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Step-By-Step Guide To The Digestive Process
The Mouth

Digestion starts in the mouth - the beginning of the digestive tract. Food smells cause
the salivary glands in the mouth to secrete saliva ("mouth-watering"), so even before we
start eating our digestive system is primed and ready for action! Saliva contains
antibacterial compounds and various enzymes to aid the breakdown of food molecules.
It also softens the food - enabling the tongue to mould it into a bolus or ball for
swallowing. The tongue, teeth and saliva work together to start digestion and aid
swallowing. Teeth chop and grind food, breaking the food down into pieces small
enough to be digested and increasing the surface area over which the digestive
enzymes in saliva can act. For more,
see Guide To Digestion In The Mouth.

The Pharynx and Esophagus

Food is then swallowed and passes into the pharynx, or throat. When we swallow,
passages to the lungs (windpipe) and the nasal cavity are automatically closed, and the
food goes into the esophagus - a muscular tube extending from the pharynx to the
stomach. Food is propelled through the esophagus and into the stomach by means of
muscular contractions called peristalsis. At the bottom of the esophagus, just before the
opening to the stomach is a ring-shaped muscle known as the lower esophageal
sphincter (LES). This muscle relaxes (opens) to let food into the stomach and then
tightens (closes) to prevent regurgitation. If the LES malfunctions and allows food in the
stomach to re-enter the esophagus, it may cause a condition called gastroesophageal
reflux disease (GERD), characterised by heartburn and regurgitation. For more, see
Guide To Digestion In The Esophagus.

The Stomach

A large pouch with strong muscular walls, the stomach serves as a temporary holding
station and food-processor for the chewed and swallowed food. It has the ability to
expand or contract depending upon the amount of food it contains. The stomach aids
digestion in two ways. Its strong muscular walls churn the food into chyme - a semi-fluid
mixture resembling porridge - while glands within the walls secrete gastric juice - a
blend of hydrochloric acid and various digestive enzymes - that helps to digest foods like
protein, fats, a few carbohydrates and alcohol. To prevent the stomach from digesting
itself(!) its walls are lined with a membrane called mucosa which secretes a protective
slimy substance called mucus. Liquids pass through the stomach in a matter of
minutes, while solid food can remain in the stomach for up to 5 hours. Chyme slowly
exits the stomach and passes into the small intestine. For more, see Guide To
Digestion In The Stomach.

The Small Intestine

Approximately 17 feet in length, the small intestine is a coiled tube made up of three
sections - the duodenum, jejunum and ileum. As the semi-digested food (chyme)
enters the duodenum from the stomach, the duodenal lining releases intestinal
hormones that stimulate the gallbladder and pancreas to release special digestive
juices (bile and pancreatic juice) which help to further break down food molecules in the
chyme. It is in the small intestine that most nutrients are digested and absorbed,
although different nutrients are absorbed at different speeds. Typically carbs are
digested most rapidly, followed by proteins and finally fats. Micronutrients (vitamins and
minerals) consist of molecules tiny enough for the body to absorb without breaking
them down first, but water soluble vitamins are absorbed faster than fat soluble ones.
The duodenum and jejunum is where the chyme is broken down, while the ileum is
responsible for absorbing nutrients into the bloodstream. The absorbed nutrients pass
through the bloodstream to the liver where they are processed and either stored or
distributed to other parts of the body. After every useful, digestible ingredient other than
water has been wrung out of the chyme, the remaining "waste" passes into the large
intestine. For more, see Guide To
Digestion In The Small Intestine.

Surplus Energy Converts To Stored Body Fat
Aside from breaking down and absorbing nutrients, the digestive system also converts
food into energy to help power the muscles and fuel the millions of chemical reactions
needed to sustain good health. After immediate energy requirements have been
satisfied, any remaining surplus is stored as glycogen (a small reserve of liquid energy
stored in the liver and muscles), or body fat. For more, see Guide To Body Fat And
Adipose Tissue/Body Fat
- How We Gain Fat.

The Large Intestine

Also known as the large bowel, the large intestine - consisting of 3 sections, the cecum,
colon and rectum - is approximately 5 feet in length and has two main functions: to
absorb all remaining water from the food waste and to compress the remaining matter
into a compact bundle (feces or stool) so that defecation (excretion of waste) is easy
and convenient. The cecum is a short pouch containing a valve which opens to receive
chyme from the ileum. The colon absorbs water and through bacterial action reduces
the bulk of fiber in the feces. The rectum is the terminal segment of the digestive tract, in
which feces accumulate just prior to discharge. They are discharged through the anus
which contains two important muscles - the internal sphincter and the external
sphincter. The internal sphincter is always tight, except when feces enter the rectum, in
order to keep us continent (eg) when we are asleep. When we get an urge to defecate,
we depend upon the external sphincter to keep the stool in until we go to the toilet. In
total, it takes about 36-48 hours or longer for waste matter to pass through the large
intestine. As in the esophagus and the small intestine, the contents of the large
intestine are pushed forward by a sequence of muscular contractions called peristalsis
(a type of motility or muscular movement). Peristalsis is regulated by a large network of
nerves, hormones and muscles. Malfunction of any of these components may lead to a
range of intestinal problems, including indigestion and constipation. For more, see
Guide To Digestion In
The Large Intestine.

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