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Chapter 38 page 987
• There are four parts to digestion:
1. Ingestion – taking in of nutrients
2. Digestion – breakdown of large organic molecules
into smaller molecules called nutrients by:
a. Mechanical digestion (physically tearing, grinding
or churning large pieces into smaller pieces)
b. Chemical digestion (separating large molecules
into smaller molecules through chemical reactions.)
• 3. Absorption – the transport of digested
nutrients from the small intestine (SI) into the
tissues of the body.
• 4. Egestion – removal of material that the
body cannot digest
• Nutrients are simple molecules used by an
organism for energy and for building materials.
Lipids (fats)
Digestive Tract or Alimentary Canal
• The human digestive tract consists of 5 major organs
and is well over 8 meters long in an adult:
1. mouth
2. pharynx
3. esophagus (also spelled oesophagus)
4. stomach
5. small intestine
• duodenum
• jejunum
• ileum
– 6. large intestine
• rectum
• anus
The digestive tract includes the organs where
the food actually goes, beginning at the
mouth and ending at the anus. It is also called
the alimentary canal.
• The digestive system includes the organs of
the digestive tract and organs that provide
secretions to those organs. For example, food
doesn’t go into the liver but the liver secretes
a substance called bile into the pancreas
which releases it into the small intestine when
it is needed to help break down lipids.
• Mechanical digestion, the breakdown of food
begins in the mouth where it is chewed and
broken apart into smaller pieces.
• This gives more surface area for enzymes to
attack and digest which speeds up chemical
• Mechanical digestion of all foods begins in the
mouth with mastication.
• It continues in the stomach with the churning
caused by the 3 layers of stomach muscles and
with the secretin of hydrochloric acid.
• Chemical digestion occurs when molecules are
broken apart by enzymes.
• Different enzymes chemically digest different
nutrients and each enzyme works best in a
certain temperature and pH level.
• As we chew, 3 pairs of salivary glands secrete
saliva into our mouths. Saliva lubricates the
food and contains an enzyme, amylase, which
starts the chemical digestion of COH.
• Mucous comes from the cells lining the
mouth, making it easier to swallow.
• Swallowing: Combined action of the throat and
mouth muscles push the clump of chewed food,
the bolus, into the esophagus. The epiglottis, a
small flap of skin covering the opening of the
trachea, prevents food from going the wrong way.
• The contractions of the muscles in the
esophagus, peristalsis, squeeze the food through
the esophagus into the stomach.
(see Figure 38-11, page 980)
• A thick ring of muscle, the cardiac sphincter,
closes the esophagus after the food enters the
stomach and keeps the stomach acid from
moving into the esophagus.
• The stomach is a large sac lined with three layers
of muscles. Glands in stomach lining produce
mucous. Other glands produce hydrochloric
• Mechanical digestion in stomach- Muscles
alternate to churn food and hydrochloric acid.
• Chemical digestion- The acid activates an enzyme
called pepsin which is secreted by a third set of
glands. Pepsin starts the digestion of protein.
Amylase is denatured.
Amylase (or any enzymes for that matter)
denatures when the temperature or pH level
strays too far from the optimal level. For
example, the amylase works in a slightly basic
pH. When the pH rapidly drops as it moves
down the esophagus and into the stomach, the
enzyme denatures and is no longer able to break
down carbohydrates.
• Chyme is the mixture of food and stomach
juices, churned by the muscles in the stomach.
Chyme leaves the stomach through the pyloric