Digestive
System
Function:
Prepares food to be used
by the cells by breaking it
down
Passage of food from the
mouth to the anus takes
about 24-33 hours
Steps of breaking down food for
the cells:
• Ingestion- taking in food, chewing, swallowing
• Digestion- mechanical grinding of food to break
it down
• Absorption- if nutrients are small enough, they
pass through the intestinal wall to the blood
• Transportation- blood carries nutrients to the
cells and stores the excess in the liver
• Utilization- cells use the nutrients from the blood
• Elimination- undigested and unabsorbed
materials are removed from the body by
defecation
Parts of the digestive system:
•
•
•
•
•
Mouth
Esophagus
Stomach
Small intestines
Large intestines
Accessory Structures:
•
•
•
•
•
•
Liver
Gall bladder
Pancreas
Salivary glands
Teeth
Tongue
Lining of the digestive system
(mouth to large intestines):
•
•
•
•
4 layers (Tunics):
Mucosa- innermost lining
-Composed of mucous
membrane
-Allows food to slide easy
Submucosa- 2nd layer
(below mucosa)
-Has a lot of nerves and
blood vessels
Muscularis- has muscles
that aid in movement of
food through the system
Serosa- outermost layer
-Coats the entire
digestive tract
Objectives:
• Explain how digestion if accomplished by
the mouth.
• Describe the function(s) of the esophagus
and explain how these functions are
accomplished.
Mouth:
• Where food enters
the digestive system
• Function: teeth,
tongue, and saliva
mechanically begin
breakdown of food
• It takes about 20
seconds to bite and
chew food enough to
swallow
3 ways the mouth breaks down
food:
• Chewing (called mastication) physical
breakdown of food
• Saliva- releases chemicals that aid in food
breakdown
• Salivary amylase- an enzyme released in saliva that breaks
down starch (carbohydrates)
• Other foods aren’t broken down by enzymes here
• Swallowing- food is formed in a ball when
chewed and then swallowed
• The ball of food is called a bolus
• When food is swallowed, it is still too large to be absorbed to
the cells
Esophagus:
• Muscular tube that
connects the mouth
to the stomach
• It takes 5-8 seconds
for food to pass
through it
How does food go thru the
esophagus?
• Food is pushed down the
esophagus by gravity and
the waves of the
esophagus called
peristaltic waves
(peristalsis)
• In peristaltic waves:
– the muscular wall of the
esophagus contracts
pushing food down the
esophagus
Why does food not enter the
windpipe when swallowing?
• The epiglottis closes
the windpipe off when
swallowing to keep
food out of the
trachea (windpipe)
• The epiglottis stays
open at all times
unless swallowing so
air can go to the lungs
Stomach:
• Football shaped organ
when full
• Food stays here 2-3 hours
• It has 3 layers of muscle
that contract to churn (mix)
food so that it further breaks
it down
Functions of stomach:
• storage of food
• mixing of the food with gastric
secretions to form chyme
• emptying of the contents into the
small intestine at a rate that is
proper for digestion and absorption
Heartburn:
• The sphincters of the
stomach close it off by
squeezing the tubes
closed
• Sometimes the sphincter
does not close
completely
• This allows the acids of
the stomach to go up into
the esophagus causing a
burning sensation
• It is called heartburn
because it feels like a
burn that is at the heart
Stomach openings:
The stomach has 2 openings:
Esophageal sphincter
(also called cardiac sphincter)-
where esophagus attaches to
stomach (food enters)
Pyloric sphincter- where it
attaches to the small intestine
(food exits)
Other parts of the stomach:
-Fundus
-Body
-Rugae (antrum)expands when
the stomach is
filled with food
How does the stomach
chemically break down food?
• The walls of the stomach have
millions of gastric glands that
secrete gastric juices
• These juices have enzymes
called pepsin and
hydrochloric acid (very
acidic)
• Pepsin- breaks down proteins
(protein breakdown does not
start until it gets to the
stomach)
• Hydrochloric acid- drops the
pH of the stomach to 2 so it
dissolves good and kills
bacteria
Stomach acid:
• The pH of stomach juices are very
acidic
• If the acids were to contact your
skin, it would result in a major
burn
Why does the stomach not
burn?
