Download Digestion Intro

6.1, H2, H3, H4
The Digestive System (Overview)
 The purpose of the digestive system is to break down
large macromolecules (carbohydrates, lipids, and
proteins from our food into smaller molecules that can
be absorbed by the body)
 Digestion – the breaking down of food into smaller
molecules usable by the cell
 Ingestion – the process of taking food into the body
 Motility- Refers to the moving and mixing of food
forward through the digestive tract
 Secretion- the production and release of chemicals
into the body.
In the digestive system, secretion includes
- digestive juices that are secreted into the system
to maintain the pH
- mucus for lubrication
- secretion of various enzymes for digestion
 Absorption – the act of taking a substance into the
cells of the body
 Egestion - the act of eliminating a substance from the
body
 Alimentation – the combined process of ingestion,
digestion and egestion
 Enzymes – biological catalysts (speed up chemical
reactions, by lowering the activation energy of a
reaction, without being used up)
- There are 3 main groups of enzymes.
Type of
Enzyme
Amylase
Ex
Salivary
amylase
Protease Pepsin
Lipase
Source
of Enzyme
Saliva – made
by salivary
glands
Site of
Release
Mouth
Gastric juice – Stomach
made by
gastric glands
Pancreatic Pancreatic
Juice – made
lipase
by pancreas
Duodenum
Reactants
(what it
breaks
down)
Products
Optimu
m pH
starch
maltose
pH 7
proteins
Smaller
protein
chains
and
amino
acids
pH 2
lipids
Glycerol pH 8
and fatty
acids
1) INGESTION
 2 major processes control ingestion:


a) nervous system (when)
b) cultural conditioning (what)
 Normally one eats as much as the body needs. This is
called the “set point” (your body’s natural tendency)
Ingestion
 3 factors involved in establishing the set point:
1) the concentration of glucose in the blood
 low glucose = hunger
 high glucose = satisfied
(hypothalamus in brain  control center)
2) the distension of the stomach (feeling of being full)
3) the amount of body fat (long term control) – not
completely understood
2) Digestion
 Refers to the breaking down of structurally complex
foodstuffs into simpler ones.
 There are two kinds of digestion possible in humans.
 Mechanical and Chemical
Digestion
Mechanical Digestion
 Physically breaking food down into smaller particles by
chewing (mouth) or churning (stomach). The purpose is
to maximize surface area upon which enzymes can work so
that digestion becomes more efficient.
Chemical Digestion
 Using secreted enzymes to break large molecules into
smaller ones. There are several places where we find
enzymes. The mouth is the first place in the digestive
process.
3) Absorption
 Refers to the movement of the structurally simple
food-stuffs from the digestive tract into the blood.
 (From the LUMEN of the intestines, to the capillaries
in the walls of the intestines)
4) Egestion
 the removal of waste food materials (unabsorbed
materials) from the body.
 Background Information:
 the digestive tract or gastrointestinal tract (GI
tract) is a simple system which moves food by
peristaltic contractions from mouth to anus
 also referred to as the Alimentary canal because it is
simply a big tube open at 2 ends (mouth and anus)
 Approximately 7-9m long (depending on age and size
of individual)
 the function of the digestive system is:
1.
to physically tear food into smaller particles
2. to chemically breakdown large molecules into
smaller molecules (done by enzymes)
3. the absorption of nutrients into the circulatory
system
 the pathway is
 Mouth Pharynx EsophagusStomach Small
intestineLarge intestine RectumAnus
 (it is like one long tube)
 along the way accessory structures add important
enzymes and molecules to help digest the food. Food
never enters the accessory glands.
Human Digestive System
Parts and Functions
A) The Mouth
 -food is taken in (ingested here) and the digestion
process begins.
1) Teeth:
 function to mechanically break down food
(mastication) into smaller pieces each tooth type
assisting in the process.
