Download Digestive System

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 major nutrients the body requires for life
• The major risks associated with excess and
deficiencies of food
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
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
Vital Nutrients
• The small intestine absorbs all
the nutrients required for life
• Some nutrients are required for
energy and for building blocks
for more cells and cell parts
– Carbohydrates
– Proteins
– Lipids
• Other nutrients are required for
homeostasis but not energy
– Water
– Vitamins
– Minerals
Carbohydrates
• A single carbohydrate molecule is
called a simple carbohydrate (such as
glucose)
– Starches are made of many bound
together and are called complex
carbohydrates
– Some are more complex than others,
and the more complex the branching,
the longer the digestion takes
• “White” starches such as white rice
and pasta contain simpler chains
– Complex carbs, since they break down
more slowly, are less likely to cause
blood sugar to spike
Proteins
• Proteins must be broken down to
the individual amino acids
• The human body can synthesize
many of the amino acids from
other amino acids (so they are
nonessential), but there are
certain ones that cannot be
synthesized
– These are called essential amino
acids
• If even one of the essential amino
acids is missing, the diet is
incomplete and no new protein
can be made
Lipids
• There are many classes of lipids
– Some vitamins and cholesterol are
both lipids, but cannot be burned for
energy
• The energy-storing lipids are fatty
acids, and these fats can be saturated
or unsaturated
– Saturated fats have no double bonds
and so pack together more tightly than
unsaturated fats
– Treating unsaturated fats with
hydrogenation can create trans fats,
which are very difficult to metabolize
and therefore very unhealthy
Vitamins and Minerals
• Vitamins are organic
compounds that serve key
purposes in cell
mechanisms but only in
small amounts
– Example: vitamin A used to
make retinal
• Minerals are inorganic
elements and compounds,
such as sodium, potassium,
calcium, iron, and iodine
– Also necessary for life
Hopefully that wasn’t too much to digest…
• Enjoy your Thanksgiving weekend!
• See you next week for the last week of new
lessons!
– Tuesday: Urinary system
– Thursday: Reproductive system