Lesson 30.2
Digestive and Excretory Systems –
Part 2
Assumption College
Mathayom 4 Universal Biology 1
Miss Anna
30.3: The Digestive System
Functions of the
Digestive System
 What
does the digestive system do
for us?
The Digestive System…
 …converts
food into small molecules
that can be used by the cells of the
body
Food is processed by the
digestive system in 4
phases:
Ingestion
2. Digestion
3. Absorption
4. Elimination
1.
Step 1: Ingestion

Putting food into your mouth (the opening
to the digestive tract)
Step 2: Digestion

2 Types:

Mechanical Digestion: physical
breakdown of food into small pieces;
chewing

Chemical Digestion: digestive enzymes
break down food into small molecules the
body can use
Step 3: Absorption

The small intestine absorbs nutrients from
the small food particles

The molecules can then enter the
circulatory system, which transports them
throughout the body
Step 4: Elimination

Some materials cannot be digested by the
body and are eliminated
The process of digestion
How do our teeth help us
ingest and digest food?

Teeth help to cut and tear bites of food

They start the process of mechanical
digestion

A small amount of absorption occurs in
the mouth
How do teeth help us ingest and
digest our food?

Teeth and saliva begin to work on
digesting food first

Chewing begins the process of mechanical
digestion

Digestive enzymes in saliva start the
breakdown of complex carbohydrates into
smaller molecules
The mouth

Do much of the mechanical work of
digestion

Incisors, cuspids and bicuspids cut into
and tear at food

Molars crush and grind food

Tongue moves food around
Teeth

Secreted by salivary glands

Helps moisten the food and make it easier
to chew

Release of saliva controlled by nervous
system
Saliva

Saliva contains an enzyme called
amylase

Amylase begins to break the chemical
bonds in starches, forming sugars
Amylase

The combined actions of the tongue and
throat muscles push the clump of food
(called a bolus) down the throat

When you swallow, the bolus first enters
the area at the back of the throat (called
the pharynx)
After chewing…
So far, the path of food:

Mouth  pharynx (back of the throat) 
down the throat 

From the throat, the bolus passes through
a tube called the esophagus

peristalis: contractions of smooth muscle
which provide the force that moves food
through the esophagus toward the
stomach
The Esophagus
Peristalsis

The stomach is a large muscular sac that
continues the chemical and mechanical
digestion of food

The lining of the stomach contains lots of
microscopic glands that release
substances to help us digest our food
Chemical Digestion in the Stomach

Some glands in the stomach release an
enzyme called pepsin

Pepsin breaks proteins into smaller
polypeptide fragments
Pepsin

Contractions of the stomach’s smooth
muscle layers churn and mix the food

Gradually, a mixture called chyme is
formed out of the bolus

After 1-2 hours, the pyloric valve between
the stomach and small intestine opens,
and chyme moves from the stomach to
small intestine
Mechanical Digestion in the
Stomach
So far, the path of food:

Mouth  pharynx (back of the throat) 
down the throat  esophagus  stomach
 small intestine

Most of the chemical digestion and
absorption of food occurs in the small
intestine

As chyme enters the duodenum (first
part of the small intestine), it mixes with
enzymes and digestive fluids from the
pancreas, liver, and duodenum itself
Digestion in the small intestine

The pancreas is a gland behind the stomach
and serves 3 important functions:
◦ 1) the pancreas produces hormones that regulate
blood sugar levels
◦ 2) the pancreas produces enzymes (lipase) that
break down carbohydrates, proteins, and lipids, in
the food we eat
◦ 3) the pancreas produces sodium bicarbonate, a
base that quickly neutralizes acid as chyme enters
the duodenum
Pancreas
Real-life pancreas

The liver assists the pancreas in fat
digestion

The liver produces bile, a fluid loaded
with lipids and salts

Bile is stored in a small organ called the
gallbladder
The liver
Question:

Where in the body does the digestion of
carbohydrates begin?
Answer:

In the mouth

The process of chewing, secreting
amylase, and producing saliva all begin
the process of digesting carbohydrates
Question: What are the 2 roles of
the pancreas in FAT digestion?
Answer: The two roles of the
pancreas in fat digestion are:
1) produces enzymes (lipase)
which breaks down fats
2) production of sodium
bicarbonate, which neutralizes
stomach acid

Enzymes are proteins, which is a kind of
macromolecule
Review: Enzymes

Although the pancreas, liver and
gallbladder serve important functions in
digestion, food does not pass through
these organs
Pancreas, liver and gallbladder
continued…

The enzymes produced by the pancreas
break down which types of nutrients?
Question

Proteins, fats, and carbohydrates
Answer

Bile, which is produced by the liver and
released by the gallbladder, affects
digestion in which type of nutrient?
Question

Fats
Answer

Most nutrients are absorbed in the small
intestine

The large intestine absorbs water and
several vitamins and prepares waste for
elimination from the body
Absorption and Elimination

The small intestine is specially adapted for
absorption of nutrients

Its folded surface and finger-like
projections provide a large surface area
for the absorption of nutrient molecules
Absorption from the small
intestine

The finger-like projections are called villi
and are covered with tiny projections
known as microvilli
Absorption from the small
intestine

By the time the chyme is ready to leave
the small intestine, it is basically nutrientfree

Complex organic molecules have been
digested leaving only water, cellulose, and
other indigestible materials behind
Absorption from the small
intestine

As material leaves the small intestine and
enters the large intestine, it passes by a
small organ called the appendix

In some mammals, the appendix
processes cellulose and other things

The only time humans notice their
appendix is when it becomes inflamed, in
which case it must be removed
Absorption from the small
intestine
Appendix

When chyme leaves the small intestine, it
then enters the large intestine (or
colon)

The large intestine is shorter than the
small intestine, but it has a greater
diameter
Absorption from the large
intestine

The primary role of the large intestine is
to remove water from the undigested
material that is left

Water is absorbed through the lining of
the large intestine, leaving indigestible
materials behind
Absorption from the large
intestine

There is a large amount of bacteria found
in the large intestine to help aid in
digestion
Absorption in the large intestine
Large Intestine

The concentrated waste material – the
feces – that remains after most of the
water has been removed passes into the
rectum and is removed from the body
through the anus
Elimination
Questions?