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Digestive System
By the end of this class you should
understand:
• The processes of physical and chemical
digestion
• The route of food, nutrients and waste
through the body
• The primary functions of the major organs of
the digestive system
• The essential process of nutrient absorption
Digestive Organs
• The digestive system is
divided into two types of
organs:
– The alimentary organs that
make up the alimentary canal
– Accessory organs that do not
make up the alimentary canal
• The alimentary canal is the
tube running through your
body
Alimentary Organs
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Mouth
Pharynx (throat)
Esophagus
Stomach
Small intestine
Large intestine
Anus
Movement
• Organs are moved from one to
the next through a process called
peristalsis (the pushing of food
through the alimentary canal by
smooth muscle)
– In the esophagus, swallowing is
initiated by skeletal muscle but
completed by smooth muscle and
qualifies as peristalsis
– The stomach, small and large
intestine all have peristalsis but it is
much slower
Digestive Function
• The digestive system has
one goal only: to put
nutrients into the
bloodstream so that all
our cells can access
those nutrients
– Many excess nutrients
can be stored
• The digestive system has
many “preparation
steps” to prepare food
for this process
The Most Vital Organ
• Only one organ performs
almost all the absorption of
nutrients (excepting water):
the small intestine
– So called because it has a
small diameter, though it is
several meters long
• If we drank liquids containing
only single molecules of
nutrients, we would need no
other digestive organ!
– This is what they inject into
your blood when you’re in a
coma
Two Types of Digestion
• Digestion is the process of breaking
down nutrients so they can be
absorbed
– Undigested nutrients cannot be
absorbed and pass through the small
intestine to the large intestine
– For more information, ask someone
who is lactose-intolerant about
drinking milk
• Mechanical (physical) digestion is
the chewing and churning of food
• Chemical digestion is the breaking
of chemical bonds in food
Mechanical Digestion
• The mouth (chewing) and stomach
(churning) perform mechanical
digestion of food
• The mouth accomplishes this with
hard calcium structures called teeth
and heavy muscles attached to the
mandible chewing
• The stomach has layers of smooth
muscles that churn the food before
it moves to the small intestine
Chemical Digestion
• The mouth and stomach also
initiate the process of chemical
digestion
– The mouth has salivary glands that
produce saliva, which begins
chemical digestion
– The stomach has pepsin, an enzyme
that denatures and breaks down
protein
• Note chemical digestion of fats
does not begin until the small
intestine
The Stomach
• The stomach produces
gastric juice, which contains
pepsin as well as other
chemicals
• This includes hydrochloric
acid, which makes the
stomach environment very
acid
• The acid itself is not for
digesting the food
– Kills bacteria
– Enables pepsin to work
Stomach Ulcers
• Stomach ulcers were once thought
to be caused by stress but are now
known to be a bacterial infection
(but stress does block the immune
system)
• The scientist who discovered the
bacterium that causes ulcers was
eventually awarded the Nobel
Prize
– No one had believed him initially so
he drank a flask of the bacterium to
gain ulcers
Gastric Juice
• Gastric juice is the secretion of the stomach
• Gastric juice is very acidic and can damage
tissues other than the stomach lining
– Repeated vomiting can cause gastric juice to erode
the enamel of the teeth
– If gastric juice is produced in excess it can
overflow into the esophagus, causing “heartburn”
• Once gastric juice mixes with and digests food
it is called chyme
Processing of Chyme
• Chyme is the partially
digested food of the stomach
mixed with acid
• Chyme is delivered into the
small intestine very slowly
– Excess acid could damage the
small intestine
– Helps the intestine absorb all
the nutrients
• The small intestine has many
digestive enzymes of its own
that complete digestion
Accessory Organs
• The small intestine has enzymes on
its absorptive cells but also
receives digestive juices from two
accessory organs
• The pancreas produces a massive
battery of digestive enzymes to
finish all digestion of food
– Most prominent enzyme is trypsin, a
protease that breaks down protein
into amino acids
• The liver produces bile, which is
stored in the gallbladder until
chyme enters the small intestine
Bile
• Bile does not contain
digestive enzymes and does
not directly perform
chemical digestion
• Bile is instead an
emulsifying agent, much like
a detergent
– To emulsify is to allow oil and
water to mix
• Bile is therefore vital for
digesting and absorbing fats
from food
Small Intestine Absorption
• Since nutrients can only be
absorbed through cell membrane,
the small intestine epithelial tissue
has adaptations for increasing
surface area
– The tissue is arranged into fingerlike
villi (singular villus) which create
more surface area
– The individual cells have a brush
border, or microvilli (singular
microvillus)
• These cells expend ATP to actively
transport nutrients into the blood
Small Intestine Activity
• When food enters the stomach it
still consists of starches, proteins
and whole fats
– These cannot be absorbed since
they are too large!
• The small intestine can only
absorb the small molecules that
make these up (simple sugars,
amino acids, and fat molecules)
– Large molecules that are not
digested cannot be absorbed!
Lactose Intolerance
• Lactose Intolerance is one of
many digestive disorders caused
by a failure to absorb nutrients
– Lactose (milk sugar) is made of a
molecule of glucose and galactose
stuck together (too large to
absorb)
– Most people express a small
intestine enzyme called lactase
that breaks them apart
• Failure to digest lactose (or other
nutrients) results in the nutrients
ending up in the large intestine
– Bacteria metabolize it and release
gas and waste acids
Liver Function
• Blood flows directly from the
small intestine to the liver
– This is called hepatic portal
circulation
– Protects the rest of the body
from whatever you just ate
• The liver has metabolic enzymes
to process many nutrients after
they are absorbed
– Production of fat and glycogen
– Storage of vitamins
– Breakdown of toxins such as
alcohol
Large Intestine
• Once all nutrients are absorbed into
the small intestine, the large intestine
(which is mostly composed of a tube
called the colon) carries remaining
indigestible materials to the anus
• The large intestine is filled with
colonies of bacteria that feast on our
leftovers (often producing gas as a
waste product)
• The large intestine is capable of
absorbing water and vitamins,
including those produced by bacteria
Hopefully that wasn’t too much to digest…
• Wednesday is nutrition lesson!