Vitamin and Mineral Digestion Taken For Granted
Copyright 2005 Kristy Haugen
The body is a complex web of systems. Most are not fully aware
of the complexity of the digestive system. However, most know of
its opposite, indigestion. Most do not realize how amazing the
body can be, especially the digestive tract. When the process of
digestion is described as fascinating, passing gas isn't going
to be the major focus (flatulence), but more so the process of
how gas is produced.
The digestive system's primary role is to convert the food into
substances that are capable of being absorbed. The digestive
system is comprised of the following structures: the mouth,
pharynx, esophagus, stomach, small intestine, large intestine,
and the anus. The liver, gallbladder, pancreas, and salivary
glands also play a role in digestion but are not considered part
of the alimentary canal (primary digestive organs).
Digestion begins in the mouth when food is ingested. This is a
mechanical process. Through mastication, (the biting and chewing
action of the teeth) the breakdown of food from larger particles
into smaller particles takes place. This process doe not
chemically alter the food, but increases the total surface area
of the food. This in turn increases the speed and efficiency of
enzyme activity. An enzyme is a protein that catalyzes, or
speeds up, a chemical reaction. Enzymes are essential to sustain
life because most chemical reactions in the body would occur too
slowly, or would lead to different products without the
assistance of enzymes.
For more information on enzymes and how enzymes work, go to
http://www.vitaminmaniac.com/liquid_vitamin_articles/ and click
on 'Liquid Vitamins Contain Enzymes Because?' article link.
Saliva also plays an important part with digestion in the mouth.
Saliva is secreted by the salivary glands which lubricates the
food to facilitate swallowing. The salivary glands begin to
produce saliva in response to food; whether stimulated by smell
or taste. Some may experience a mouth watering sensation in
response to a big juicy steak. Also, saliva initiates the
digestion of carbohydrates. Amylase is the digestive enzyme
found in saliva that helps with carbohydrate digestion. Once the
food has been sufficiently chewed, the tongue rolls it into a
ball (bolus) and pushes it into the pharynx (the cavity that
leads from the mouth to the esophagus). Swallowing (deglutition)
propels the bolus downward into the upper esophagus using a
peristaltic contraction (wavelike motion). At this time, the
epiglottis blocks the trachea (airway) to prevent food from
entering the lungs and interfering with breathing. Peristaltic
contractions continue to move the bolus (food) downwards to the
lower esophageal sphincter. This is the ring of smooth muscle
fibers at the junction of the esophagus and stomach; it is also
referred to as the cardiac sphincter. When food approaches, the
sphincter relaxes to allow food into the stomach. After the food
has passed through the sphincter, the muscle fibers contract to
keep the food and digestive juices from re-entering the
esophagus. Heartburn results when the cardiac sphincter relaxes
and allows the digestive juices to re-enter the esophagus. When
this happens too often, the smooth muscle of the esophagus is
eroded, which can cause bleeding and persistent heartburn
referred to as GERD (gastro-esophageal reflux disease). This can
become a serious condition.
The stomach is a large muscular organ; the walls are lined by a
thick gastric mucosa. The stomach is also lined by two types of
glands: gastric and pyloric glands. These glands contain mucous
cells which secrete mucus that protects the stomach lining from
the harsh stomach acid (pH of 2). Chief cells located in the
gastric glands secrete pepsinogen, which is a zymogen. A zymogen
is an inactive form of an enzyme. The gastric glands also
contain parietal cells which secrete hydrochloric acid. This
aides in the conversion of pepsinogen to the active enzyme
pepsin, and secrete intrinsic factor which helps to absorb
vitamin B12. Hydrochloric acid is essential to kill bacteria in
the food, and to help breakdown the food into an absorbable
form. The pyloric glands contain peptic cells which also secrete
the zymogen pepsinogen. Gastrin cells are located in the pyloric
glands. These cells secrete the hormone gastrin for hydrochloric
acid production in the parietal cells; and stimulate the
churning of the stomach to help produce the acidic, semi-fluid,
partially digested mixture referred to as chyme. Protein
digestion is initiated in the stomach.
The chyme then empties into the small intestine by way of the
pyloric sphincter. The pyloric sphincter is the ring of smooth
muscle fibers located at the joining of the stomach and small
intestine. The small intestine consists of three regions: the
duodenum, jejunum, and ileum. The bulk of digestion will be done
in the duodenum. The jejunum and ileum have a primary function
of absorption.
The small intestine has the perfect anatomy for absorption. The
extended length, highly coiled structure, along with surface
villi (small finger like projections), and epithelial cells with
a brush border microvilli allow for increased surface area for
absorption. Nutrients are absorbed across the epithelium villi
and are carried to the bloodstream through capillaries (small
blood vessels) or lacteals (small lymph vessels that serve as
extensions of the lymphatic vessel in the villi). Goblet cells
located in the small intestine secrete mucus on the surface
epithelium of the villi for protection from the digestive juices.
The pancreas releases a pancreatic juice in response to the
hormone secretin that is secreted by the duodenum. This hormone
cholecystokinin (CCK) is secreted in response to the acidity of
the chyme in the small intestine. The pancreatic juice that is
secreted has an alkaline pH to neutralize the acidity of the
chyme. The pancreatic juice contains many enzymes (inactive &
active) that digest carbohydrates, proteins, and lipids (fats).
The liver plays the role of secreting and synthesizing bile.
Bile is a non-enzymatic digestive fluid that is used to
breakdown (emulsify) fats. The gallbladder simply stores and
concentrates the bile. Bile is made up of bile salts, bile
pigments, and cholesterol.
Meals high in fat tend to spend a longer amount of time in the
stomach since it takes more time to digest. The hormone
enterogastrone is released by the duodenum. This hormone
inhibits the peristalsis in the stomach, slowing the release of
chyme into the small intestine. This also gives more time for
the bile to properly digest the fats.
The remaining food passes from the small intestine to the large
intestine. The large intestine consists of three parts: cecum,
colon, and rectum. The large intestine plays a smaller role of
digestion, mainly to absorb any electrolytes and water that has
not already been absorbed. This process is done in the colon.
Many normally harmless bacteria colonize the large intestine,
such as E. coli. E. coli is important because this type of
bacteria produces vitamin K as a byproduct. This is a good
source of vitamin K. Also, the amount of time spent in the large
intestine determines the consistency of the stool. If too little
time is spent in the colon, diarrhea and dehydration result. If
too much time is spent in the colon, constipation will result.
Lastly, the stool passes into the rectum. The rectum stores the
feces (stool), which consist of unabsorbed digestive secretions
(enzymes), water, undigested food (cellulose and fiber, etc.).
The anus is the opening through which wastes are eliminated. The
anus is separated from the rectum by two sphincters that
regulate elimination.
The digestive process is just one extremely complex process that
occurs without question. It is often taken for granted. The
complexity isn't easy to understand. The next time you eat a
juicy steak, understand the journey the meal is sent on is much
farther than the trip to the restaurant.