The Human
Digestive System
Mechanical & Chemical
Digestion
A Tour of the
Mammalian
Digestive System
The Oral Cavity
 Presence of food triggers a nervous
reflex that causes salivary glands to
deliver saliva.
 Even before food is actually in the mouth,
salivation may occur in anticipation
because of learned associations between
eating and the time of day, cooking,
odors, or other stimuli.
…continued
 More than 1 L of saliva is secreted into
the oral cavity each day.
 Saliva contains the following:
 mucin: glycoprotein that lubricates food
 buffers: prevents tooth decay
 salivary amylase: digestive enzyme that
breaks down starch and glycogen into
smaller polysaccharides and disaccharides
The Pharynx
 The swallowing reflex is triggered
when a bolus of food reaches the
pharynx
… continued
 The esophageal sphincter (a muscle that
is usually contracted) relaxes, allowing
the esophagus to open and the bolus of
food to enter the esophagus.
 The larynx moves upward and tips the
epiglottis over the glottis, preventing food
from entering the trachea.
 After the food has entered the
esophagus, the larynx moves downward
and opens the breathing passage
The Esophagus
 Conducts food from the pharynx down to
the stomach.
 Peristalsis squeezes a bolus along the
narrow esophagus.
 Salivary amylase continues to break
down starch and glycogen as the bolus
passes through the esophagus.
Peristalsis
 Waves of muscular
contraction move
the bolus down the
esophagus to the
stomach.
The Stomach
 Located on left side of the abdominal
cavity, just below the diaphragm.
 This organ has very elastic walls and
accordion-like folds which allow it to
stretch to accommodate about 2 L of
food and fluid.
Anatomy of the Stomach
Secretion of Gastric
Juices
 The epithelium that lines the lumen of the
stomach secretes gastric juices - a
digestive fluid that mixes with the food.
 Contains a high concentration of
hydrochloric acid (HCl) - pH ~ 2
 Function of acid is to help break down
meat and plant material aswell as to kill
most bacteria that are swallowed with the
food.
… continued
 Gastric Juice also contains pepsin, an
enzyme that breaks down protein.
 Pepsin works well in acidic environments.
 The precursor to pepsin is pepsinogen.
 HCl in gastric juice converts pepsinogen
to pepsin.
… continued
 The mucosa that lines the stomach
consists of simple columnar epithelium
organized into gastric glands.
 Gastric pits lead into the gastric glands
which have three types of secretory
cells.
 Mucous Cells
 Chief Cells
 Parietal Cells
Control of Gastric
Secretion
 Controlled by a combination of nervous
impulses and hormones.
 When we see, smell, or taste food,
impulses from the brain to the stomach
initiate the secretion of gastric juices.
 Substances in the food itself stimulate
the walls of the stomach to release a
hormone called gastrin.
… continued
 The production of gastrin further
stimulates the secretion of gastric juices
(example of positive feedback).
 If the pH of the stomach contents
becomes too low, the acid will inhibit the
release of gastrin thus decreasing the
secretion of gastric juices.
 3 L of gastric juices are secreted every
day.
Chyme
 Every 20 seconds, the stomach contents
are mixed by the churning action of the
smooth muscle.
 As a result of mixing and enzyme action
is a nutrient broth called acid chyme.
Rate of Release from
Stomach
 Much of the time, the stomach is closed
off at either end.
 The cardiac orifice normally only dilates
when a bolus is driven through by
peristalsis.
 The occasional backflow of acid chyme
from the stomach into the lower end of
the esophagus causes heartburn.
Anatomy Review
… continued
 The pyloric sphincter helps regulate the
passage of the chyme into the small
intestine.
 A squirt at a time, it takes about 2 - 6
hours for the stomach to empty its
contents following a meal.
The Small Intestine
 Although some digestion takes place
in the mouth and stomach, most
enzymatic digestion of food occurs in
the small intestine.
 Most absorption also takes place in the
S.I.
 Name is based on diameter not length.
 Approx. 6 metres in length in humans.
The Liver
 Performs many important functions
including the production of bile.
 Bile is a mixture of substances that is
stored in the gall bladder until needed.
 Bile contains no digestive enzymes, but
does contain salts which aid in the
digestion of and absorption of lipids.
The Pancreas
 Produces several digestive
enzymes.
 Carbohydrases - break down
carbohydrates (eg. Pancreatic
amylase and maltase)
 Proteases - break down proteins
(eg. Trypsin, Chymotrypsin, and
Carboxypeptidase)
… continued
 Lipases - break down lipids (eg.
Pancreatic lipase)
 Nucleases - break down nucleic acids
(into nucleotides which are then broken
down into bases, sugars and
phosphates.
Summary: Digestive
Juices
… Specifics
The Duodenum
 First segment of small intestine.
 Approx. 25 cm in length.
 Receives digestive juices from the
pancreas, liver, gall bladder, and gland
cells of the intestinal wall itself.
Hormone Regulation
 At least 4 regulatory hormones help ensure
that digestive secretions are present only
when needed.
 Gastrin (already discussed)
 Secretin (stimulates pancreas to release
buffer)
 Cholecystokinin or CCK (stimulates
pancrease to release enzymes and gall
bladder to release bile)
 Enterogastrone (inhibits peristalsis and
acid secretion of stomach)
Jejenum & Ileum
 Remaining portions of small intestine.
 Function mainly in the absorption of
nutrients.
Absorption
 Large folds of epithelium line the small
intestine.
 Villi project outward from the folds.
 Each villus has a small lymphatic vessel
called a lacteal and a network of blood
capillaries surrounded by a layer of
epithelial cells
 Microvilli extend into the intestinal
lumen to create a “brush border”.
Intestinal Wall
… continued
 Nutrients are absorbed across the
epithelium and then across the unicellular
wall of the capillaries or lacteals.
 In some cases the transport is passive
(down concentration gradient).
 In some cases the transport is active
(ATP used).
… continued
 Sugars and amino acids are carried away
from the intestine by the bloodstream.
 After glycerol and fatty acids are absorbed by
epithelial cells, they are recombined to form
fat again.
 The fats are then mixed with cholesterol and
coated with special proteins, forming
chylomicrons. (HDL or LDL).
 Chylomicrons are then transported into
lacteals.
Fate of Macromolecules
 The capillaries and veins that drain
nutrients away from the villi converge into
a single circulatory channel, the hepatic
portal vessel.
 Liver has the ability to interconvert
various organic molecules.
 Once the liver makes changes, blood
flows to the heart to be pumped around
the body.
Hepatic Portal System
Large Intestine
 a.k.a. colon.
 Major function is to reabsorb water.
 About 7 L of fluid are secreted into the lumen
of the digestive tract each day.
 Most water reabsorption occurs along with
nutrient absorption in the S.I.
 The colon finishes the job by reclaiming most
of the water that remains in the lumen.
 Together, the S.I. & colon reclaim 90%.