Download The Digestive System

(Students: Bolded, italicized, and underlined sections will be on the exam)
The Digestive System:
I. Function of the System:
A. The digestive system is a physical and chemical process that breaks down food
into basic nutrients (including water, carbohydrates, proteins, fats, vitamins,
and minerals) for the body’s cells, and packages the unused portion for waste
disposal. When we eat such things as bread, meat, and vegetables, they are not in
a form that the body can use as nourishment. Our food and drink must be
changed into smaller molecules of nutrients before they can be absorbed into the
blood and carried to cells throughout the body. Digestion is the process by which
food and drink are broken down into their smallest parts so that the body can use
them to build and nourish cells and to provide energy.
II. The Main Parts of the System:
System Overview: The digestive system is a series of hollow organs joined in a
long, twisting tube from the mouth to the anus. This tube is called the alimentary
canal. The alimentary canal includes the mouth (with salivary glands),
esophagus, stomach, small intestine, large intestine, rectum, and anus. Food does
not travel through the liver, appendix, nor pancreas. Inside this tube is a lining
called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains
tiny glands that produce juices to help digest food. Two solid organs, the liver and
the pancreas, produce digestive juices that reach the intestine through small tubes.
The gall bladder stores one of these juices from the liver called bile, which digests fats.
The appendix may also play a role in storing toxins from the system. In addition, parts
of other organ systems (for instance, nerves and blood) play a major role in the
digestive system. Here are the parts, in more detail.
A. Mouth: The mouth is the beginning of the digestive tract; and, in fact, digestion
starts here when taking the first bite of food. The teeth and tongue begin by
breaking down the food. Chewing, called mastication, breaks the food into
pieces that are more easily digested, while the salivary glands secrete saliva
which mixes with food to begin the process of breaking it down into a form your
body can absorb and use.
B. Salivary Glands: Three pairs, parotid, submandibular, sublingual. There are
salivary glands in the cheek and others that open into the floor of the mouth.
Saliva is a mixture of mucus and enzymatic fluids, each produced to various
extents in various glands. It also contains the enzyme salivary amylase, (starts to
break down starch) lysozyme (antibacterial) and IgA antibodies. In some mammals
(and snakes!) saliva may be poisonous, ‘quieting down’ living prey.
C. Esophagus: It is a food passage that connects the mouth to the stomach. It is
located in your throat near your trachea (windpipe), the esophagus receives food
from your mouth when you swallow. By means of a series of wavelike muscular
contractions called peristalsis, the esophagus delivers food to your stomach.
D. Stomach: The stomach is a hollow organ, or "container" that holds food while
it is being mixed with enzymes that continue the process of breaking down food
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into a usable form called chyme (chime). Cells in the lining of the stomach
secrete hydrochloric acid and powerful enzymes that are responsible for the
breakdown process. When the contents of the stomach are sufficiently processed,
they are released into the small intestine.
Small Intestine: The small intestine is a 22-foot (approximately) long muscular
tube that breaks down food using enzymes released by the pancreas and bile
from the liver, and it then absorbs the digested nutrients into the bloodstream.
It is made up of three segments — the duodenum, jejunum and ileum. Peristalsis
also is at work in this organ, moving food through and mixing it with the digestive
secretions from the pancreas and liver. The duodenum is largely responsible for the
continuous breaking-down process, with the jejunum and ileum mainly are
responsible for absorption of nutrients into the bloodstream. Contents of the small
intestine start out semi-solid (called chyme), and end in a liquid form after passing
through the organ. Water, bile, enzymes and mucous contribute to the change in
consistency. Once the nutrients have been absorbed and the leftover-food residue
liquid has passed through the small intestine, it then moves on to the large
intestine, or colon.
Pancreas: The pancreas secretes digestive enzymes into the duodenum, the first
segment of the small intestine. These enzymes break down protein, fats and
carbohydrates. As part of the endocrine system, the pancreas also makes
insulin, secreting it directly into the bloodstream. Insulin is the chief hormone
for metabolizing sugar.