It is lined with a layer of mucous
and mucous glands which work as
a barrier against the acids
Ulcer- hole in the lining of the
stomach where acid exits
Ulcers
Small intestines:
• About 6 meters long
(18 feet)
• Narrow in width
(small)
• Food is here 2-3
hours
• When food enters the
small intestine from
the stomach, it is a
soupy mixture called
chyme
Parts of the small intestine:
3 segments:
• Duodenum- 1st
segment-joins
stomach
-About 22 cm long
• Jejunum- 2nd
segment- the next 6 ft
-Curves downward
• Ileum- last segmentjoins large intestine
Small intestine functions:
• When food enters the intestines from the stomach, it is
still too large to be absorbed into the blood
2 processes (functions) occur in the small
intestine:
1. Digestion:
-The small intestine finishes digestion
2. Absorption:
-when molecules are broken down small enough,
molecules move across the lining of the intestines to
the blood
Digestion in the small intestine:
• Completed in small intestine
How?
• The pancreas secretes enzymes which a
released in the small intestine that finish the
breakdown of carbohydrates and proteins.
• It also releases the enzyme lipase that breaks
down fat (lipids)
• Also, the liver releases bile that breaks down
fats
Absorption process:
• When molecules are digestion small enough, they go to the
blood through the lining of the small intestine
How?
• The walls of the intestines are
lined with fingerlike projections
called villi.
• The villi have blood vessels
within them.
• They increase the surface area
allowing the digested particles to
contact the intestines, soaking
through to the blood (absorption)
Absorption through villi:
food molecules
inside small intestines
to cells
outside intestines (blood vessels carry
absorbed nutrients to cells of the body)
Exiting the small intestine:
• Once food reaches the end of the small
intestine, all food that can be digested is
broken down
• Only food that we cannot breakdown such
as cellulose remains
• The mixture is still soupy (chyme) when it
reaches the large intestine
Large intestine:
• Also called the colon
• It is about 5 -6 ft long
(it is wide)
• Undigested food
stays here about 18
hours
Parts of the large intestine:
3 main segments
• Ascending colon- 1st
segment
connects to small
intestine
• Transverse colon- 2nd
segment
• Descending colonlast segment
connects to rectum
Cecum- where small and
large intestines join
3 Functions of large intestine:
• Absorb sodium and water
-Undigested material enters the large intestine
(chyme) and water is removed from it
• Incubate bacteria
-Bacteria grow and undigested material and
convert some of it to vitamins (the bacteria is
what produces the smell)
• Eliminate waste
-Once water is removed, the undigested material
(feces) is eliminated out of the rectum called
defecation
How is material moved thru the
large intestines?
• Mass movement- the colon
contracts in sections forcing
feces to the next section until it
exits the body
• Mass movement is triggered
by food ingestion and stomach
expansion
The rectum has 2 sphincters
that control release of feces
• Internal sphincterinvoluntary-can’t control it
• External sphincter- voluntarycan control it
Appendix
• Worm-like extension of
the large intestine
• It is at the top of the colon
where it bends
• Purpose:
-Unknown
-Thought to be a
reservoir to incubate
bacteria
• Appendicitisinflammation of appendixdangerous because if it
ruptures it will release
bacteria
Large intestine disorders:
• Diarrhea- occurs when the material moves too
fast through the large intestine
-Unable to remove the water
-It is the body’s natural response to expel foreign
matter (like viruses) so they won’t make you
sicker
• Constipation- occurs when the material passes
to slow absorbing too much water
-Can be triggered by not eating enough fiber, not
taking in enough water, digestive disorders
Accessory
Structures:
•
•
•
•
•
•
Liver
Gall bladder
pancreas
Salivary glands
Teeth
tongue
Liver
• Weighs around 3 lbs.
• Has multiple functions:
–
–
–
–
–
–
–
–
–
Secretes bile
Removes nutrients from blood (storage)
Converts excess glucose to glycogen when needed
Stores iron and vitamins
Converts protein to fatty acids and urea
Metabolizes fats, proteins, carbohydrates
Makes plasma proteins
Detoxifies drugs
Removes bacteria and worn out red blood cells
Bile:
• Secreted by liver to the digestive tract
Components:
• 97% water
• Salts, pigments, cholesterol
– The pigments give it the green color
– Some of the pigments help breakdown food
Jaundice- condition where
the skin turns yellow due
to the build up of bile in the liver
How does bile get to the intestines?
