THE DIGESTIVE SYSTEM

COMPILED BY HOWIE BAUM

DIGESTIVE
SYSTEM

People are probably
more aware of their
digestive system
than of any other
system, not least
because of its
frequent messages.

Hunger, thirst,
appetite, gas ☺, and
the frequency and
nature of bowel
movements, are all
issues affecting daily
life.

The Digestive Tract
• Six Functions of the Digestive System

1. Ingestion

2. Mechanical processing

3. Digestion

4. Secretion

5. Absorption

6. Excretion

The Digestive Tract
• Ingestion

– Occurs when materials enter digestive tract via the
mouth

• Mechanical Processing
– Crushing and shearing
– Makes materials easier to propel along digestive

tract

• Digestion
– The chemical breakdown of food into small organic

fragments for absorption by digestive epithelium

The Digestive Tract
• Secretion

– Is the release of water, acids, enzymes, buffers, and
salts

– By epithelium of digestive tract
– By glandular organs

• Absorption
–Movement of organic substrates, electrolytes,

vitamins, and water
–Across digestive epithelium tissue
–Into the interstitial fluid of digestive tract

• Excretion
– Removal of waste products from body fluids
– Process called defecation removes feces

AN INTRODUCTION TO THE DIGESTIVE SYSTEM

• The Digestive Tract

• Also called the gastrointestinal (GI) tract or
alimentary canal

• Is a muscular tube

• Extends from our mouth to the anus

• Passes through the pharynx, esophagus, stomach,
and small and large intestines

The digestive system is one
of the most clearly defined in
the body.

It consists of a long
passageway, the digestive
tract, and associated glands.

These include the liver and
pancreas, which are
connected to the main tract
by ducts, or tubes, and
empty their products, such
as enzymes, into the tract.

Major Organs of
the Digestive Tract
Oral Cavity (Mouth)
Ingestion, mechanical processing with
accessory organs (teeth and tongue),
moistening, mixing with salivary secretions
Pharynx
Muscular propulsion of materials into
the esophagus

Esophagus
Transport of materials to the stomach

Stomach
Chemical breakdown of materials by acid
and enzymes; mechanical processing
through muscular contractions

Small Intestine
Enzymatic digestion and absorption of
water, organic substrates, vitamins, and ions

Large Intestine
Dehydration and compaction of indigestible
materials in preparation for elimination

Anus

Accessory Organs of
the Digestive System
Teeth
Mechanical processing by
chewing (mastication)

Tongue
Assists mechanical processing
with teeth, sensory analysis

Salivary Glands
Secretion of lubricating fluid
containing enzymes that
break down carbohydrates

Liver
Secretion of bile (important
for lipid digestion), storage of
nutrients, many other vital
functions

Gallbladder
Storage and concentration of
bile

Pancreas
Exocrine cells secrete buffers
and digestive enzymes;
Endocrine cells secrete
hormones

Your intestines are about 22 feet long– the surface area
would cover almost 3000 square feet, the size of a
tennis court!

There’s a good reason for its size –

Our digestive system is vital for our survival.

 It’s our nutrient feed

Our main defense system

Our main elimination route for waste

 The location for a million different vital biochemical
processes.

The Oral Cavity
• Functions of the Oral Cavity

1. Sensory analysis
• Of material before swallowing

2. Mechanical processing
• Through actions of teeth, tongue, and palatal

surfaces

3. Lubrication
• Mixing with mucus and salivary gland

secretions

4. Limited digestion
• Of carbohydrates and lipids

Figure 24-7a The Salivary Glands.

Parotid duct

Openings of
sublingual Salivary Glands

ducts
Parotid salivary

Lingual gland
frenulum Sublingual

salivary gland
Opening of left
submandibular Submandibular

duct salivary gland

Submandibular
duct

The oral cavity is bounded by the teeth, tongue, hard palate, and soft palate.
These structures make up the mouth and play a key role in the first step of
digestion, called ingestion.

This is where the teeth and tongue work with salivary glands to break down
food into small masses that can be swallowed, preparing them for the
journey through the alimentary canal.
https://www.visiblebody.com/learn/digestive/digestive-oral-cavity#saliva

Saliva Moistens Food &
Begins the Process of
Chemical Digestion

Saliva secreted by salivary
glands aids the mechanical
and chemical process of
digestion.

Saliva is about 99% water
and not only moistens food
but cleanses the mouth,
dissolves food chemicals so
they can be tasted, and
contains enzymes that start
the chemical breakdown of
starchy foods.

There are three pairs of
salivary glands: parotid,
sublingual, and
submandibular (also called
the submaxillary gland).

