Learning Objectives
At the end of this lesson you should be able
to:

Identify the important organs in the
digestive system and their functions;

Trace the pathway of food in the digestive
system

Appreciate the importance of the digestive
processes of the human body.
What is the digestive system?
Your digestive system is a
network of organs that help
you digest and absorb
nutrients from your food. It
includes
your
gastrointestinal (GI) tract
and your biliary system. Your
GI tract is a series of hollow
organs
that
are
all
connected to each other,
leading from your mouth to
your anus. Your biliary
system is a network of three
organs that deliver bile and
enzymes through to your GI
tract your bile ducts.
Digestion is the process of mechanically and
enzymatically breaking down food into
substances for absorption into the bloodstream.
Mechanical digestion involves
physically breaking down food
substances into smaller particles
to more efficiently undergo
chemical digestion.
The role of chemical digestion is
to
further
degrade
the
molecular structure of the
ingested
compounds
by
digestive enzymes into a form
that is absorbable into the
bloodstream.
Why is digestion important?
Digestion is important because your body needs
nutrients from food and drink to work properly and
stay healthy. Your digestive system breaks nutrients
into parts small enough for your body to absorb and
use for energy, growth, and cell repair.
Stages to food processing
1. Ingestion: taking in/eating of food
2. Digestion: breaking down
food(mechanically/chemically) into
nutrients
3. Absorption: taking in of nutrients by
cells
4. Elimination: removal of undigested
food and waste products of digestion
Organs of the Digestive System

Digestion begins immediately in
the mouth with both mechanical
and chemical digestion.

Mechanical digestion in the oral
cavity consists of grinding of food
into smaller pieces by the teeth, a
process called mastication.

Chemical digestion in the mouth is
minor but consists of salivary
amylase
(ptyalin,
or
alphaamylase) and lingual lipase, both
contained in the saliva.

After sufficient digestion in the oral
cavity, the partially digested
foodstuff, or bolus, is swallowed
into the esophagus
Organs of the Digestive System
 A tube (about 25 cm long)
that connects the mouth
to the stomach.
 Located in your throat near
your trachea (windpipe),
the esophagus receives
food from your mouth
when you swallow.
A
series
of
muscular
contractions within the
esophagus
called
peristalsis delivers food to
your stomach.
Organs of the Digestive System
 The
stomach
is
the
expanded organ located
between the esophagus and
the small intestine.
 It is a muscular organ, elastic,
pear-shaped organ that is
about 30.5 cm long and 15.2
cm wide. The stomach’s
capacity is about 1 liter.
 The stomach performs three
functions: stores swallowed
food; mixes the food with the
digestive juice it reduces;
and conveys its contents
slowly into the small intestine.
Organs of the Digestive System
 Mechanical digestion in
the stomach occurs via
peristaltic contractions of
the smooth muscle from
the fundus towards the
contracted pylorus,
termed propulsion.
 There
is
significant
chemical digestion in the
stomach. Two types of
glands exist in the gastric
mucosa that aid in
chemical
digestion:
oxyntic
glands
and
pyloric glands.
Organs of the Digestive System
The majority of chemical
digestion occurs in the
small intestine. A muscular
tube that is small in
diameter (2.5 cm) but
approximately 7 meters in
length.
The muscles of the small
intestine mix food with
digestive juices from the
pancreas, liver, and
intestine, and push the
mixture forward for further
digestion.
Organs of the Digestive System

The walls of the small
intestine absorb water
and
the
digested
nutrients
into
your
bloodstream.
As
peristalsis continues, the
waste products of the
digestive process move
into the large intestine.

It secretes enzymes such
as maltase, sucrase, and
lactase for the digestion
into monosaccharides;
Organs of the Digestive System

Peptidase that digests
peptide molecules into
amino
acids;
and
nuclease
for
the
digestion
of
nucleic
acids into sugar and
nitrogenous bases.

By the time food leaves
the
small
intestine,
around
90%
of
all
nutrients
have
been
extracted from the food
that entered.
Organs of the Digestive System
 The duodenum (25 cm) is the
first segment of the small
intestine.
It’s
largely
responsible for the continuous
breaking-down process.
 To help break food down, the
duodenum receives digestive
juices from other organs in
your
digestive
system,
including
your
liver,
gallbladder and pancreas.
Ducts from these organs feed
into the duodenum.
 The absorption of vitamins,
minerals, and other nutrients
begins in the duodenum.
Organs of the Digestive System
 The liver produces bile
(green liquid) to emulsify
fats in the chyme in the
small intestine into globules.
 Bile is stored in the
gallbladder.
 The
pancreas
secretes
digestive enzymes to break
down protein, fats, and
carbohydrates.
It
also
converts
starch
into
maltose; fats into fatty
acids and glycerol; proteins
into amino acids; and
nucleic
acids
into
nucleotides.
Organs of the Digestive System
 The main function of the
jejunum ( 1.4 m) absorption
of important nutrients such
as sugars, fatty acids, and
amino acids.
 Peristalsis, the involuntary
contraction
of
smooth
muscles that moves nutrients
through
the
digestive
system, is vigorous and quick
in the jejunum. Nutrients
absorbed by the jejunum
enter
the
bloodstream,
where they can then be
distributed to the organs of
the body.
Organs of the Digestive System
 The last portion of the
small intestine is the ileum
, which has fewer villi and
basically compacts the
leftovers to pass through
cecum through large
intestine.
 It
absorbs
any
final
nutrients,
with
major
absorptive
products
being vitamin B12 and
bile acids.
Organs of the Digestive System
 Waste products from the
digestive
process
include
undigested parts of food, fluid,
and older cells from the lining
of your GI tract. The large
intestine or colon absorbs
water and changes the waste
from liquid into stool. Peristalsis
helps move the stool into your
rectum.
 Water
is
removed
and
bacteria break down some
undigestible
materials,
producing
important
compounds (such as vitamin K
and B with water). The
concentrated waste material
that remains is called feces.
Organs of the Digestive System
 All leftover waste is
compacted
and
stored at the end of
the large intestine
called the rectum.
 When full, the anal
sphincter
loosens
and
the
waste,
called feces, passes
out of the body
through the anus.