Transcript DIGESTIVE SYSTEM

DIGESTIVE SYSTEM
• Gastroenterology - the study of
the digestive sytem
Structures of Digestive System
• 1. Gastrointestinal tract (GI)
▫ Also called the alimentary canal
▫ Long continuous tube from the mouth to
the anus (~ 9 meters long)
▫ Includes mouth, pharynx, esophagus,
stomach, small intestine, large intestine,
rectum and anus
• 2. Accessory Organs
▫ Teeth, tongue, salivary glands, liver, gall
bladder, pancreas (most do not come in
direct contact with the food)
Functions of Digestive System
• 1. Ingestion - eating/drinking
• 2. Secretion - ~ 7L of H2O, acid,
buffers, enzymes per day
• 3. Mixing/Propulsion - alternate
contraction & relaxation of
smooth muscle lining the GI tract
▫ Mixes food & secretions, propels
them toward the anus
• 4. Digestion
▫ Mechanical - begins in the mouth
with teeth chewing & grinding food
▫ - continues in the stomach with
churning & mixing food with
digestive enzymes
▫ Chemical – carbohydrates, proteins
and lipids are broken down
▫ - digestive enzymes speed up this
process
• 5. Absorption – fluids and
molecules from chemical
digestion pass through the
lining of the small intestine into
the bloodstream
• 6. Defecation - elimination of
waste material (feces…more
commonly known as “poop”) 
Walls of the Digestive Tract
• 1. mucosa – innermost mucus
membrane layer
• 2. submucosa – surrounds mucosa
• 3. muscularis - outside the
submucosa (2 layers of smooth
muscle)
• 4. serosa – below the diaphragm
(secretes serous fluid for
lubrication)
Upper GI Tract
•Includes:
▫ mouth (salivary glands, tongue,
teeth, hard & soft palate)
▫ Pharynx
▫ Esophagus
▫ Stomach
Salivary Glands
• 3 main glands that produce and
secrete saliva
▫ 1. parotid gland – largest;
anterior & inferior to the ear
▫ 2. sublingual gland – under the
tongue
▫ 3. submandibular gland – floor of
the mouth
Composition of Saliva
• 99.5% H2O
• 0.5% solutes, including Na+, K+, Cl-, HCO3-,
PO4-3, bacteria inhibitors, enzymes and
mucus
• Salivary amylase – breaks down starches
• Lingual lipase – breaks down lipids
• HCO3-, PO4-3 act as buffers to keep pH
between 6.35-6.85
• Mucus – acts as lubrication for food
Upper GI Tract Secretions
Substance
Secretion
Saliva
Classification
Secreted
By
Secrete
d Into
Acts On
Function
Salivary Enzyme Salivary
Amylase
glands
Mouth
starch
Breaks
down
starch to
polysaccharides
Lingual
Lipase
Enzyme Salivary
glands
Mouth
Lipids (in Breaks
stomach) down
lipids
Mucus
~~~~~~ Salivary
glands
mouth
Food
Moistens
food
Functions of Saliva
• 1. cleansing the mouth
•2.
•3.
•4.
•5.
