Digestive System
Food Processing
Gastrointestinal (GI) Tract
● Tube where food is processed,
beginning at the mouth and ending
at the anus
○ Also called alimentary canal
Stages of Digestion
Four steps to digestion:
1. Ingestion
2. Digestion
3. Absorption
4. Elimination / egestion
1. Ingestion
The initial taking in or eating of food.
2. Digestion
Humans have a mouth to mechanically (teeth and tongue) and
chemically (saliva) start to break down food.
2. Digestion
Mastication (chewing) of food into
smaller particles.
● Job of teeth and tongue
● Mechanical breakdown
2. Digestion
Saliva produced by salivary glands:
● contains amylase to begin break
down of carbohydrates
● stimulates taste buds
● lubricates food for swallowing
2. Digestion
● Lingual lipase
○ Begins breakdown of lipids in
the mouth
Additional Structure
The tongue is covered in papillae which are covered in
taste buds.
The following structures are not directly related to digestion:
1. When you swallow, the uvula blocks food from entering the nasal
cavity.
2. The epiglottis is a flap of tissue which covers the pathway to the
lungs.
2. Digestion
The now softened mass of food (called bolus) is swallowed and goes to
your esophagus. The esophagus is surrounded by muscles which
pushes food down the tube in a wave-like manner call peristalsis.
2. Digestion
Food from the esophagus is pushed
into the stomach.
Three layers of muscle squeeze
and churn the contents of the
stomach.
● Mechanical digestion
2. Digestion
The stomach stores food and
secretes gastric juice, which
converts a meal to acidic chyme.
Chyme = thick, semi-fluid mass of
partially digested food
2. Digestion
The stomach is lined with
mucous membrane that secretes
protective mucus.
Small pores called gastric pits
contain cells that secrete gastric
juice.
2. Digestion
Gastric juice is made up of
hydrochloric acid (HCl), the enzyme
pepsin, and mucus.
● Parietal cells secrete hydrogen and
chloride ions separately
● Chief cells secrete inactive
pepsinogen, which is activated to
pepsin when mixed with
hydrochloric acid in the stomach
2. Digestion
Hormones such as gastrin are stimulated by the
presence of food and vagal nerve stimulation →
triggers increased secretion of gastric juice
Hydrochloric acid activates the precursor
pepsinogen and transforms it into pepsin.
2. Digestion
How does your stomach not digest itself?
Mucus coats the lining of the stomach to
protect it from powerful enzymes.
2. Digestion
Have you ever vomited and felt that your throat was
burning?
What stops that acid from regularly moving into the
esophagus?
2. Digestion
cardiac sphincter
What is heartburn?
pyloric sphincter
2. Digestion
After chemical and
mechanical digestion in the
stomach, the chyme is
produced and is a
slurry-like substance.
Chyme enters the small
intestines.
2. Digestion
Rugae
● Folds in stomach which allow for
the expansion of the stomach and
increased surface area for
digestion
2. Digestion & Absorption
The small intestines is where most
absorption takes place.
Three sections of small intestines:
1. duodenum
2. jejunum
3. ileum
2. Digestion & Absorption
1. Duodenum
Both the pancreas and the gallbladder
secrete additional enzymes into the
duodenum to complete digestion.
• carbohydrases
• proteases
• lipases (mainly gallbladder)
2. Digestion & Absorption
Pancreatic Secretions
● The pancreas produces proteases
trypsin and chymotrypsin,
protein-digesting enzymes that are
activated after entering the duodenum
● Its solution is alkaline and neutralizes
the acidic chyme
2. Digestion & Absorption
Pancreatic Secretions
● Produces lipase to chemically
break down lipids
● Produces pancreatic amylase to
chemically break down
carbohydrates to glucose
molecules
2. Digestion & Absorption
Bile Production by the Liver
● In the small intestine, bile aids in
digestion and absorption of fats. Bile
emulsifies fat. This is physical NOT
chemical digestion
● Bile coats the lipid and makes it easier
for the lipase to break the lipid down
2. Digestion & Absorption
Bile Production by the Liver
● Fat emulsification increases the
surface area for chemical digestion of
fats by lipases
● Bile is made in the liver and stored in
the gallbladder
2. Digestion & Absorption
2. jejunum
This section of the small intestines
marks the end of digestion where the
last of the digestive enzymes may be
found.
3. Absorption
Absorption occurs mostly at the small
intestines.
The inside surface is folded.
Why?
A folded surface has a greater
surface area.
3. Absorption
Each fold is covered by finger-like projections (villi). Why?
3. Absorption
Each fold is covered by finger-like
projections (villi). Why?
To speed up the process of absorption by
increasing surface area. Each villus is
covered with many fine brush-like
microvilli.
3. Absorption
3. ileum
The ileum is the final section of the small
intestines where absorption may take
place.
Additional Structures
The thin tissue holding the small intestines in place is called the mesentery.
4. Elimination
What is eliminated?
The process of elimination begins in the
large intestines (colon).
4. Elimination
Sections of the large intestines:
● caecum
● ascending colon
● transverse colon
● descending colon
● rectum
● anus
4. Elimination
All the components which are not absorbed must pass through the ileocecal
valve.
4. Elimination
The caecum is the pouch that marks the
beginning of the colon.
The appendix is a projection at the
bottom of the caecum.
Appendix Removal
4. Elimination
Large intestines:
● 90% of the water passing through the
colon is reabsorbed
● Indigestible plant matter is broken
down by the bacteria in the colon
● Gases released by bacteria
contributes to the foul odor of feces
4. Elimination
Feces is stored in the rectum until it is
eliminated through the anal sphincter
(anus).
Internal → involuntary control
External → voluntary control