The Tongue Creates a Bolus of food so it can travel down the
Pharynx and Esophagus
https://www.youtube.com/watch?time_continue=13&v=6PymJS9
7XdY

Peristaltic Waves Move
Nutrients and Waste
Through the Intestines

• Peristalsis
• Consists of waves of muscular

contractions
• Moves a bolus along the length of

the digestive tract

• Peristaltic Motion
1. Circular muscles contract behind

bolus
• While circular muscles ahead of

bolus relax

2. Longitudinal muscles ahead of bolus
contract

• Shortening adjacent segments

3. Wave of contraction in circular
muscles

• Forces bolus forward

STOMACH
STRUCTURE

The stomach is the widest
part of the digestive tube.
It is a muscular-walled, J-
shaped sac in which food is
stored, churned, and
mixed with gastric juices
secreted by its lining.

This process begins
moments after food enters
the stomach from the
oesophagus, through the
gastro-oesophageal
junction.

Gastric juices include
digestive enzymes and
hydrochloric acid, which
not only breaks down food
but also kills potentially
harmful microbes.

Food that is chewed in the
oral cavity then swallowed
ends up in the stomach
where it is further digested
so its nutrients can be
absorbed in the small
intestine.

The salivary glands, liver
and gall bladder, and the
pancreas aid the processes
of ingestion, digestion, and
absorption.

These accessory organs of
digestion play key roles in
the digestive process. Each
of these organs either
secretes or stores
substances that pass
through ducts into the https://www.visiblebody.com/learn/digestiv
alimentary canal. e/digestive-accessory-organs#saliva

The Liver Secretes
Bile to Emulsify Fats

in the Small Intestine

The liver is one of the
largest organs in the body
and it is continuously
producing bile.

This yellowish-brown fluid
aids chemical digestion by
emulsifying fats in the
duodenum.

Bile flows out of the liver into
the right and left hepatic
ducts, into the common
hepatic ducts, and toward
the small intestine to help
with digestion and the
absorption of fats.

The Gall Bladder
Stores Bile

If bile is not
immediately needed for
digestion, it flows up
the cystic duct to the
gall bladder.

The gall bladder is a
green, pear-shaped sac
about 10 cm or 4 in.
long that stores and
concentrates excess
bile secreted by the
liver.

Bile is released by the
gall bladder as needed
into the small intestine.

The pancreas secretes
pancreatic juice, a mix of
digestive enzymes, water,
buffers (bicarbonates),
and electrolytes produced
by acinar and epithelial
cells.

Pancreatic juice drains
through the main
pancreatic duct (duct of
Wirsung) into the
common bile duct and
then into the small
intestine.

There it buffers stomach
acids and breaks down
protein, fats, and
carbohydrates.

Nutrients In, Waste Out:
How the Human Body Absorbs

Nutrients and Eliminates Waste

Villi that line the walls of the small
intestine absorb nutrients into
capillaries of the circulatory system
and lacteals of the lymphatic
system.

Villi contain capillary beds, as well
as lymphatic vessels called
lacteals.

Fatty acids absorbed from broken-
down chyme pass into the lacteals.

Other absorbed nutrients enter the
bloodstream through the capillary
beds and are taken directly to the Nutrients In, Waste Out: How the

Human Body Absorbs Nutrients and
liver, via the hepatic vein, for Eliminates Waste
processing.

Figure 24-3 The Structure of the Digestive Tract (Part 2 of 2).

Circular
fold

Mucosal
epithelium

Mucosa
Lamina
propria

Villi
Mucosal glands
Submucosal gland
Muscularis
mucosae

Mucosa Lymphatic vessel

Submucosa Artery and vein
Submucosal
plexus

Muscularis Circular muscle
externa layer
Serosa Myenteric plexus

(visceral
peritoneum) Longitudinal

muscle layer

Chyme passes from the small
intestine through the ileocecal
valve and into the cecum of the
large intestine.

Any remaining nutrients and
some water are absorbed as
peristaltic waves move the
chyme into the ascending and
transverse colons.

This dehydration, combined
with peristaltic waves, helps
compact the chyme.

The solid waste formed is called
feces.

It continues to move through
the descending and sigmoid
colons.

The large intestine temporarily
stores the feces prior to
elimination.

The body expels waste
products from digestion
through the rectum and anus.

This process, called
defecation, involves contraction
of rectal muscles, relaxation of
the internal anal sphincter, and
an initial contraction of the
skeletal muscle of the external
anal sphincter.

The defecation reflex is mostly
involuntary, under the
command of the autonomic
nervous system. https://www.youtube.com/watch?v=_exsrs

0STtU 3-1/2 minutes
But the somatic nervous
system also plays a role to
control the timing of WHAT HAPPENS IN 1 MINUTE
elimination. https://www.youtube.com/watch?v=GjSTiWcqyF

I

THE END !!