solvent for taste
lubricate food
breakdown starches
inhibits bacterial growth
Salivation
• Controlled by the nervous system
• Touch, taste, smell, sight & sound of
food cause increases in salivation
• Normal conditions – moistens mouth &
esophagus and cleanses teeth
• stress – secretions stop (mouth
gets dry)
• Eating – secretions increase
Function of the Tongue in
Digestion
• Move food in the mouth and to
back of mouth for swallowing
(degluttition)
• Contains taste buds – found on
the papillae (bumps) of the
tongue
• Gustation - taste
Mechanical Digestion in the
Mouth
• Mastication – chewing
▫ Food is manipulated by the
tongue, ground by the teeth and
mixed with saliva to form a
bolus (chewed up, soft mass)
Chemical Digestion in the Mouth
• Salivary amylase – breaks down
polysaccharides (starch) into maltose
▫ Continues in the stomach ~ 1 hour
after being swallowed
• Lingual lipase – breaks down
(triglycerides) lipids into fatty acids
▫ Becomes activated in the acidic
stomach environment, so happens
after food is swallowed
Teeth
• Two sets
▫ Deciduous (baby teeth) – 20
▫ Permanent (including wisdom) -32
• 3 types of teeth
▫ Incisors - sharp edges
(biting,cutting)
▫ Cuspids – pointed (grasp, tear)
▫ Molars - flat (grinding)
Parts of teeth
• Crown - visible portion of tooth
• Enamel - hardest substance in the body;
surrounds dentin in the crown
• Root -portion that is in the sockets of
mandible and maxilla
• Pulp - central core of tooth; blood vessels
& nerves
• Dentin – surrounds pulp cavity; a living
cellular substance similar to bone
• Neck –region between crown and root
• Root canal - pulp cavity
Other parts of the Mouth
• Hard palate - roof of the mouth
• Soft palate - skeletal muscle &
connective tissue; ends in the
uvula
• Uvula - moves food away from
the nasal cavity & into the oral
pharynx
Pharynx
• 3 parts of pharynx
▫ Nasopharynx – upper region;
contains tonsils & adenoids;
functions in respiration
▫ Oropharynx – behind oral cavity;
first region where food enters
when swallowed
▫ Laryngopharynx - behind larynx
Stages of Swallowing
• 1. Voluntary Stage – bolus to
back of mouth and into
oropharynx
• 2. Pharyngeal Stage – bolus
from oropharynx into esophagus
 Involuntary
 Airway closes, breathing is interrupted
 Soft palate & uvula close off
nasopharynx
 Epiglottis blocks respiratory tract
• 3. Esophageal Stage - bolus goes
from esophagus into stomach
▫ Peristalsis - rhythmic contractions
that move the bolus along the
digestive tract
▫ When bolus is at the end of the
esophagus, the cardiac sphincter
relaxes and food enters the
stomach
Functions of the Stomach
• 1. mix saliva, food & gastric juices
to form chyme (semisolid mixture)
• 2. hold food until it moves into the
small intestine
• 3. secretes gastric juice into
stomach
• 4. secretes gastrin into blood
Mechanical Digestion in the
Stomach
• Food enters stomach through a
valve called the gastroesophageal
sphincter, which keeps food from
going back into the esophagus
• Mixing waves in the body of the
stomach & the pylorus move the
chyme into the duodenum
Chemical Digestion in the Stomach
• Gastric Juice consists of:
▫ 1. Mucus – protects stomach lining
▫ 2. Pepsinogen - breaks down proteins
▫ 3. Gastric lipase – breaks down fats
▫ 4. HCl – kills bacteria; stimulates
secretion of gastrin (a hormone)
▫ 5. Intrinsic factor - necessary to
absorb Vitamin B12 in the small
intestine
Upper GI Tract Secretions
Substance
Secretion Classification
Secreted
By
Secreted
Into
Acts On
Function
Gastric
Juice
Pepsin
(Pepsinog
en)
Enzyme
Gastric
Glands
stomach
Proteins
Breaks
proteins
into
peptides
Gastric
Lipase
Enzyme
Gastric
Glands
Stomach
Lipids
Breaks
down
lipids
HCl
Acid
Gastric
Glands
Stomach
Contents
of
stomach
Decreases
pH; kills
bacteria
Mucus
~~~~~~
Gastric
Glands
Stomach
Walls of
stomach
Protects
from acid
Gastrin
Hormone
Gastric
Glands
Blood
Gastric
Stimulate
glands;
s gastric
sphincters glands &
cardiac
