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The Digestive System
The Basics of Nutrition
Heterotrophs (can NOT synthesize organic nutrients)
Digestion involves:
• ingestion:
taking in food
• digestion:
breaking down the food into a form that can be transported
across the cell membrane
• egestion:
getting rid of undigested wastes
The Basics of Nutrition
Nutrient: any substance that provides energy for
growth and repair:
–
–
–
–
–
–
Carbohydrates
Proteins
Lipids
Vitamins
Minerals
Water
Food molecules must be converted into
subunits that can pass easily through cell
membranes
large molecules
end products
of digestion
1. Carbohydrates
(polysaccharides)
Monosaccharides
2. Proteins
(polypeptides)
Amino acids
3. Lipids
Fatty acids and
Glycerol
Structure of Digestive System
• Continuous tube extending from mouth to
anus
• Accessory organs attached along the way
(appendix, gall bladder, liver, pancreas)
Organs of Digestive System
1.
2.
3.
4.
5.
6.
Mouth – “oral cavity”
Pharynx – “throat”
Esophagus
Stomach
Small intestine
Large intestine – “colon”
The Mouth
• Ingestion occurs here
• Contains teeth, tongue, and salivary glands
• Both chemical and mechanical digestion occur
here.
– Chemical: salivary amylase is an enzyme in
saliva that breaks down ( Starch) carbohydrates.
– Mechanical: teeth grind food increasing surface
area for enzyme action
• saliva also lubricates the food by adding water
and mucus
a. food we swallow is called a bolus
Pharynx
•
•
•
Back of the throat
Where openings to trachea and esophagus
meet
Epiglottis: flap of connective tissue that
closes over the trachea when swallowing
Esophagus
•
•
tube that connects the oral cavity with
stomach
peristalsis: muscular contractions that
squeeze food towards the stomach
Stomach
•
•
muscular organ that mechanically digests
food
lined with cells that secrete
- mucus to protect stomach lining
- protease ( Pepsin ) to chemically digest
protein
- hydrochloric acid (HCl)
 HCl has several functions
- lowers the pH in the stomach (pepsin works
best at pH 2)
- helps to kill microorganisms ingested in the
food
The acidic chyme leaves the stomach and
enters the small intestine
• Absorption of nutrients into the
bloodstream occurs here
• VILLI - finger-like projections
lining the small intestines for
increased surface area for……
DIFFUSION of nutrients to occur
Small Intestines:
- length is approximately 9 feet: food
remains here for a long time
- villi: Increases the surface area to about
300 m2 (size of tennis court)
Most of the chemical digestion of food occurs
in the SI by the time food passes through the
duodenum – (first 25 cm of the SI)
Digested end products are taken into the villi
by active and passive transport
Accessory Organs
• Pancreas:
- produces enzymes that break down
carbohydrates,, lipids, proteins and releases
them into the SI
- produces sodium bicarbonate – neutralizes
stomach acid
- Also produces hormones that regulate blood
sugar levels (Endocrine Hormones)
pancreatic juice
contains:
proteases - digest proteins
lipases - digest lipids
amylases - digest carbohydrates
Gall Bladder
• Small, non-vital organ –
storage of bile
• Bile – emulsifies fat
• Tucked on underside of liver
Gallbladder
The gallbladder releases bile into the SI
Bile is NOT an enzyme - its function is to break
down fat globules by a emulsification
Emulsification: mechanical digestion
of fats molecules dissolved into smaller
droplets
The Liver
• Largest internal organ – opposite stomach
• Function:
– Produces bile (breaks down fats)
– Converts glucose to glycogen
– Produces urea (the main substance of urine)
– Filters harmful substances from the blood (such as
alcohol)
– Responsible for producing cholesterol
Bile is produced in the liver then stored in the
gallbladder
Bile is a mixture of 3 chemicals
1. cholesterol - excess cholesterol removed
from the blood by the liver
2. bilirubin - a yellow pigment formed from dead
red blood cells
3. bile salts - to help break up the fat structure
Appendix
• Pouch located where small and large intestines
meet
• A vestigial organ:
– Non-vital – we notice no side effects when it is
removed
– Thought to house important bacteria needed for
balance in body
– Could possibly play a role in immunity
– In evolutionary past – scientists believe that
there were 180 vestigial structures – today there
are virtually none!!!
• All indigestible waste at this point – no
nutritional value
• Known as the colon
• Regulates water concentration of fecal matter
before EGESTION through the RECTUM and
ANUS
• Function is to reabsorb most of the water that
was not picked up by the SI
• Too much or too little water can be
problematic…
A large colony of bacteria known as E. Coli
lives in the LI
They Help produce vitamin K - needed for
blood clotting
1. Erosion of the
stomach lining by
digestive enzymes
or acid due to lack
of protective
mucus.
• Large intestine is
difficult to empty
due to too much
water being
reabsorbed in the
colon – waste
hardens.
• Cardiac sphincter
stretches and stomach
acid splashes walls of
esophagus.
• Burning and pressure in
chest
• Caused by overeating,
acidic foods, or
caffeinated beverages
• Not enough water is absorbed out of
waste before egestion
• Usually caused by bacteria
• Results in watery feces
• Could cause SEVERE DEHYDRATION
• Inflammation of the
appendix
• Could be very
dangerous if it
explodes – infection
will spread in
bloodstream
throughout body
• Accumulation of
hardened cholesterol in
the gall bladder
• Hepatitis (inflammation of the liver)
• Type A , B, & C
• Cirrhosis
• Distorted or scarred liver as a result of chronic
inflammation
Question #1
The principal function of the large
intestine is to:
1.Produce salivary amylase
2.Produce bile
3.Store bile
4.Reabsorb water from feces
Question #2
Choking on food is most likely
caused by an interference with the
proper functioning of the
1. Diaphragm
2. Oral cavity
3. Esophagus
4. Epiglottis
Question # 3
What is the role of teeth in digestion?
1. They break down food mechanically,
increasing the surface area of the food for
enzyme action.
2. They break food down mechanically,
stimulating enzyme synthesis in the food.
3. They break down food chemically, aiding in
the movement of food down the esophagus.
4. They break down food chemically, mixing
saliva with the food for easier digestion.
Question # 4
In the human body, hydrochloric acid is
responsible for the low pH of the
contents of the
1. Lungs
2. Kidneys
3. Pancreas
4. Stomach
Question # 5
In humans villi that absorb small, soluble
nutrients into the bloodstream are found in
the
1. Large Intestine
2. Small Intestine
3. Esophagus
4. Stomach
Question # 6
Bile is a secretion produced by the liver,
which aids in the emulsification of
1. Lipids
2. Proteins
3. Sugars
4. Starches
Question #7
Feces is usually about 40% water and 60%
solid matter. Reducing the water
content to 20% would most likely result
in
1. Diarrhea
2. Ulcers
3. Gallstones
4. Constipation
Question # 8
Which nutrient should provide the largest
percentage of calories in a wellbalanced diet?
1. Proteins
2. Saturated fats
3. Water
4. Carbohydrates