The Digestive System
Digestive Function
The digestive system is the organ system that processes food
extracts nutrients from it, and eliminates the residue.
It does this in four stages:
ingestion, the selective intake of food --
digestion, the mec
hanical and chemical breakdown of food --
absorption, the uptake of nutrient molecules into the --
epithelial cells of the digestive tract and then into the blood
. or lymph
. defecation, the elimination of undigested residue --
The digestion stage itself has two facets
Mechanical digestion is the physical breakdown of food
into smaller particles.
Chemical digestion is a series of hydrolysis reactions that
break dietary macromolecules into their monomers
(residues). It is carried out by digestive enzymes.
Some nutrients are already present in usable form in the
ingested food and are absorbed without being digested:
vitamins, free amino acids, minerals, cholesterol, and water.
Digestion involves the processes of motility, secretion, and
. membrane transport
General Anatomy
The digestive system has two anatomical subdivisions, the
. digestive tract and the accessory organs
The digestive tract is a tube extending from mouth to anus
measuring about 9 m long. It is also known as the
alimentary canal.
,It includes the oral cavity, pharynx, esophagus, stomach
small intestine, and large intestine.
Part of this, the stomach and intestines, constitute the
. gastrointestinal (GI) tract
The accessory organs are the teeth, tongue, salivary
. glands, liver, gallbladder, and pancreas
The esophagus, stomach, and intestines have a nervous
network called the enteric nervous system, which regulates
. digestive tract motility, secretion, and blood flow
Mastication (chewing) breaks food into pieces small
enough to be swallowed and exposes more surfaces to the
action of digestive enzymes. It is the first step in mechanical
digestion. Mastication requires little thought because food
stimulates receptors that trigger an involuntary chewing
. reflex
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