Liver: The liver is the largest organ (by mass) in the digestive system and it has
multiple functions. Its main function within the digestive system is to process
the nutrients absorbed from the small intestine. Bile, a green liquid from the
liver, is secreted into the small intestine and plays an important role in
digesting fat. In addition, the liver is the body’s chemical "factory." It takes the
raw materials absorbed by the intestine and makes many of the various chemicals
the body needs to function. The liver also de-toxifies and breaks down potentially
harmful chemicals, including alcohol.
Gallbladder: The gallbladder stores and concentrates bile, and then releases it
through the bile duct into the small intestines (duodenum part) to help absorb
and digest fats.
Colon (Large Intestine): The colon is a 6-foot long muscular tube that connects
the small intestine to the rectum, and is largely responsible for the absorption of
water and minerals into the bloodstream, as well as collecting food residue for
excretion The large intestine is wider than the small intestines, and is made up of
the cecum, the ascending (right) colon, the transverse (across) colon, the
descending (left) colon and the sigmoid colon, which then connects to the rectum.
The appendix is a small tube attached to the cecum, and its function is not known,
although many speculate that it stores some chemical and bacterial wastes. The
large intestine is a highly specialized organ that is responsible for processing
wastes so that emptying the bowels is easy and convenient. Stool, or waste left
over from the digestive process, is passed through the colon by means of
peristalsis, first in a liquid state and ultimately in a solid form called feces. As stool
passes through the colon, water is removed. Stool is stored in the sigmoid (Sshaped) colon until a "mass movement" empties it into the rectum once or twice a
day. It normally takes about 36 hours for stool to get through the colon. The
stool itself is mostly food debris and bacteria. These ‘good’ bacteria perform
several useful functions, such as synthesizing various vitamins, processing waste
products and food particles, and protecting against harmful bacteria. When the
descending colon becomes full of stool, or feces, it empties its contents into the
rectum to begin the process of elimination.
J. Rectum: The rectum (Latin for "straight") is an 8-inch chamber that connects
the colon to the anus. It is the rectum's job to receive stool from the colon, to
‘let the person know’ that there is stool to be evacuated and to store the stool
until evacuation happens. When anything (gas or stool) comes into the rectum,
sensors send a message to the brain. The brain then decides if the rectal contents
can be released or not. If they can, the sphincters relax and the rectum contracts,
disposing its contents. If the contents cannot be disposed, the sphincter contracts
and the rectum accommodates so that the sensation temporarily goes away.
K. Anus: The anus is the last part of the digestive tract. It is a 2-inch long canal
consisting of the pelvic floor muscles and the two anal sphincters (internal and
external). The lining of the upper anus is specialized to detect rectal contents. It
lets you know whether the contents are liquid, gas or solid. The anus is
surrounded by sphincter muscles that are important in allowing control of stool.
The pelvic floor muscle creates an angle between the rectum and the anus that
stops stool from coming out when it is not supposed to. The internal sphincter is
always tight, except when stool enters the rectum. It keeps us continent when we
are asleep or otherwise unaware of the presence of stool. When we get an urge to
go to the bathroom, we rely on our external sphincter to hold the stool until
reaching a toilet, where it then relaxes to release the contents.
III. Interactions With Other Systems:
The digestive system interacts with every system, since it provides nutrients for
every cell in every organ system. Some examples of systems that directly interact
with the system include:
• The Excretory System: The rectum and anus are part of the body’s excretory
system, since they store and expel waste products from the body.
• The Nervous System: The nervous system controls the movement of the food
through the digestive tract, as well as the secretion of digestive enzymes in
the system.
• The Circulatory System: The nutrients from the digestive system are absorbed
into the blood in the intestines, and are then pumped to all of the cells
throughout the body.
• The Endocrine System: The pancreas is a part of both the digestive and
endocrine systems, since it secretes digestive enzymes into the digestive
tract, and insulin into the bloodstream.
IV. Some Disease and Disorders:
• Constipation: A very common disorder where the rectum stores solid fecal matter,
and the body fails to expel it ‘in a timely fashion. People who live an easy, sedate
life in a city and consume rich food are more prone to it than others. Treatment
includes an array of medications and eating foods (roughage including fruits and
certain grains) that soften stools and ‘encourage’ it to pass.
• Peptic Ulcer: Excess hydrochloric acid in the stomach begins to ‘eat away’ at
the muscle lining inside of the stomach, causing bleeding and severe pain.
