The energy required for all the processes
and activities that take place in our bodies is derived from the foods we
ingest. The digestive system allows us to utilize food from such diverse
sources as meat from an animal and the roots of a plant, and utilize them as an
energy source. Whether it is the ability to coordinate the chewing of the food
without injuring our tongue and lips or the propulsion of the food from the
stomach into the duodenum while releasing the appropriate enzymes, our
digestive system allows us to manage the process without much thought and often
while performing other tasks.
What is digestion?
The process of digestion is a fascinating
and complex one that takes the food we place in our mouth and turns it into
energy and waste products. This process takes place in the gastrointestinal
tract, a long, connected, tubular structure that starts with the mouth and ends
with the anus. The food is propelled forward within the system, altered by
enzymes and hormones into usable particles and absorbed along the way. Other
organs that support the digestive process are the liver, gallbladder, and
pancreas. The time it takes for food to travel from entering the mouth to be
excreted as waste is around 30 to 40 hours.
The mouth is the entry point for food, but
the digestive system often gets ready before the first piece of food even
enters our mouth. Saliva is released by the salivary glands into our oral
cavity when we smell food. Once the food enters the mouth, chewing
(mastication) breaks food into smaller particles that can be more easily
attacked by the enzymes in saliva. Our teeth can perform a cutting as
well as grinding function to accomplish this task. The tongue assists in mixing
the food with the saliva and then the tongue and roof of the mouth (soft
palate) help move the food along to the pharynx and esophagus.
The pharynx and esophagus
The pharynx (throat) is the transition area
from the mouth to the esophagus. From the pharynx there are two paths that the
food bolus can take; 1) the wrong path, which is down the windpipe into
the lungs, or 2) the correct path into the esophagus and then the stomach.
The act of swallowing is a complex process that closes the windpipe (to protect
our lungs) and moves food into the esophagus. This process is mostly automatic
(reflex) but it is also partially under our direct control.
Once it enters the esophagus, food is moved
down the esophagus and into our stomach. The esophagus is a muscular tube that
contracts in a synchronized fashion (peristalsis) to move food down towards the
stomach. While the muscles behind the food product contract, the muscles ahead
of the food relax, causing the forward propulsion of the food. Peristalsis is
the main mechanism by which food moves through our digestive system.
Once the food approaches the stomach, a
muscular valve (the lower esophageal sphincter) relaxes and lets the food pass
into the stomach. This sphincter has the important function of closing the
stomach so no food or stomach acid reenters the esophagus (and therefore
avoiding heartburn or regurgitation).
The stomach and small intestine
From glands that line the stomach, acid and
enzymes are secreted that continue the breakdown process of the food. The
stomach muscles further mix the food. At the end of this process, the food you placed
in your mouth has been transformed to a thick creamy fluid called chyme.
This thick fluid is then pushed into the
duodenum (the first part of the small intestine). With the help of enzymes from
the pancreas and bile from the liver, further breakdown of the food occurs in
the small intestine.
The small intestine has three segments. The
first segment is the duodenum where further breakdown of the food takes place.
The next two parts of the small intestine (jejunum and ileum) are mostly
responsible for the absorption of nutrients from the processed food into the
bloodstream through the walls of the intestine.
After the small intestine, the leftover
waste leaves the upper gastrointestinal tract (upper GI tract) which is made up
of everything above the large intestine, and moves into the large
intestine or colon (the beginning of the lower GI tract).
The colon, rectum, and anus
The role of the lower GI tract is to
solidify the waste product (by absorbing water), store the waste product until
it can be evacuated (going to the bathroom) and help with the evacuation
process.
The large intestine (colon) has four parts:
1. ascending
colon,
2. transverse
colon,
3. descending
colon and
4. sigmoid
colon.
All together the colon is approximately
7 feet long and connects to the rectum. Here as in most other parts
of the GI system, the waste product is moved along by peristalsis. As the waste
product passes through the colon, water is absorbed and stool is formed.
The stool from the colon is stored in the
rectum. The anal sphincter provides the control over releasing stool or holding
it. Once stool arrives in the rectum, a feedback to the brain makes the person
aware of the need for a bowel movement. Voluntary control over the anal
sphincter lets us hold the stool until we go to the toilet.
Three other organs are instrumental in the
digestive process.
1. Pancreas: Although
the pancreas is mostly known for its blood sugar regulatory function
with the production of insulin (as part of the endocrine system -- he
insulin goesdirectly from the gland into the bloodstream), it is the main
producer of digestive
enzymes as part of the exocrine system (the enzymes produced by
the gland pass through a duct into the intestines). These enzymes are
released into the duodenum and help with the digestion of fats, proteins,
and carbohydrates..
2. Liver: The
liver produces bile for fat digestion and elimination. In addition, nutrients
are stored in the liver, and toxins and chemicals are filtered by liver.
3. Gallbladder: Bile
is stored and released from the gallbladder. When fatty food enters the
duodenum, the gallbladder contracts and releases bile.
