three enzymes in pancreatic juice that break down
carbohydrates, fats, and proteins. The gall bladder, located next to the
liver, stores bile produced by the liver. While bile does not contain enzymes,
it contains bile salts that help to dissolve fats. The gall bladder empties bile
into the duodenum when chyme enters that portion of the intestine.
The jejunum is about 8 feet long. The digested carbohydrates, fats, proteins, and most of the vitamins and
minerals are absorbed in this section. The inner lining of the small intestine is composed of up to five million tiny,
fingerlike projections called villi. The villi increase the rate of absorption of nutrients into the bloodstream by
greatly increasing the surface area of the small intestine.
The ileum, the last section of the small intestine, is the longest section of the digestive system, measuring about
11 feet. It absorbs nutrients into the bloodstream.
Absorption and Elimination in the Large Intestine
The large intestine is wider and heavier than the small intestine. It is also much shorter, only about 5 feet long. It
rises up on the right side of the body, crosses over to the other side underneath the stomach, descends on the left
side, then forms an S shape before reaching the rectum and anus. The muscular rectum, about 6 inches long,
expels feces through the anus, which has a large muscular sphincter that controls the passage of waste matter.
The large intestine removes water from the waste products of digestion and returns some of it to the
bloodstream. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract.
Millions of bacteria in the large intestine help to produce certain B vitamins and vitamin K. These vitamins are
absorbed into the bloodstream along with the water.
"Digestive System." UXL Encyclopedia of Science, edited by Amy Hackney Blackwell and Elizabeth Manar, 3rd ed., UXL, 2015. Research in
Context, go.galegroup.com/ps/i.do?p=MSIC&sw=w&u=pioneer&v=2.1&id=GALE%7CCV2644300344&it=r&asid=104473f919bdfe9146c4adab
8c574824. Accessed 24 Feb. 2017.