• Common bile ductreleased bile to the
duodenum (small
intestine)
• Common hepatic
duct- takes bile to
gall bladder to store
• Gall bladder- where
excess bile is stored
Pancreas:
• Releases enzymes
that aid in digestion
and absorption of
food
• Releases these juices
to the duodenum
(small intestine)
• Pancreatic ducttakes the pancreatic
juices to the intestine
Salivary glands:
• Secrete saliva (about 1 liter/day).
Very little is secreted while we
sleep, this is why you awaken with a
dry mouth
3 main pairs of salivary glands:
1. Parotid- largest pair-in front of and
below ears
-Secretes to the cheek area
-Mumps- swelling of these glands
2. Submandibular- at an angle to the
jaw
-Secretes under the tongue
3. Sublingual- under the tongue and
secretes under the tongue
Saliva:
• Secreted in mouth
• Smell, taste, and sight of food stimulates
the secretion of saliva to prepare the
mouth for ingestion of food (“makes my
mouth water”)
• Sour pickles and lemonade are said to be
the strongest stimulates of saliva secretion
Components of Saliva:
• 1. serous- thin watery secretion
– Contains digestive enzymes
– Secreted by the parotid and submandibular
glands
• 2. mucous- lubricates
– Secreted by the sublingual and
submandibular gland
• 3. salts, antibodies, and substances
that kill bacteria
5 Functions of saliva
• Lubricate
• Moisten food to make
easier to swallow
(forms bolus)
• Aids in taste
• Washes away
particles and bacteria
• Releases salivary
amylase (breaks
down carbohydrates)
• Aids in digestion by
mechanically
grinding food to
break it down
• Teeth are rooted in
sockets on bony
ridges of the jaw
bones
• Gingivae- gumscover the projections
that extend from the
socket
• Periodontal
ligaments- attach
teeth to the sockets
Teeth:
• Crown- part above the gums
• Roots- part beneath the gum
• Dentin- substance that teeth
are made of
-gives them shape and rigidity
• Enamel- tough protective
covering of the crown
-hardest substance in body
-prevents wear
• Cementum- covers dentin part
of root
• Pulp cavity- inside the dentin
-filled with pulp
• Pulp- area that contains the
blood vessels and nerves
-called the nerve of the tooth
-very sensitive
• Root canal- the part of the
pulp that extends to the roots
Tooth structures:
Types of teeth:
• 1. deciduous- called milk
teeth or baby teeth
-Start to come in around
6 months and have a full
set by 2 years
• 2. permanent- replaces
deciduous by pushing
them from underneath out
of socket
-Adults have a max. of 32
Kinds of teeth based on
function:
• Incisors- used to bite and
tear- front teeth
-8 of these
• Canines- used to tearpointed
-4 of these
• Premolars- used to grind
and crush
-Behind canines
-8 of the these
• Molars- used to grind and
crush
-12 of these
-The last molars are the
wisdom teeth which
usually come in at around
age 18
Teeth disorders:
• Impacted teeth- often
wisdom teeth are too large to
fit in the area at the back of
the mouth
– They cannot come thru so they
press against other teeth
causing pain
• Dental plaque- light colored
film formed by bacteria and
saliva near the gum line
• Caries- progressive decay
that produces cavities
– Sugar on teeth is decomposed
by bacteria which sometimes
damages the enamel
Tongue
• Moves food around to aid
in digestion and taste
• Movement of the tongue
is limited by a layer of
mucous lining called the
lingual frenunculum
• Papillae- projections on
the top of the tongue that
hold the taste buds
Taste buds:
• Taste buds are located all
over the surface of the
tongue
• However, taste buds for
certain taste are found more
concentrated in specific
areas
4 main taste regions:
• 1. sweet (tip)
• 2. salty (sides)
• 3. bitter (back center)
• 4. sour (mid area, behind sweet)
Enzymes that breakdown nutrients
(proteins, fats, carbohydrates):
• Salivary amylase- breaks down
carbohydrates in the mouth to
monosaccharides
• Pepsin- breaks down proteins in the
stomach to amino acids
• Lipase- breaks down fats to glycogen in
the small intestine
*Bile- (not an enzyme) breaks down fats
Form that nutrients must be in to
be in to be small enough to be
absorbed to the cells:
• Proteins- broken down to amino acids
• Fats- broken down to glycogen
• Carbohydrates- broken down to
monosaccharides (glucose)