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Digestion. Why Do We Need to Digest?. Nutrients from food provides us with the energy and materials we need for work, growth and repair The problem is that most of the nutrients we need cannot be used in the form they are eaten

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why do we need to digest
Why Do We Need to Digest?

Nutrients from food provides us with the energy and materials we need for work, growth and repair

The problem is that most of the nutrients we need cannot be used in the form they are eaten

Nutrients need to be broken up into their smallest components in order to diffuse into our blood stream

what needs to be broken up
What needs to be broken up?
  • Carbohydrates  simple sugars
  • Proteins  amino acids
  • Fats  Glycerol and fatty acids
  • Enzymes
  • Type of protein that breaks food molecules into smaller units.
the process of digestion
The Process of Digestion

Divided into 3 stages:

  • Physical Digestion
    • Taking food in – Cutting and chewing food
  • Chemical Digestion
    • Enzymes break down food even further
  • Absorption
    • Wastes are excreted

Physical digestion starts starch digestion


Rapid passage of food to stomach

  • Digestion of proteins


  • Digestion of proteins

Production of many enzymes which digest all types of food

Small Intestine

Production of more enzymes and absorption of most end products

large intestine
Large Intestine

Reabsorption of water


Temporary storage of undigested waste


Removal of undigested wastes

where does digestion begin
Where does digestion begin?

On your plate with a fork and knife

Your saliva initiates chemical digestion with an enzyme that breaks carbohydrates down into simple sugars

Your teeth and tongue break up food into smaller pieces and grind it up

physical digestion teeth
Physical Digestion - Teeth

Adult humans have 4 different kinds of teeth (32 total)

  • Incisors at the very front (4 on top, 4 on bottom)
  • Canines (cuspids) beside the incisors and are pointed (4 total)
        • Used for tearing or shredding
  • Premolars (8 total)
  • Molars (12 total incl. wisdom teeth)

Premolars + molars – flattened on upper surface, used for grinding and chewing tough food

parts of the teeth
Parts of the Teeth

Crown - visible part above the gum

Root – Part below the gum line, holds tooth in place

Enamel – protective coating – hardest substance in the body, cannot be replaced.

parts of the teeth16
Parts of the Teeth

Dentine – hard bone that gives teeth shape and strength. It is sensitive to temperature, sugar, touch, acids, etc.

Pulp Chamber – message center for sensation in the dentine

Gum line – help hold and protect teeth

parts of the teeth17
Parts of the Teeth

Root Canal – carries blood and nerve endings

Cementum – connect tooth to the jaw bone

Carnivores – Sharp teeth for grabbing food and ripping it apart

Herbivores – Flat teeth for grinding food

Omnivores – Flat and sharp teeth

what causes tooth decay
What causes tooth decay?
  • Bacteria called plaque live in your mouth
  • Plaque eats any food that stays on your teeth
  • Produce acid that eats away the enamel, creating pits called cavities
  • Build up of a hard crust called tartar

NOTE: Sugar does not cause tooth decay, it feeds the bacteria that do

how to maintain healthy teeth

How to maintain healthy teeth

Brushing and flossing removes plaque and keeps gums healthy

Wear mouth guards in sports

Regular dental checkups (clean away tartar)

Diet: calcium, phosphorus, vitamins A, C & D all help maintain teeth and gums

some dental problems
Some Dental Problems
  • Sensitivity due to the removal of enamel by cavities and exposing dentine
  • Abscesses – bacteria get into the root and infect it
  • Peridontal Disease – Affect the tissue around the teeth. Includes retreating gum line, sore gums or bleeding. Usually caused by poor diet or hygiene.
  • Halitosis – bad breath caused by smoking, infections, tooth decay, sinus infections, etc.
begins in the mouth
Begins In the Mouth


  • 1st enzyme to act.
  • Produced by saliva
  • Helps break down starch into sugar molecules.
  • Produced by 3 pairs of glands inside the mouth
  • Is slightly acidic
  • Approx 1000 mL produced per day!
  • 99% is water
function of saliva
Function of Saliva
  • Moistens dry food
  • Binds the loose crumbs together so bits do not get into the respiratory system
  • Softens food so rough edges will not scratch the walls
  • Enzyme amylase begins chemical digestion
structures in the mouth
Structures in the Mouth

Hard Palate

Soft Palate


the tongue
The Tongue
  • Attached to the floor of the mouth
  • Helps to move food to the molars
  • Mixes food with saliva
the tongue30
The Tongue

Once the food is moist and soft, the tongue rolls it into a ball called a bolus.

This prepares the food to pass into the pharynx to be swallowed.


