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The Digestive System
Major Function
 To break down food into nutrients that can be used
by the cells of our body.
Types of Nutrients
Simple Molecules
(can be absorbed directly by cells)
Complex Molecules
(must be broken down before
being absorbed)
Vitamins
Carbohydrates
Minerals
Proteins
Water
Fats
Some foods such as bread and
meat usually contain several of
these nutrients
Carbohydrates
 Substances that are made mostly of sugars OR
release sugars during digestion (starch)
Foods that contain sugars
Foods that contain starch
Fruits
Pasta
Vegetables
Bread
Honey
Juice
Candy Bars
Corn
Tubers (Potatoes, Beets, and
Turnips)
Legumes (peas and beans)
These are ALL examples of carbohydrates
The Role of Carbohydrates
 They are a short-term energy source for cells.
 They provide energy that can be used quickly
 Ex. Gatorade
 Ex. Carbo-loading (video)
The Carb that’s not really a Carb
 Fiber
 Although it is a complex sugar, our body cannot
digest it and so it isn’t a source of energy
 BUT it still has an important role!
 Helping us to go to the bathroom by absorbing water
and stimulating contractions of the intestines which help
to eliminate stool
Fats
 Can come from plants or animals
 Examples: oils, butter, margarine, cream, nuts,
cheese, meat, chips
 Also a source of energy, however this energy is
usually stored in adipose tissue until needed.
 Not the first source of energy that we use (which are
carbs)
Proteins
 Large molecules of amino acids
 Found mainly in:
 meat, poultry, fish, meat substitutes
 Meat substitutes
 Eggs, nuts, legumes, tofu
Role of Proteins
 They allow our bodies to build and repair tissues
(muscle, bone, and skin)
 They can also provide energy if the body has a
shortage of carbohydrates and fats.
 What about Gluten? A protein!
Vitamins
 Help to regulate metabolism (the chemical reactions
in our bodies)
 13 types
 Found in many different foods
 Fruits, vegetables (especially dark green and orange
ones), eggs, meat, milk, grains, nuts
 We only need a small amount of each
 That’s why it’s important to eat a variety of foods
Some Examples of
Vitamins
 Vitamin C:
 Gums and teeth
 Vitamin A:
 Night vision
 Vitamin D:
 bones
 Vitamin K:
 Blood clotting
Minerals
 Simple chemical elements (what are their symbols?)





Calcium
Potassium
Sodium
Phosphorus
Iron
 We only need a small amount
 Makeup various tissues (cartilage, bone and teeth)
 Also play a role in regulating metabolism
Water
 Makes up about 60% of the body
 Almost everything we eat contains water
 Our bodies need 2 -3 L a day
 Important to many chemical reactions
 Regulates metabolism
 Transports various substances (ex in the blood)
Dietition
 Advising people on their food choices
 Planning meals for people with various health
conditions
 Monitoring the quality of new food products
 You need a physiology and psychology background
 DEC in health Sciences
 Bachelor’s degree in dietetics and nutrition
Energy Value of Nutrients
 How much energy that nutrient supplies to the
body
 Measured in kilojoules (kJ)
 1 gram of fat provides 37 kJ of energy
 1 gram of protein or carb provides 17kJ of energy
 A calorie is equal to 4.18 kJ
Energy Needs
 Everyone has their own unique energy
requirements
 Adolescents between the ages of 13 and 15 need an
average of 10 500 kJ of energy each day
(approximately 2100 calories)
 To determine this we add up the person’s energy
requirements for their:
 Basal metabolism (vital functions of the body)
 Men usually need more energy for this than women
 Physical activities
 Digestion and Absorption of Food
How to maintain your
weight
 The number of calories you use must be equal to the
number of calories you consume.
Anatomy of the
Digestive System
 Between the time it enters and exits the digestive
system, food undergoes mechanical and chemical
transformations.
 The various organs of the digestive system are
responsible for the transformations that occur.
 These organs are divided into two groups
 Digestive tract
 Digestive glands
Pharynx
See page 75
of textbook
Digestive Tract
 Mouth
 Pharynx
 Esophagus
 Stomach
 Small Intestine
 Large intestine (colon)
Mouth
 Where food enters the body
 Includes the tongue and teeth
 Enclosed by the lips cheeks and palate
 The uvula is located at the back of the mouth. It is a
small muscle that lifts to close the passage to the
nasal cavities when we swallow
Mouth
uvula
Pharynx
 Both part of the digestive and respiratory systems
Esophagus
 An elastic muscular tube that runs along the
vertebral column behind the trachea
 It connects the pharynx to the stomach
Stomach
 Muscular J-shaped pocket
 Left side of the abdomen
 At the entrance of the stomach is a muscle called the
cardiac sphincter which prevents food from
travelling back up the esophagus
 At the exit of the stomach is the pyloric sphincter
which controls the entry of food into the small
intestine.
Stomach
Small Intestine
 Approximately 3 m long
 Divided into 3 sections
 Duodenum
 Jejunum
 ileum
 Folded several times
 Attaches to the large intestine on the right side of
the abdomen
Small Intestine
Small Intestine
 Its inner surface is lined with microscopic folds
called intestinal villi
 These folds increase the contact surface between the
small intestine and the food that passes through it
 The surface area of the intestinal mucous membrane
is about 200 squared meters, which is approximately
the size of a tennis court!
