Transcript The Digestive System

The Digestive System
Objectives:
• List and describe the major functions of the
digestive system.
• Describe why digestion of food is necessary and
compare mechanical and chemical digestion
• Identify the major digestive organs, the digestive
accessory glands, and their functions
• Describe the absorption of food in the small
intestine and the absorption of water in the large
intestine
• List and describe disorders and diseases of the
digestive system.
Function
• Digestion
– Breakdown of ingested food
– Absorption of nutrients into the blood
• Once the nutrients are absorbed by the digestive
system they are transported by the blood to the
tissues for metabolism.
– Production of cellular energy (ATP)
– Constructive and degradative cellular activities
Types of Digestion
• Digestion is a catabolic process in which large complex
molecules (carbohydrates, lipids, proteins, nucleic acids)
are broken down into simpler monomers
(monosaccharides, glycerol and fatty acids, amino acids,
and nucleotides) which can be absorbed by the body.
• There are two forms of digestion:
a. mechanical: In mechanical there is no
chemical change in the food. The food is simply broken
down into smaller pieces and mixed with digestive juices
secreted in the body. Ex. Mastication (chewing)
b. chemical: In chemical digestion the is a
chemical change in the food. The polymers are broken
down into monomers commonly by hydrolysis reactions
carried out by enzymes contained within the digestive
juices.
Six Processes of Digestion
• Ingestion – getting food into the mouth
• Propulsion – moving foods from one region of the digestive system to
another
• Mechanical digestion
– Mixing of food in the mouth by the tongue
– Churning of food in the stomach
– Segmentation in the small intestine
• Chemical Digestion
– Enzymes break down food molecules into their building blocks
– Each major food group uses different enzymes
• Carbohydrates are broken to simple sugars
• Proteins are broken to amino acids
• Fats are broken to fatty acids and alcohols
• Absorption
– End products of digestion are absorbed in the blood or lymph
– Food must enter mucosal cells and then into blood or lymph capillaries
• Defecation
– Elimination of indigestible substances as feces
Digestive Processes
Divisions of Digestive System
Organs
• Two main groups
– Alimentary canal – continuous coiled hollow
tube that runs from the mouth to the anus
– Accessory digestive organs- secrete digestive
juices by ducts (exocrine glands) into the
alimentary canal.
Organs of the Alimentary Canal
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Mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Anus
Accessory Digestive Organs
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Salivary glands
Teeth
Pancreas
Liver
Gall Bladder
Digestive Organs
Mouth Oral Cavity (Alimentary
Canal)
• Mastication (chewing)
of food
• Mixing masticated
food with saliva
• Initiation of
swallowing by the
tongue
• Allowing for the sense
of taste
Salivary Glands (Accessory Organs)
Salivary Glands:
• Saliva-producing glands
– Parotid glands – located anterior to ears
– Submandibular glands
– Sublingual glands
Saliva:
• Mixture of mucus and serous fluids
• Helps to form a food bolus
• Contains salivary amylase to begin starch digestion
• Dissolves chemicals so they can be tasted
Teeth (Accessory Organs)
• The role is to masticate (chew)
food
• Humans have two sets of teeth
– Deciduous (baby or milk) teeth
– 20 teeth are fully formed by age
two
• Permanent teeth
– Replace deciduous teeth
beginning between the ages of
6 to 12
– A full set is 32 teeth, but some
people do not have wisdom
teeth
• Types of teeth:
Incisors - cutting
Canines - tearing
Premolars – shearing,
shredding
Molars - grinding
Tooth Structure
• Crown – exposed part
– Outer enamel
– Dentin
– Pulp cavity
• Neck
– Region in contact with the
gum
– Connects crown to root
• Root
– Periodontal membrane
attached to the bone
– Root canal carrying blood
vessels and nerves
Pharynx (Alimentary Canal)
• Serves as a passageway for
air and food
• Food is propelled to the
esophagus by two muscle
layers
– Longitudinal inner layer
– Circular outer layer
• Food movement is by
alternating contractions of
the muscle layers
(peristalsis)
Esophagus (Alimentary Canal)
• Runs from pharynx to stomach through the
diaphragm
• Conducts food by peristalsis
(slow rhythmic squeezing)
• Passageway for food only (respiratory
system branches off after the pharynx)
Alimentary Canal Organ Structure
and Tissue Arrangement
• Mucosa
– Innermost layer
– Moist membrane
• Surface epithelium
• Small amount of connective tissue
(lamina propria)
• Small smooth muscle layer
• Submucosa
– Just beneath the mucosa
– Soft connective tissue with blood
vessels, nerve endings, and
lymphatics
• Muscularis externa – smooth muscle
– Inner circular layer
– Outer longitudinal layer
• Serosa
– Outermost layer – visceral
peritoneum
– Layer of serous fluid-producing
cells
Stomach (Alimentary Canal)
• Located on the left side of the abdominal cavity
• Food enters at the cardioesophageal sphincter
• Regions of the stomach
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Cardiac region – near the heart
Fundus
Body
Pylorus – funnel-shaped terminal end
• Food empties into the small intestine at the pyloric
sphincter
• Rugae – internal folds of the mucosa
• External regions
– Lesser curvature
– Greater curvature
Stomach (Alimentary Canal)
• Acts as a storage tank
for food
• Site of food
breakdown
• Chemical breakdown
of protein begins
• Delivers chyme
(processed food) to the
small intestine