Type of tooth Incisor
Function
Number in
Adult Mouth
Canine
Premolar
Molar*
Bite or
cut food
Tear or
shred food
Grind food
Crush food
8
4
8
12 (8 molars and 4
wisdom teeth)
2) Saliva:
 chemical digestion starts with saliva (about 1.0 L/day is




produced)
has buffers that help prevent tooth decay
contains digestive enzymes secreted from three
salivary glands
salivary amylase – enzyme that catalyzes the
breakdown of polysaccharides into smaller
polysaccharides and maltose
saliva contains mucus, a protective secretion that acts
as a lubricant and aids in swallowing.
3) Tongue:
 functions to place food onto the most appropriate




teeth.
contains taste buds (papillae)
taste buds send information to the brain about the
taste of the food
bitter taste detected at back of tongue, sour at the
sides and sweet and salty near the tip
the tongue mixes food with the saliva and rolls the
food up into a ball called a bolus
4) Swallowing:
 after chewing food is placed near the back of the
tongue
 soft palate and uvula move to seal the nasal passage
 at the same time, the trachea moves upward to the
epiglottis, this action blocks the opening of the glottis
(opening of the trachea) preventing food from
entering the lungs
 the above actions are voluntary, the rest of the
digestive system is involuntary
 http://www.pennmedicine.org/encyclopedia/em_Disp
layAnimation.aspx?gcid=000126&ptid=17
B) Esophagus
 a muscular tube about 25 cm long and is about 2.5 cm wide
that conducts food from the pharynx to the stomach
 located directly behind (posterior) to the trachea
 It is collapsed when there is no food in the tube
 secretes a non-enzymatic mucous that acts like a lubricant
 liquids take about 1 sec and solids take about 3 sec to travel
through the esophagus
 peristalsis, the involuntary wave-like muscular action
(contraction) that moves food through the esophagus
esophagus is made up of three layers:
 From inside out:
1) mucosa: inside layer covered with mucous (slippery)
2)submucosa  contains blood, lymph vessels, nerves
(surrounds mucosa)
3) muscularis  2 muscular layers - circular muscle
- longitudinal muscle
 at the end of the esophagus is a ring of tightly closed
muscle called the cardiac sphincter or
gastroesophageal sphincter.
 (A sphincter is a circular muscle that contracts to close
and opening.) This sphincter prevents the stomach
acid from damaging the esophagus as it keeps the
bottom of the tube closed. (acts like a valve)
 http://www.pennmedicine.org/encyclopedia/em_Disp
layAnimation.aspx?gcid=000097&ptid=17
C) Stomach
 located on the left side of the abdominal cavity, below
the diaphragm
 food enters at the cardiac sphincter
 the stomach is a muscular sac that can expand in size
to accommodate approx. 2 L of food
 The stomach has 4 layers with special structures and
functions
 1st (inner) layer of the stomach: the mucosa, has many
folds called ruggae
 in between these folds are gastric pits and gastric glands
which produce gastric juice
 2nd layer: submucosa, contains nerves and blood vessels
 When food enters the stomach, nerve cells detect the food
and initiate release of the hormone gastrin into the blood
stream.
 Gastrin acts on the gastric glands, telling them to release
gastric juice.
 3rd layer: muscularis, is a layer of muscles that
contract frequently, churning and mixing the food
with gastric juices to produce a semi-solid mixture
called chyme
 Has three muscle layers (circular, longitudinal,
oblique)
 4th layer: serosa, holds the stomach in place and
secretes a lubricating fluid that eliminates friction
between organs.
Gastric Juice
 about 3L per day is secreted
 the secretion known as gastric juice contains a variety
of substances that aid in the digestion of food
including enzymes that help in the breakdown of food
 contents of gastric juice are: hydrochloric acid, pepsin,
lipase
Gastric Juice
 Hydrochloric acid
 loosens tough fibrous material
 kills bacteria that enters stomach with food
 Pepsin
 breaks down proteins into amino acids and polypeptides
 Actually made as pepsinogen, an inactive precursor, and
activated in the presence of HCl.
 If made active, it would destroy the cells that make it!
 Lipase
 breaks down triglycerides into fatty acids and glycerol
 Mucus secreted by the epithelial cells of the inner stomach
protect the stomach lining from the hydrochloric acid.
