Transcript The Digestive System

The Digestive System
By: D. Reis
The digestive tract,
also called the
gastrointestinal tract
(GI) or alimentary
canal is a tube that
is open at both
ends.
The 4 Stages of Food
Processing
• Ingestion – taking in nutrients
• Digestion – physically and chemically
breaking down food molecules
• Absorption – absorbing digested
molecules into cells
• Egestion – removal of waste food
materials
Ingestion and Digestion: Mouth
• Incisors (8) –
bite
• Canine teeth (4) –
tear
• Premolars – (8)
• Molars (12) – crush
and grind
• Teeth assist in the
physical digestion of
food.
Tooth Anatomy
• Enamel crown – hard
substance containing
calcium compounds
• Dentin – bonelike
substance encasing
pulp cavity
• Pulp cavity –
contains nerves and
blood vessels
Salivary Glands
• The salivary glands
secrete saliva, water
and amylase.
• Amylase – an enzyme
released by the
salivary glands while
chewing. Amlyase
breaks down starch
(amylose) into
maltose subunits.
Pharynx
• A bolus enters the
pharynx.
• Pharynx – dualpurpose passage for
food and air
• The epiglottis acts like
a trap door that
prevents food from
entering the trachea
(windpipe) so that
choking does not
occur.
Peristalsis
• From the pharynx the
bolus of food
stretches the walls of
the esophagus and
initiates peristalsis
(contractions helping
to move food along GI
tract)
• Peristalsis is
independent of gravity
Digestion: Stomach
• Sphincters – circular
muscles that control
movement of food
into and out of the
stomach
• Stomach – contains
ridges called rugae
that allow it to
expand.
Cardiac
sphincter
Pyloric sphincter
Gastric Pits
• Gastric pits are
openings in the
stomach lining
which extend into
the mucosa as
straight and
branched tubules,
forming gastric
glands. Dog
stomach to the left
Stomach Secretions
• Stomach secretions include :
• HCl – helps break down food, kills bacteria that
may have been ingested with food and provides an
acidic environment for pepsin to function
• Mucus – Protects the stomach lining from acidic
gastric juices
• Pepsin – Enzyme that works at a pH of 2 to help
break down protein molecules
• Chemical and physical digestion in the stomach
changes the bolus of food to partially digested food
that contains water and gastric juices with a very
low pH called chyme.
Factors Affecting Enzymes
Stomach Ulcers
• Without mucus, the gastric
juice would digest the
stomach lining
• Damage to the mucus
barrier results in gastric
ulcers
• Most stomach ulcers have
been linked to the
bacterium H. pylori, which
prevents the lining of the
stomach from producing
mucus
Digestion: Small Intestine
• Duodenum – the first
25cm portion of the
small intestine where
most digestion occurs
• Measure up to 7 m in
length and 2.5 cm in
diameter
• In the duodenum chyme
is mixed with gall
bladder and pancreas
contents.
Small Intestine: Digestion and
Absorption
• Duodenum – food
digestion
• Jejunum – structure
immediately following
the duodenum, finalize
food digestion
• Ileum – structure
immediately following
the jejunum; absorptive
function; linked to large
intestine by ileocecal
valve
Absorption: Small Intestine
• 80% of the absorption of nutrients takes place
within the small intestine.
• Villi – small fingerlike projections of the mucosa
that project into the lumen.
• Microvilli – cells making up the lining of each
villus
• Both increase the surface area of the small
intestine for maximum absorption of nutrients
into the intestinal wall.
• Lacteal – capillary network that allow for
diffusion and transport from intestinal cells into
the blood.
Villi
Mesentery
• Anchors the small
intestine to the back of
the abdominal wall.
Blood vessels, nerves,
and lymphatics branch
through the mesentery
to supply the intestine.
• Prevents the
entanglement of the
small intestine.
Small Intestine
• The small intestines secretes the following
enzymes:
• Disaccharidases – breaks down
disaccharides into monosaccharides (ex:
maltase)
• Peptidases – complete the digestion of
protein into amino acids
Large Intestine
• The large intestine or colon is 1.5 m long
and is twice the diameter of the small
intestine
• Houses bacteria E. coli, which uses waste
to synthesize vitamin K and B.
• Main function is to reabsorb water (90 %
of water is reabsorbed back into the blood)
and to remove feces.
• Feces consist of cellulose, bacteria and
water. Cellulose provides bulk.
Large Intestine
Accessory Organs
J
• The liver is the 2nd largest
organ in the human body
divided into 2 lobes, left
and right.
• Under the right lobe of
the liver is the
gallbladder.
• The pancreas lies behind
the stomach and is about
15cm in length.
Gall Bladder
• The entry of fats into the duodenum stimulates the release of
the hormone cholecystokinin (CCK). CCK causes the gall
bladder to contract and send bile through the common bile
duct to the duodenum.
• Bile is synthesized by the liver and stored in the gall bladder.
Bile empties into the small intestine where it acts like
detergent by physically breaking down large fat globules to
smaller fat globules in the small intestine.
• The principal components of bile are cholesterol, bile salts,
and the pigment bilirubin. Bilirubin is a brownish yellow
substance found in bile. It is produced when the liver breaks
down old red blood cells. Bilirubin is then removed from the
body through the stool and gives stool its normal brown
color.
Gallstones
• An imbalance between these components
of bile cholesterol, bile salts, and bilirubin - leads to the formation of gallstones.
• Gallstones can block the normal flow of
bile by lodging in any of the ducts that
carry bile thus impairing fat digestion.
• Jaundice is a yellowish discoloration of the
skin caused by the accumulation of bile in
the blood.
The Liver
• Catalase found in high concentrations in
the liver helps detoxify many harmful
substances present in the blood.
• Cirrhosis of the liver occurs when
damaged liver cells are replaced by
connective tissue and fat due to
excessive alcohol.
Pancreas
• Once chyme enter duodenum the hormone
secretin released from the duodenum stimulates
the release of Bicarbonate ions (HCO-3)from the
pancreas which raises the pH of the chyme from
2.5 – 9, protecting the cells of the duodenum
• Because bicarbonate ions increase the alkalinity
in the duodenum pepsin becomes inactive.
• Lipase – fat digesting enzyme released from the
pancreas
• Pancreatic amylase – carbohydrate digesting
enzyme released from the pancreas
• Trypsin and Chymotrypsin – Protein digesting
enzyme released from the pancreas