Human Digestion
• This part of the life process of NUTRITION process
by which an organism obtains and utilizes food
Food processing in four stages
1. Ingestion: taking in food
2. Digestion: mechanical and chemical breakdown of
food so that it can be absorbed by the cells
3. Absorption: cells lining the digestive tract take up
(absorb) small nutrient molecules
4. Elimination: undigested material passes out of the
digestive tract
Human Digestion
Digestion ≡ 2 part process that changes food into a form
useable by the body cells
1. Mechanical digestion – physical breakdown of large
pieces of food into smaller ones
2. Chemical digestion – hydrolysis – the splitting of large
insoluble molecules into small, soluble molecules with
the use of water and enzymes ( in other words;
breaking complex molecules into simple ones)
• The process of chemical digestion (hydrolysis) is
regulated by enzymes
Examples of chemical Digestion
• Carbohydrates + water  simple sugars ( e.g.
glucose)
• Proteins + water  amino acids
• Lipids + water  3 fatty acids + glycerol
HUMAN DIGESTIVE SYSTEM
• Human digestive tract
= GI (gastrointestinal)
Consists of a one-way
continuous tube
(mouth to anus)
Mouth
• Functions
– mechanical digestion
• teeth
–break up food
– chemical digestion (saliva)
• amylase enzyme
–digests starch
All that
in spit!
Mouth
• mucus
–protects soft lining of
digestive system
–lubricates food for easier
swallowing
• buffers
–neutralizes acid to prevent
tooth decay
• anti-bacterial chemicals
–kill bacteria that enter mouth
with food
All that
in spit!
HUMAN DIGESTIVE SYSTEM
• Mouth: (oral cavity)
ingests food
• Teeth: function in
mechanical breakdown of
food, increases surface
area of food for enzyme
action
• Tongue: acts as a plunger
to push food back into the
throat (pharynx)
– Taste buds are located
on the surface of the
tongue
mouth
break up food
digest starch
kill germs
moisten food
HUMAN DIGESTIVE SYSTEM
• Pharynx: food is pushed by tongue to back of throat,
initiates swallowing – food is now in the form of a
bolus
– Epiglottis: flap that prevents choking
• Esophagus: muscular tube that moves food from
mouth to the stomach by process of peristalsis:
– wave of muscular contractions that moves chewed
food to stomach
Swallowing (& not choking)
• Epiglottis ≡ flap of cartilage
– closes trachea (windpipe) when
swallowing
– food travels down esophagus
• Peristalsis ≡ involuntary muscle contractions
to move food along
Stomach
• Structure: muscular, expandable
bag
• Functions
–disinfect food
• hydrochloric acid ≈pH 2
–kills bacteria
–food storage
• can stretch to fit ≈2L food
–Digestion: pepsin (enzyme)
proteins  A.A.
But the stomach is made out of protein!
What stops the stomach from digesting itself?
mucus secreted by stomach cells protects
stomach lining
mouth
break up food
digest starch
kill germs
moisten food
stomach
kills germs
break up food
digest proteins
store food
Cardiac
sphincter
Pyloric
sphincter
Ulcers
• Used to think all
ulcers were caused
by stress
–tried to treat
with antacids
• Now know some
ulcers caused by
bacterial infection
of stomach
–H. pylori bacteria
–now treat with
antibiotics
Colonized by
H. pylori
inflammation of
stomach
Free of
H. pylori
inflammation of
esophagus
H. pylori
inflammatory
proteins
(CagA)

cytokines
neutrophil cells
cell damaging
proteins
(VacA)

helper T cells
white blood cells
Small intestine
• Functions
– digestion
• digest carbohydrates
–amylase from pancreas
• digest proteins
–trypsin & chymotrypsin from
pancreas
• digest lipids (fats)
–bile from liver & lipase from
pancreas
This is
where all the
work is done!
Small intestine
• Functions
– absorption
• nutrients move into body
cells by:
–diffusion
–active transport
This is
where all the
work is done!
Absorption in Small Intestines
• Absorption through villi & microvilli
– finger-like projections
– increases surface area for absorption
SMALL INTESTINE
6 meters long,
but can stretch
to cover a
tennis court
mouth
break up food
digest starch
kill germs
moisten food
stomach
kills germs
break up food
digest proteins
store food
small intestines
breakdown food
- proteins
- starch
- fats
absorb nutrients
Pancreas: accessory organ
• Produces digestive enzymes
– digest proteins
• trypsin, chymotrypsin
– digest starch
• amylase
– digest lipids
• lipase
• Produces buffers
– buffers neutralize
stomach acid
small
intestine
pancreas
mouth
break up food
digest starch
kill germs
moisten food
pancreas
produces enzymes to
digest all foods
stomach
kills germs
break up food
digest proteins
store food
Liver & Gall Bladder: accessory organs
• Liver produces bile
– Bile breaks down fats
– gallbladder only stores bile
• that’s why you can have your gall bladder removed
bile contains colors
from old red blood cells
collected in liver =
iron in RBC rusts &
makes feces brown
mouth
break up food
digest starch
kill germs
moisten food
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes
to
digest all foods
stomach
kills germs
break up food
digest proteins
store food
Large Intestines
• Functions to re-absorb water
• use ≈ 9 liters of water every
day in digestive juices
–if don’t reabsorb water
would die of dehydration
Large Intestines
• Function
– 90% of water re-absorbed
• not enough water re-absorbed:
–diarrhea
–can be fatal!
• too much water re-absorbed:
–constipation
• reabsorb by diffusion
You’ve got company!
• Living in the large intestine is a community of
helpful bacteria
– Escherichia coli: E. coli
• digest cellulose
–digests fruits & vegetables
• produce vitamins
–vitamin K & B vitamins
PEE-YOO!
• BUT generate gaseous (flatus) by-products of
bacterial metabolism
–Methane (CH4), hydrogen sulfide (H2S)
–STINKY! Everyone makes 1L to 2L daily
mouth
break up food
digest starch
kill germs
moisten food
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes to
digest all foods
stomach
kills germs
break up food
digest proteins
store food
small intestines
breakdown food
- proteins
- starch
- fats
absorb nutrients
large intestines
absorb water
Rectum
• Last section of large intestines
– after the colon (s-shaped holding area)
– eliminate feces through anus
– what’s left over?
• undigested materials
So don’t forget
– mainly cellulose from plants
to wash
your hands!
– called roughage or fiber
– keeps everything moving & cleans out
intestines
• masses of bacteria, mucus
Vermiform Appendix
• Remnant from ancient grass-eating ruminant ancestor
Vestigial organ
Digestive Homeostasis Disorders
• Constipation– person has uncomfortable or infrequent
bowel movements results from sluggish peristalsis that
allows excess water to be removed from feces (fecal
matter hardens)-may result from insufficient fiber in
diet
• Diarrhea– opposite of constipation– associated with
intestinal disturbances caused by infections or stress–
prolonged diarrhea may result in severe dehydration
• Gall stones– small hard particles made of cholesterol
which form & collect in gall bladder- may block the bile
duct and cause pain
• Acid reflux -backflow of stomach contents upward into
esophagus
• Appendicitis- inflammation of vermiform appendix