 (The enzymes and HCl are quite strong, so the cells of the
mucosa divide rapidly to heal damage – the stomach lining
is replaced ~2-3 every days!)
 the acidic chyme passes through the pyloric sphincter (a
ring of tightly closed muscle) out of the stomach and into
the first part of the small intestine
 The stomach is a holding sac that only lets out a little bit of
chyme into the small intestine at a time.
D) Small Intestine
 The site of most digestion and absorption of nutrients
 a long, coiled tube 6-7 m long in humans
 approximately 2.5 cm wide
 attached by a thin membrane called the mesentery to
the rear wall of the abdomen
 consists of three parts: duodenum, jejunum, and
ileum
Duodenum
 the first 25 cm
 receives acidic chyme from the stomach and continues
digestion with the help of enzymes
 chyme is mixed with secretions from the pancreas
(pancreatic juice) liver, gall bladder, via the common
bile duct as well as glands cells from duodenal wall
Jejunum
 about 3m long
 contain many more intestinal folds and glands
 enzymes produced here remain attached to the intestinal wall
and the peristaltic contractions bring the chyme in contact
with the enzymes
 function of jejunum is digestion and absorption of nutrients
 nutrient absorption is enhanced by the increased surface area
How Surface Area of the Small
Intestine is Increased
1)
2)
folds – increase S.A. 3X
villus – increase S.A. 10X
 villi are tiny finger-like projections along the internal surface of the
intestine
 villi contain microvilli
 function of villi is to absorb small nutrient molecules back into
circulatory via capillaries and lymphatic system via lacteals preventing
them from being excreted from the body
 (lacteals absorb and transport fat)
3) microvillus – increases S.A. 20X
(Total 600X)
Ileum
 about 4m long
 has fewer digestive enzymes than jejunum as well as
smaller and fewer villi
 main function is absorption of nutrients
 once the nutrients have been removed from the food
mass the term feces is applied.
 peristaltic contractions move the digested food along
the small intestine pushing the feces through the
ileocaecal sphincter into the large intestine
E) The Large Intestine
 a tube that is 1.5 m in length and is approximately 7.6
cm in diameter
 U shaped (caecum, ascending, transverse,
descending, sigmoid colon, rectum, anus)
 receives secretions from the ileum via the ileocaecal
sphincter (valve)
 the feces first enters a sac-like pouch called the
caecum. (In herbivores the caecum is used to
breakdown cellulose but it has lost that function in us)
 the appendix, a finger-like tube extends from the
caecum.
 It is a vestigial organ (it has no known function) but
food can get trapped there causing bacteria to lodge in
the sac.
 The bacteria release toxins that cause the appendix to
swell causing appendicitis.
 If the appendix bursts and releases the bacteria into
the abdominal cavity it is much more serious and is
called peritonitis.
 feces enters the caecum and rhythmic contractions
move the feces up the ascending colon, across the
transverse colon, down the descending colon, to the
sigmoid colon and it is then pushed into the rectum
 large intestine has 3 major functions:
1) reabsorption of water
 20L of fluid (from food, saliva, and other digestive
juices) pass through the l. intestines daily, most of it is
reabsorbed
2) vitamin production – vitamin K and some B
vitamins made by harmless bacteria that reside in
the intestines
 Another by-product of bacterial action is gas (a
mixture of methane and carbon dioxide and hydrogen
sulfide). Most of this gas is reabsorbed in the intestine
but some may escape through the anus as flatulence.
3) Egestion: formation and expulsion of feces (stool)
which includes the undigested and indigestible
materials such as cellulose
Egestion
 feces is stored until the rectum sends out a nerve
impulse telling the brain it is full (stretching of the
walls of the rectum signals elimination) The brain
coordinates the information and you feel the need for
a bowel movement.
 elimination is controlled by sphincter muscles in the
anus (anal sphincter)
Animations
 http://www.pennmedicine.org/encyclopedia/em_Disp
layAnimation.aspx?gcid=000041&ptid=17
 http://highered.mcgraw-
hill.com/sites/0072507470/student_view0/chapter24/a
nimation__organs_of_digestion.html