The tongue moves the bolus of swallowed food to the back of the mouth

The soft palate moves upward to partially seal off the nasal passage

At the same time, the epiglottis closes the opening into the respiratory passage

amylase in action
Amylase in Action
  • Place cracker in mouth
  • Note the taste
  • Leave on tongue until you notice a taste change (can take up to 5 min)
  • What do you taste?
the sense of taste
The Sense of Taste

Humans detect taste with taste receptor cells

These are clustered into taste buds

Taste buds are clustered into bumps called papillae

There are 5 primary taste sensations

  • salty
  • sour
  • sweet
  • bitter
  • Umami (Savory)
muscular contractions
Muscular Contractions

The movement of food from the tongue down into the pharynx is under voluntary control

The second stage, involving the epiglottis and the movement of the food into the esophagus, is involuntary

the esophagus
The Esophagus
  • Flexible tube – approx. 25 cm long
  • Leads from the pharynx to the stomach
  • Walls have 2 layers of muscle
  • The inner lining covered with mucus – helps food pass through easily

Bolus is moved through peristaltic action

Peristalsis - the rhythmic contractions of muscles – like squeezing a tube of tooth paste



  • Ring of muscle that controls thepassage of bolus into stomach

– like pulling a drawstring

the stomach
The Stomach
  • Large muscular bag that stretches as it fills with food
  • Can hold 1.5 L
  • Made up of many layers, including 3 layers of muscle

Gastric glands produce gastric juice.

    • Pepsin (an enzyme) – Breaks down protein
    • Hydrochloric Acid
  • The muscular walls of the stomach contract to mix food with gastric juice, producing a mixture called chyme.
hydrochloric acid hcl
Hydrochloric Acid (HCL)
  • Lowers the pH of the stomach to allow enzymes to work efficiently
  • Helps kill bacteria
  • Mucus secretions protect the stomach walls from HCl
  • BUT sometime mucus is not enough and HCL and digestive enzymes eat away at the lining, resulting in a peptic ulcer.
heart burn
Heart Burn

Presence of stomach acid in the esophagus

Cardiac sphincter is not working properly – opens allowing acidic stomach contents into the esophagus




The pyloric sphincter is located at the lower end of the stomach

  • Controls the flow of partially digested food (Chyme) out of the stomach
the small intestine
The Small Intestine
  • 2.5 cm in diameter, and is about 7m in length


Small Intestine


small intestine
Small Intestine
  • A long coiled and looped tube
  • Fills most of the abdomen
  • Held in place by a membrane called mesentery
4 functions of the small intestine
4 functions of the Small Intestine
  • Keeps the food moving by peristalsis
  • Secretes enzymes which continue the digestive process
  • The site where digestion by chemicals from the pancreas and liver take place
  • Nutrients are absorbed into the bloodstream
digestive processes
Digestive processes…

3 basic food substances in the small intestine:

  • Carbohydrates
  • Fats
  • Proteins

They are broken down into molecules that are small enough to pass through the wall and enter the circulatory system

the pancreas
The Pancreas
  • Produce pancreatic juice – which contain about 28 enzymes and sodium bicarbonate
    • Lipase – breaks down fat
    • Amylase – completes carbohydrate digestion
    • Trypsin and peptidase – complete protein digestion
  • Sodium bicarbonate neutralizes the acidic chyme as it comes out of the stomach
great test question

Great Test Question!

How are our bodies built to handle the acid in our stomach?

the liver
The Liver
  • Produces bile that is stored in the gall bladder
  • Bile emulsifies fat – which means that it helps fat dissolve in water so it can be digested and absorbed
absorption of nutrients
Absorption of Nutrients
  • Surface of the small intestine is folded
  • Covering the surface are projections called villi


  • Increase surface area for absorption
  • Collect the nutrients and transport them to where they are needed in the body
what happens next
What Happens Next?

Reabsorption and Elimination: The Large Intestine

the large intestine consists of
The Large Intestine consists of:
  • Cecum
  • Appendix
  • Colon
  • Rectum
the large intestine
The Large Intestine
  • Approx. 1.5 m in length
  • 7.6 cm in diameter
  • Functions mainly to reabsorb water

Cecum – Where the small intestine empties into the large intestine

  • Appendix
    • Believed to have no function
    • Can become infected –called appendicitis
large intestine function
Large Intestine function
  • Reabsorbs water and maintains the fluid balance of the body
  • Absorbs certain vitamins
  • Undigested food is dried into suitable consistency for defecation
  • Stores waste before it is eliminated
the rectum
The Rectum
  • Last section of the digestive tract
  • Ends with the anal sphincter (like a drawstring)
  • When full there is a mild feeling of discomfort, which tells us that the feces is ready to be eliminated
what your feces can tell you
What Your Feces Can tell you…
  • Feces are 75% water and 25% solids
  • Diet lacking fiber = drier, compacted feces which can result in constipation
  • Sufficient fiber = Holds more water and is much softer which allows it to pass through easily
quick recap
Quick Recap…

Small Intestine:

  • Breaks down food and absorbs nutrients
  • Villi increase’s surface area = more absorption
  • Moves the rest to the large intestine (through peristalsis)
quick recap66
Quick Recap

Large Intestine:

  • Removes water
  • Moves undigested food (peristalsis) to be released as waste