sphincter;
3 Phases of Stomach Digestion
• 1. Cephalic Phase
▫ Controlled by nerve stimuli
▫ Sight, smell, sound or taste of food
causes increased nerve impulses
▫ Stomach secretes pepsinogen, HCl
and mucus into the chyme
▫ Gastrin is secreted into the
bloodstream
▫ Stomach churning increases
• 2. Gastric Phase
▫ Stomach stretches to accommodate food
▫ pH in stomach increases
▫ Gastric juices are secreted
▫ Gastrin – hormone secreted by the stomach
 Stimulates gastric glands
 Strengthens the contraction of the
cardiac sphincter
 Relaxes pyloric & ileocecal sphincters
▫ pH below 2 - secretions inhibited
▫ pH above 2 – secretions stimulated
• 3. Intestinal Phase
▫ Activated by the small intestine
▫ Inhibitory effect on the stomach
▫ Slowing of stomach emptying
▫ 2 hormones secreted
 Secretin – decreases gastric secretions
 CCK (cholecystokinin) - inhibits stomach
emptying
▫ Stomach empties in 2-4 hours…first
carbs, then proteins, lastly fats
Lower GI Tract
•Duodenum through Anus
Small Intestine
• Duodenum (approx 10 inches)
▫ Recieves chyme from the stomach
▫ Pancreatic juice & bile enter here
• Jejunum (approx 3 feet)
▫ Begins where duodenum turns downward
• Ileum (approx 6 feet)
▫ Ends at ileocecal sphincter, which prevents
feces from backing into small intestine
mesentary – thin tissue that prevents small
intestines from twisting or tangling
Functions of Small Intestine
1. Digestion
2. Absorption
-food remains in intestine for 3-5
hours
-Intestinal glands secrete intestinal
juice, which contains enzymes:
- sucrase, lactase, maltase,
peptidase & enterokinase
Digestive Secretions – Lower GI
Substance
Secretion Classification
Secreted
By
Secreted
Into
Acts On
Function
Intestinal
Juice
Sucrase
Enzyme
Small
Intestine
Small
Intestine
Sucrose
(breaks
into - )
Glucose &
fructose
Intestinal
Juice
Lactase
Enzyme
Small
Intestine
Small
Intestine
Lactose
(breaks
into-)
Glucose &
galactose
Intestinal
Juice
Maltase
Enzyme
Small
Intestine
Small
Intestine
Maltose
(breaks
into -)
2 or 3
molecules
of glucose
Intestinal
Juice
Peptidase Enzyme
Small
Intestine
Small
Intestine
Peptides
(breaks
into -)
Amino
acids
Intestinal
Juice
Enterokinase
Small
Intestine
Small
Intestine
Trypsinogen
(breaks
trypsin
Enzyme
Characteristics of Small Intestine that
increase surface area for absorption
• 1. Length - ~ 10 feet long
• 2. Villi – finger-like folds in mucosa
• 3. Microvilli – hair-like projections in villi
• 4. Circular folds – causes chyme to “spiral”
instead of moving straight through
- 90% of absorption occurs in small intestine
- nutrients, electrolytes, water, Vitamins A, B, C,
D, E and K are absorbed (Vitamin B12 must be
bound to intrinsic factor in order to be
absorbed)
Hormones of the Small Intestine
• These secretions act to stimulate
the pancreas
▫ 1. Secretin – tells the pancreas to
secrete sodium bicarbonate ions to
increase pH
▫ 2. CCK (Cholecystokinen) – tells
the pancreas to secrete digestive
enzymes & gall bladder to secrete
bile
Digestive Secretions – Lower GI
Substance
Secretion
Classification
Secrete
d by
Secreted
Into
Acts On
Function
Intestinal
Juice
Secretin
Hormone Small
Circulatory Stomach
intestine system
&
(blood)
pancreas
Inhibit
gastric
secretions,
stimulate
pancreatic
buffers
Intestinal
Juice
CholeHormone Small
Circulatory Stomach
cystokinin
intestine system
pancreas
(CCK)
(blood)
& gall
bladder
Inhibit
stomach
emptying,
stimulate
pancreatic
enzymes,
stimulate
release of
bile
Pancreas
• Produces and secretes insulin,
which regulates blood sugar
• Produces pancreatic juice which
is secreted into the small
intestine
Pancreatic Juice
• Contains water, salts, sodium
bicarbonate and enzymes
▫ sodium bicarbonate – keeps pH
between 7.1-8.2 to buffer the
acidic chyme from the stomach,
stop the action of pepsin, and
activate digestive enzymes in
the small intestine
Pancreatic Enzymes
• There are pancreatic enzymes that act to
break down carbohydrates, proteins and
lipids.
