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Biology 12
9.1 The Digestive Tract
The digestive system is involved in the ingestion and digestion of food and elimination of
indigestible material.
• Digestion takes place within the digestive tract, which begins with the mouth and ends
with the anus.
• Digestion involves mechanical and chemical digestion.
o Mechanical digestion: chewing of food, and churning and mixing of food in the
stomach
o Chemical digestion: enzymes break macromolecules down into small organic
molecules
Biology 12
The Mouth
The mouth takes food into the body.
• Teeth: involved in chewing food
• Tongue: composed of skeletal muscle and
involved in forming the bolus (a mass of
food that is ready for swallowing)
• Roof of the mouth: composed of a hard palate and a soft palate; prevents ingested food
from entering the nasal cavity
• Tonsils: contain lymphoid tissue that protect against infections
• Salivary glands: produce saliva to keep the mouth moist; saliva contains an enzyme that
digests starch
The Pharynx
The pharynx is a passageway that
receives air from the nasal cavities
and food from the mouth.
Swallowing (a reflex action) occurs in the pharynx.
• The soft palate moves back to close off the nasopharynx
• The trachea moves up under the epiglottis to cover the glottis (the opening to the larynx
(voice box))
• During swallowing, food enters the epiglottis because the air passages are blocked
The Esophagus
The esophagus is a long muscular tube that moves food
from the mouth to the stomach by peristalsis.
• Peristalsis (rhythmic muscular contractions) pushes
food along the esophagus and through the digestive
tract to the stomach.
Biology 12
The Stomach
The stomach is an organ that receives food from the esophagus, mechanically and chemically
digests food, and moves food into the small intestine.
Structure and Function of the Stomach
The human stomach has thick walls with folds (called
rugae) that allow it to expand and fill with food.
• Lining of the stomach: has gastric glands that
secrete gastric juice containing mucus and
digestive enzymes
• Wall of the stomach:
• Three muscle layers (longitudinal, circular,
oblique)
• Responsible for moving and churning food
(mechanical digestion), and mixing food
with gastric juice to break it down
(chemical digestion)
• Alcohol and other liquids are absorbed in the
stomach, but most solid food is not.
• When food leaves the stomach, it is a thick, soupy
liquid called chyme.
• Chyme enters the small intestine by way of
the pyloric sphincter.
The Small Intestine
The small intestine receives chyme from the stomach and completes the digestion of food.
Macromolecules are broken down into nutrients, which are absorbed in the small intestine and
pass into the blood.
Structure and Function of the Small Intestine
The small intestine is composed of three parts.
• Duodenum: upper part of the small intestine
o Contains a bile duct that delivers bile from the liver and pancreatic juice from the
pancreas
− Enzymes in pancreatic juice complete food digestion
• Jejunum: middle part of small intestine
• Ileum: lower part of small intestine
o Contains lymphoid tissues involved in immune response to intestinal pathogens
The wall of the small intestine contains villi, which increase the surface area to improve the
absorption of nutrients.
• Villi: fingerlike projections that contain microvilli
o Microvilli increase the surface area of the villus for the absorption of nutrients
Biology 12
o Each villus contains blood capillaries and a small lymphatic capillary called a
lacteal
o Nutrients are absorbed into the blood capillaries and the lacteals, which carry
them to body cells
Regulation of Digestive Secretions
Digestive secretions are controlled by the nervous system and hormones.
After eating a meal:
• The stomach produces the hormone gastrin
o Gastrin: stimulates the gastric glands to secrete more gastric juice
• The duodenal wall produces the hormones secretin and CCK
o Secretin and CCK: stimulate the pancreas to secrete pancreatic juice and the gall
bladder to secrete bile
The Large Intestine
The large intestine absorbs water, salts, and some vitamins. It also stores indigestible material
until it is eliminated as feces.
Structure and Function of the Large Intestine
The large intestine includes the cecum, colon,
rectum, and anal canal.
• Cecum: a small pouch that forms the first
part of the large intestine
o Contains the appendix or
vermiform appendix, which may
play a role in fighting infection
• Colon: includes the ascending, transverse,
descending, and sigmoid colon
• Rectum: the last part of the large intestine;
opens at the anus
• Anus: rectum opening; site of defecation (expulsion of feces)
Defecation Reflex
• When feces are forced into the rectum by peristalsis, a defecation reflex occurs
• Eliminating feces from the body is a way the digestive system maintains homeostasis
• Feces are about three-quarters water and one-quarter solids
o Solids: bacteria, fibre, and other indigestible materials
o Bacteria break down some indigestible material, and produce some vitamins that
our bodies can absorb
o Water that is unsafe for drinking has a high number of coliform (intestinal)
bacteria, indicating that a significant amount of feces has entered the water