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Digestive System
Functions
 Ingest food
 Breaks it down to absorb nutrients
 Eliminates what cannot be digested
Ingestion
 Mechanical digestion – involves chewing food to break it down into smaller
pieces
o Includes the action of smooth muscles in the stomach and small intestines
which churn the food
 Chemical digestion – action of enzymes to break down large molecules into
smaller molecules that can be absorbed by cells
o Ex. Amylase – enzyme found in saliva which begins the process of
chemical digestion by breaking down starches into sugars
Esophagus
 When the tongue pushes chewed food to the back of the mouth, the swallowing
reflex is stimulated
 Tongue forces food into the upper portion of the esophagus
o Muscular tube that connects to the pharynx, or throat, to the stomach
o Lined with smooth muscles that contract rhythmically to move food
through the digestive system in a process called peristalsis
 Continues throughout the digestive tract
 Not even affected by gravity! (you could still digest food upside
down)
 When a person swallows, the epiglottis covers the trachea (windpipe)
o If food enters the windpipe, choking occurs because the body initiates the
coughing reflex in an attempt to expel the food to keep it from entering the
lungs
Stomach
 A circular muscle, called a sphincter, controls the movement of food from the
esophagus into the stomach
 Walls of the stomach are composed of three overlying layers of smooth muscle
which are involved in mechanical digestion
o As the muscles contract, they further break down the food and mix it with
the secretions of glands that line the inner wall of the stomach
 Very acidic environment
 Gastric glands secrete an acidic solution at pH 2
 Any leakage of stomach acid through the sphincter muscles can cause acid to
reach the esophagus, causing heartburn
 Pepsin – enzyme involved in the process of chemical digestion of proteins in the
stomach
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o Cells in the lining of the stomach secrete mucus to help prevent pepsin
damage to the stomach muscles
Some substances like alcohol and aspirin are absorbed by stomach cells
When empty, stomach capacity is about 50 mL
o When full, it can expand to 2-4 L
Muscular stomach walls contract and push food farther along the digestive tract
Empties through another sphincter into the small intestine
Small Intestine
 7 m in length
 Longest part of the digestive system
 Small intestine because the diameter is 2.5 cm compared to the diameter of the
large intestine at 6.5 cm
 Smooth muscular walls continue the process of mechanical digestion and
peristalsis
 Chemical digestion is completed by accessory organs
o Pancreas
 Produces enzymes that digest carbohydrates, proteins, and fats
 Produces hormones
 Secretes an basic solution to raise the pH in the small intestine to
pH 7 (neutral) for the optimum environment for enzyme action in
the small intestine
o Liver
 Largest internal organ of the body
 Produces bile which breaks down fats
 About 1 L of bile is produced each day
 Excess is stored in the gall bladder
o Gall bladder
 Stores excess bile
 Releases bile into the small intestine when needed
 Nutrients from food are absorbed from the small intestine into the bloodstream
through villi
o Fingerlike projections
o Increase the surface area of the small intestine
 Gives the small intestine as much absorption room as a tennis
court!
 When digestion is complete, any remaining food, in the form of a semi-liquid
solution called chyme moves into the large intestine
o Chyme is composed of materials that cannot be digested or absorbed by
the villi in the small intestine
Large Intestine
 End portion of the digestive tract
 1.5 cm long
 Includes the colon, rectum and appendix
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Colon normally contains bacteria which produce vitamin K and some B vitamins
o Primary function is to absorb water from chyme
Indigestible material becomes more solid, forming feces
Peristalsis continues to move feces to the rectum, causing the walls of the rectum
to stretch
o Initiates a reflex causing the final sphincter muscle to relax, eliminating
the feces from the body through the anus
Digestive Structure
Mouth
Esophagus
Stomach
Small Intestine
Large Intestine
Primary Function
Mechanical and chemical
digestion
Transport (swallowing)
Mechanical and chemical
digestion
Mechanical and chemical
digestion
Water absorption
Nutrition
Nutrition – process by which a person takes in and uses food
Calories
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Carbohydrates
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Fats
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Proteins
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Food Pyramid
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Vitamins and Minerals
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Nutrition Labels
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Time Food in Structure
5-30 seconds
10 seconds
2-24 hours
3-4 hours
18 hours – 2 days