▫ Pancreatic amylase - acts on carbohydrates
▫ Trypsin & chymotrypsin – act on proteins, but
are secreted in an inactive form so as to
protect the pancreas from their effects
(trypsinogen & chymotrypsinogen)
 Trypsinogen & entrokinase react to form trypsin
 Trypsin and chymotrypsinogen react to form
chymotrypsin
▫ Pancreatic lipase - act on lipids
Digestive Secretions – Lower GI
Substance
Secretion
Classificatio
n
Secreted
By
Secreted
Into
Acts On
Function
Pancreatic
Juice
Pancreatic
Amylase
Enzyme Pancreas
Small
Intestine
Carbohydrates
Break
into
disaccharides
Pancreatic
Juice
Trypsin
(trypsinogen)
Enzyme Pancreas
Small
Intestine
Proteins
Break
into
peptides
Pancreatic
Juice
Chymotryp- Enzyme Pancreas
sin
(chymotrypsinogen)
Small
Intestine
Proteins
Break
into
peptides
Pancreatic
Juice
Pancreatic
Lipase
Small
Intestine
Lipids
Break
into fatty
acids
Enzyme Pancreas
Liver
•
•
•
•
Heaviest gland, 2nd largest organ in the body
Multi-lobed organ
Hepatocytes – liver cells
Functions of liver
▫ 1. produces bile - a yellow-greenish fluid consisting
of water, bile salts (helps break down fats), bile
pigments (gives urine & feces their color) and
cholesterol (a by-product of lipid metabolism)
▫ 2. stores glycogen
▫ 3. filtration of blood absorbed from small intestine
▫ 4. detoxification – alters chemical composition of
harmful substances to make them less harmful
Gall Bladder
• Function is to store and concentrate bile
• Fats in chyme cause CCK to be secreted into
the blood…CCK, in turn, causes secretion of
bile
Summary of Carbohydrate Digestion
• 1. Salivary Amylase - secreted in mouth;
breaks starches into maltose (a disaccharide)
• 2. Pancreatic Amylase - secreted into small
intestine; breaks polysaccharides into
disaccharides
• 3. Secreted into small intestine; breaks
disaccharides into monosaccharides
▫ Sucrase – sucrose into glucose and fructose
▫ Lactase – lactose into glucose and galactose
▫ Maltase - maltose into glucose (2-3 molecules)
Summary of Protein Digestion
• 1. Pepsin – produced & secreted in stomach in
inactive form of pepsinogen; breaks proteins
into peptides
• 2. Trypsin & Chymotrypsin - secreted from
pancreas into small intestine in inactive form
of trypsinogen and chymotrypsinogen; breaks
proteins into peptides
• 3. Peptidases – produced & secreted in small
intestine; breaks peptides into amino acids
Summary of Lipid Digestion
• All three break lipids into fatty acids
• 1. Lingual lipase – secreted in the mouth
• 2. Gastric lipase - produced & secreted in
the stomach
• 3. Pancreatic lipase - secreted from pancreas
into small intestine
Large Intestine
• Approx. 5 ft long, 2 ½ inches in diameter
• Begins at ileocecal sphincter (where material
passes from small intestine into large
intestine)
• Cecum – inferior to ileocecal sphincter;
basically a blind pouch
• Colon – longest portion; ascending, transverse,
descending
• Sigmoid – S-shaped curve
• Rectum - last 8 inches
• Anal Canal - last inch of rectum; opens to
outside at the anus
• No villi present nor enzymes secreted
• Goblet cells – secrete mucus for lubrication
• Haustra – a series of pouches that give the
colon a “puckered” appearance
• Functions of Large Intestine
▫ 1. abosorb water
▫ 2. produce and excrete solid waste
Formation and Expulsion of Feces
• Water and electrolytes are absorbed
• Bacteria (normally present) - break down any
remaining carbs and proteins
▫ - they also produce hydrogen, carbon dioxide and
methane gases, which contribute to flatus (gas in
the colon)
• Feces – consists of water, salts, sloughed off
cells, bacterial, unabsorbed or indigestible parts
of food & mucus
• Defecation – discharge of feces from the rectum
▫ - under voluntary control of external anal sphincter