Stress may contribute to increased acid production. Treatment includes various
medications, and eating ‘softer’, ‘easier to digest’ (such as yogurt and fruits)
foods.
• Gallstones: Stones formed by excess cholesterol that hardens into a ‘stone’ that
blocks the bile duct, creating pressure and pain that is felt below the right ribs.
In extreme cases, the gall bladder may need to be removed, and the person may
have to eat less fatty foods for the remainder of their lives (since the bile stored
in the gall bladder assists with the breakdown of fat).
• Acid Reflux: Excess acid from the stomach travels through the ‘weakened’
sphincter muscle that separates the stomach from the esophagus, and up into the
esophagus, causing chest pain and discomfort. Antacids may alleviate the pain.
• Dyspepsia (Indigestion): Dyspepsia or indigestion is the term applied to difficulty
in digesting food. Dyspepsia may be associated with pain in the region of the
stomach, flatulence, eructations, a foul taste in the mouth, heartburn, and a general
feeling of discomfort.! There is also loss of appetite in some cases of dyspepsia.!
Irregular food habits in the form of overeating or eating at the wrong time, or the
wrong type of food leads to indigestion. The general term applied to the
symptoms associated with dyspepsia in Ayurveda is Agitimandya, which is a
decreased digesting capacity of agnis, particularly, jatharagni, which aids
digestion.! The aggravation of the doshas vata, pitia, and kapha-leads to this
disease.! Excess of vata leads to pain in the stomach; that of pitta causes a burning
sensation; and if kaplia is vitiated, there is nausea and vomiting.
V. Interesting Facts about the System:
• Swallowing: In leaving the mouth a bolus of food must cross the respiratory tract
(trachea is anterior to the esophagus) by a complicated mechanism known as
swallowing, or deglutination. During one of the stops of swallowing, the epiglottis
is a flap of muscle that closes the trachea during swallowing, preventing food from
entering the windpipe, allowing it to continue down the esophagus and into the
stomach.
• More on Peristalsis and the Movement of Food Through the System: The large,
hollow organs of the digestive system contain muscle that enables their walls to
move. The movement of organ walls can propel food and liquid and also can mix
the contents within each organ. Typical movement of the esophagus, stomach, and
intestine is called peristalsis. The action of peristalsis looks like an ocean wave
moving through the muscle. The muscle of the organ produces a narrowing and
then propels the narrowed portion slowly down the length of the organ. These
waves of narrowing push the food and fluid in front of them through each hollow
organ.
• The first major muscle movement occurs when food or liquid is swallowed.
Although we are able to start swallowing by choice, once the swallow begins, it
becomes involuntary and proceeds under the control of the nerves.
• The esophagus is the organ into which the swallowed food is pushed. It connects
the throat above with the stomach below. At the junction of the esophagus and
stomach, there is a ring like valve closing the passage between the two organs.
However, as the food approaches the closed ring, the surrounding muscles relax
and allow the food to pass.
• The food then enters the stomach, which has three mechanical tasks to do. First,
the stomach must store the swallowed food and liquid. This requires the stomach
to relax and accept large volumes of swallowed material. The second job is to mix
up the food, liquid, and digestive juice produced by the stomach. The lower part of
the stomach mixes these materials by its muscle action. The third task of the
stomach is to empty its contents slowly into the small intestine.
• Several factors affect emptying of the stomach, including the nature of the food
(mainly its fat and protein content) and the degree of muscle action of the
emptying stomach and the next organ to receive the contents (the small intestine).
As the food is digested in the small intestine and dissolved into the juices from the
pancreas, liver, and intestine, the contents of the intestine are mixed and pushed
forward to allow further digestion.
• Finally, all of the digested nutrients are absorbed through the intestinal walls. The
waste products of this process include undigested parts of the food, known as
fiber, and older cells that have been shed from the mucosa. These mouth, soft
palate closes off the nose and the larynx rises so that the epiglottis closes off the
trachea. Food thus moves into the pharynx and onwards by peristalsis aided by
gravity. If we try to talk whilst swallowing food may enter the respiratory passages
and a cough reflex expels the bolus.