The digestive system includes the digestive tract and its
accessory organs, which process food into molecules that can
be absorbed and utilized by the cells of the body. Food is broken
down, bit by bit, until the molecules
are
small enough to be absorbed and the
waste products are eliminated. The digestive tract,
also called the alimentary canal or gastrointestinal
(GI) tract, consists of a long continuous tube that
extends from the mouth to the anus. It includes the
mouth, pharynx, esophagus, stomach, small intestine,
and large intestine. The tongue and teeth are accessory
structures located in the mouth. The salivary glands,
liver, gallbladder, and
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pancreas are major accessory organs that have a role in digestion.
These organs secrete fluids into the digestive tract.
Food undergoes three types of processes in the body:
- Digestion
- Absorption
- Elimination
Digestion and absorption occur in the digestive tract. After
the nutrients are absorbed, they are available to all cells
in the body and are utilized by the body cells in metabolism.
The digestive system prepares nutrients for utilization by
body cells through six activities, or functions.
Ingestion
The first activity of the digestive system is to take in food
through the mouth. This process, called ingestion, has to
take place before anything else can happen.
Mechanical Digestion
The large pieces of food that are ingested have to be broken
into smaller particles that can be acted upon by various enzymes.
This is mechanical digestion, which begins in the mouth with
chewing or mastication and continues with churning and mixing
actions in the stomach.
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Chemical Digestion
The complex molecules of carbohydrates, proteins,
and fats are transformed by chemical digestion into
smaller molecules that can be absorbed and utilized
by the cells. Chemical digestion, through a process
called hydrolysis, uses water and digestive enzymes
to break down the complex molecules. Digestive enzymes
speed up the hydrolysis process, which is otherwise
very slow.
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Movements
After ingestion and mastication, the food particles move from
the mouth into the pharynx, then into the esophagus. This
movement is deglutition, or swallowing. Mixing movements occur
in the stomach as a result of smooth muscle contraction. These
repetitive contractions usually occur in small segments of
the digestive tract and mix the food particles with enzymes
and other fluids. The movements that propel the food particles
through the digestive tract are called peristalsis. These
are rhythmic waves of contractions that move the food particles
through the various regions in which mechanical and chemical
digestion takes place.
Absorption
The simple molecules that result from chemical digestion pass
through cell membranes of the lining in the small intestine
into the blood or lymph capillaries. This process is called
absorption.
Elimination
The food molecules that cannot be digested or absorbed need
to be eliminated from the body. The removal of indigestible
wastes through the anus, in the form of feces, is defecation
or elimination.
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