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36-2 The Digestive System
Digestive Tract
• Alimentary canal
– one way passage through the body
• Function:
– to convert food into simple molecules that
can be absorbed and used by the cells of the
body
The Mouth
• Start of digestion
• Chewing of teeth tear/ crush moistened
food into a fine paste
• Teeth
– anchored in the bones of jaw and connected
to a network of blood vessels and nerves
– Coated with mineralized enamel
The Mouth
• Types of teeth
– Incisors – sharp to cut thru meat
– Cuspids and bicuspids – grasp and
tear food
– Molars – large flat surface for
grinding food
• Herbivores – molars are most
common
• Carnivores – incisors are
common
• Our teeth reflect a mixed diet of
meats and plants
The Mouth
Salivary glands
• 3 pairs –moistens food and make it easier to chew
• controlled by nervous system, triggered by smell
• Amylase
– enzyme to break down starches releasing sugars
– Results in sweet taste
• Lysozyme
– fights infection by digesting the cell walls of many bacteria
• result: chewed clump of food called the bolus
• Last – food is swallowed and safely passes the epiglottis
closed over the trachea
The Esophagus
• 25 centimeters long
• Receive bolus from the mouth after
swallowed and carries it to the stomach.
• Peristalsis
– the contractions of smooth muscle that occur
throughout the alimentary canal the squeezes
the food through the esophagus
The Stomach
• Cardiac sphincter
– thick ring of muscles the close the esophagus after the food has
passed into the stomach preventing back flow.
• Functions
– has the ability to store food
– Produce enzymes and strong acids, hydrochloric acid
– Churn the food through muscle contractions
• Gastric glands
– line the stomach
– Produce mucus – lubricates and protects stomach wall
– Produce pepsin – digests proteins
• End product of stomach – chyme
The Small Intestine
• The pyloric valve opens allowing chyme to enter
the duodenum
• Duodenum
– 1st part of S.I., site where digestive fluids are added and
most of the chemical digestion occurs
• Pancreas
– located just below the stomach
– Produces hormones that regulate blood sugar
– Produces enzymes that break down carbohydrates,
proteins, lipids and nucleic acids.
– Releases sodium bicarbonate – neutralizes stomach
acid and allows enzymes to function
Liver
• large gland just above the
stomach
• Produces bile – fluid loaded
with lipids and salts
• Dissolves and disperses
droplets of fat
• Regulates the levels of nutrients
in the blood
• When food reaches the
remaining portion of the S.I. it is
ready to be absorbed
The Small Intestine
• Jejunum and ileum – covered with villi specialized for absorption of food
• villi are covered w/ tiny projections called
microvilli
• Absorbed sugars, amino acids and other
nutrients are passed directly into the
bloodstream, Fats enter lymph then bloodstream
• The blood passes to the liver where excess
sugar stored as glycogen, until levels return to
normal regulating the blood sugar level.
The Large Intestine
• All nutrients have been removed
• Also known as the colon
• Function
– remove water from undigested material
producing feces
• Waste is then eliminated through the rectum
• Bacteria live in intestine receiving nutrients
from the feces and providing the body with
vitamin K and other compounds.
Regulating Nutrient levels - Sugar
Too little sugar – organs suffer from loss of energy
Islets of Langerhans – (pancreas) – produce insulin
• High levels of sugar – insulin is released and causes liver,
muscles and fatty tissue to remove sugar form the blood
stream and store it as glycogen and fat.
• Levels of sugar drop – glucagen is released and stimulates
the liver, muscles and fatty tissues to break down glycogen
and fats to release sugar back into the blood.
Diabetes mellitus
• person is unable to produce enough insulin and requires a
special diet or injections