 See p. 77 of textbook
Large intestine
 Also known as the colon
 Bumpy muscular tube
 No villi
 Contains mucus secreting cells to help the passage
of solid residue
Large intestine
 Contains several sections



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Caecum
The ascending colon
The transverse colon
The descending colon
The rectum
 Ends with a circular muscle called the anus
 Appendix projects from the caecum
Large Intestine
Digestive Glands
These glands secrete substances that
aid in the digestion of food
2 Types of Glands
 Accessory Glands
 Located outside of the digestive tract
 Connected to the tract by small ducts
 Ex. salivary glands, liver, pancreas
 Integrated Digestive Glands
 Microscopic
 Located directly in the walls of the digestive tract
 Ex. gastric glands, intestinal glands
Salivary Glands
 3 pairs of them
 Parotid
 Sublingual
 Submaxillary
 They secrete 1 to 1.5 L of saliva a day
 They play a role in the digestion of carbohydrates
 They also have antibacterial properties
Liver
 Located on the right side of the abdomen
 Largest gland in the human body
 Produces 1 L of bile each day
 Some of this bile is stored in the gall bladder which
is right beneath the liver
 During digestion, the gall bladder releases bile into
the duodenum
Pancreas
 Located on the left side of the abdomen beneath the
stomach
 Leaf-shaped gland
 Secretes about 1.3 L of pancreatic juices each day
 The pancreatic juices are emptied into the duodenum
through a duct
 The pancreas also secretes insulin which helps to
metabolize sugar
*type 1 vs. type 2 diabetes video
Gastric Glands
 Scattered throughout the surface of the stomach
 About 35 million of them
 Each day they secrete between 2 and 3 L of gastric
juice
Intestinal Glands
 Located at the bottom of the villi that line the inner
surface of the small intestine
 There are approx. 50 million of these glands
 Each day they secrete up to 2 L of intestinal juices
into the small intestine
The Physiology of the
Digestive System
 Show video
The Food that you Eat Undergoes
Physical and Chemical Changes
 Physical Changes
 The nature of the substance remains the same, only its
physical appearance changes
 Chemical Changes
 The bonds of the molecules are broken, which gives
rise to new substances
Mouth
 Physical/Mechanical Digestion
 The action of teeth that tear and grind our food into
smaller fragments (chewing)
 The action of the tongue that moves the food around
the mouth into a soft moist ball called the BOLUS.
 The action of our saliva moistening our food which
aids in turning our dry food into a bolus
Mouth
 Chemical Digestion
 Saliva contains an enzyme called amylase which is
released by the salivary glands
 Amylase begins the digestion of starches (long chains
of glucose) into smaller chains.
 However the continued breakdown of the smaller
chains of glucose occurs later on
Pharynx
 Physical/Mechanical Digestion
 The act of swallowing (deglutination) takes place in
the pharynx.
 The tongue voluntarily pushes the bolus towards the
back of the mouth.
 Then everything else happens by reflex.
 The uvula rises and closes the nasal cavity
 The epiglottis lowers to block the trachea
 Dysphagia is when someone has a problem
swallowing
Esophagus
 Physical/Mechanical Changes
 Food moves by muscular contractions called
peristalsis
 Peristalsis is a rhythmic contraction of muscles
surrounding the digestive tract
 Ex. tube of toothpaste
Stomach
 Physical/Mechanical Changes
 The muscles in the stomach’s wall contract, thus
resulting in the movement of food, which allows it to
mix with gastric juices.
 This mixing is called churning.
Stomach
 Physical/Mechanical Changes (continued)
 Gastric juices, containing HCl, unravel undigested
proteins and prepare them for digestion
 After of few hours of churning, the food becomes a
liquid called chyme
 The chyme then travels to the duodenum by
peristaltic movements of the stomach.
Stomach
 Chemical Changes
 The gastric juices also contain the enzyme pepsinogen.
 Pepsinogen reacts with HCl to form the proteindigesting agent PEPSIN.
 Pepsin breaks down long polypeptide chains into
short chains of amino acids.
Stomach
 The stomach’s protective mechanism
 The cells of the stomach secrete mucin (mucous)
which coats the walls of the stomach to protect it from
its own digestive enzymes.
Small Intestine
 Physical/Mechanical Changes
 Fat does not dissolve in water and so travels through
the digestive tract in the form of large droplets
 In order to facilitate the chemical digestion of fats,
those large droplets need to be broken down into
smaller droplets before being digested.
Small Intestine
 Physical/Mechanical Changes
 Bile, that was made in the liver and stored in the
gallbladder, is released into the small intestine.
 This bile breaks down the fat droplets into even
smaller droplets (EMULSIFICATION).
Small Intestine
 Physical/Mechanical Changes
 Food is churned with digestive juices to facilitate
absorption
 Peristalsis moves the food through the small intestine
Small Intestine
 Chemical Changes
 Pancreatic juices that have entered the small intestine
complete the digestion of carbs, proteins, and fats.
 Pancreatic amylase completes the digestion of carbs
 Pancreatic lipase completes the digestion of fats
 Trypsin and peptidase complete the digestion of
proteins
Small Intestine
 Absorption of Nutrients
 Inside the small intestine there are small, finger-like
projections called VILLI which have even smaller
projections called MICROVILLI
 Each VILLI has blood vessels and lymphatic vessels
 Both function to increase the surface area to maximize
the nutrients that can pass into the blood and lymph
(fats)
 Video on small intestine
Large Intestine
 Physical/Mechanical Changes
 Churning continues to help facilitate the absorption of
nutrients
 Food continues to move through the digestive tract
via peristalsis
Large Intestine
 Absorption of nutrients
 Water is reabsorbed to facilitate the formation of stool
Where are nutrients
absorbed?
Small Intestine
Large Intestine
Glucose
Water
Amino Acids
Vitamins
Fatty Acids
Minerals
Glycerol
Water
Vitamins
Minerals
Rectum and Anus
 Feces is stored in the rectum
 Feces is eliminated through defecation via the anus
 Hemorrhoids are enlarged rectal veins near the anal
opening that can make the passage of feces difficult
and painful
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