Stomach: Mucosal Layer
• Simple columnar epithelium
– Mucous neck cells (goblet cells)
– produce a sticky alkaline
mucus
– Gastric glands – secrete gastric
juice
– Chief cells – produce proteindigesting enzymes
(pepsinogens)
– Parietal cells – produce
hydrochloric acid
– Endocrine cells – produce
gastrin
• Gastric pits formed by folded
mucosa
• Glands and specialized cells are
in the gastric gland region
Small Intestine (Alimentary Canal)
• The body’s major digestive organ all digestion of food is completed in
this organ
• Site of nutrient absorption into the blood
• Muscular tube extending form the pyloric sphincter to the ileocecal
valve
• Suspended from the posterior abdominal wall by the mesentery
• Duodenum (25cm = 10 inches) “12 finger widths long”
– Attached to the stomach
– Curves around the head of the pancreas
– Where bile and pancreatic juices enter the alimentary canal
• Jejunum (2.5m = 8 feet) “empty”
– Attaches anteriorly to the duodenum
• Ileum (3.6m = 12 feet) “twisted”
– Extends from jejunum to large intestine
Duodenum and Accessory Glands
Connected
Small Intestine Internal Structure
• Villi are small fingerlike
structures formed by the
mucosa
• Give the small intestine more
surface area for absorption
• Fold in the intestine are called
circular folds or plicae circulares
• Deep folds of the mucosa and
submucosa
• Do not disappear when filled
with food
• The submucosa has Peyer’s
patches (collections of
lymphatic tissue)
Villi Internal Structure and Function
• Absorptive cells are found on
the surface epithelium which are
simple columnar microvilliated
epithelium
• Blood capillaries are below the
surface epithelium and this is
where monosaccharides, amino
acids, and nucleic acids enter
into the blood stream and are
taken to the liver for processing
• Lacteals (specialized lymphatic
capillaries) where lipids are
absorbed and eventually re-enter
the blood stream to be taken to
the liver for processing.
Pancreas (Accessory Organ)
• Produces a wide spectrum of digestive
enzymes that break down all categories of
food
• Enzymes are secreted into the duodenum
• Alkaline fluid introduced with enzymes
neutralizes acidic chyme
• Endocrine products of pancreas
– Insulin
– Glucagon
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Liver and Gall Bladder (Accessory
Organs)
Largest gland in the body
Located on the right side of the body under
the diaphragm
Consists of four lobes suspended from the
diaphragm and abdominal wall by the
falciform ligament
Connected to the gall bladder via the
common hepatic duct
Produced by cells in the liver
Composition
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Bile salts
Bile pigment (mostly bilirubin from the
breakdown of hemoglobin)
Cholesterol
Phospholipids
Electrolytes
Sac found in hollow fossa of liver
Stores bile from the liver by way of the cystic
duct
Bile is introduced into the duodenum in the
presence of fatty food
Gallstones can cause blockages
Large Intestine (Alimentary Canal)
• Larger in diameter, but shorter
than the small intestine
• Frames the internal abdomen
• Cecum – saclike first part of the
large intestine
• Appendix
– Accumulation of lymphatic
tissue that sometimes becomes
inflamed (appendicitis)
– Hangs from the cecum
• Colon
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Ascending
Transverse
Descending
S-shaped sigmoidal
• Rectum
• Anus – external body opening
Functions of Large Intestine
• Absorption of water
• Eliminates indigestible food from the body as
feces
• Does not participate in digestion or absorption of
digested food
• Goblet cells produce mucus to act as a lubricant
• Site of production of Vitamin K by symbiotic
bacteria which live off the remains of food that
have not been digested or absorbed in the small
intestine. These bacteria produce over 50% of
fecal matter.
Nutrition
• Nutrient – substance used by the body for growth,
maintenance, and repair. Macronutrients are those which
are required in large amounts. Micronutrients required in
smaller amounts.
• Categories of nutrients
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Carbohydrates ( macro)
Lipids (macro)
Proteins (macro)
Vitamins (micro)
Mineral (micro)
Water
• A lack of the proper nutrients or an imbalance in the
correct amounts of each is called malnutrition. Even
though a person is obese they often suffer from
malnutrition!
Source of Nutrients
• Carbohydrates
– Most are derived from plants
– Exceptions: lactose from milk and small amounts of glycogens from meats
• Lipids
– Saturated fats from animal products
– Unsaturated fats from nuts, seeds, and vegetable oils
– Cholesterol from egg yolk, meats, and milk products
• Proteins
– Complete proteins – contain all essential amino acids
• Most are from animal products
– Legumes and beans also have proteins, but are incomplete
• Vitamins
– Most vitamins are used as cofactors and act with enzymes many are
produced by plants
– Found in all major food groups
• Minerals
– Play many roles in the body
– Most mineral-rich foods are vegetables, legumes, milk, and some meats
Diseases and Disorders of the
Digestive System
• Heartburn (Acid Reflux)
This is due to acid from the
stomach entering into the
esophagus which results in
a burning sensation. In
chronic severe cases this
can lead to damage,
ulceration, scarring, and
possibly cancer of the
esophagus if not treated.
Diseases and Disorders of the
Digestive System
• Ulcers: Ulcers occur when the
lining of the stomach or the
duodenum becomes weakened
and exposed to the effects of
digestive enzymes and stomach
acid. It eventually will digest a
hole through the mucosa and
may cause severe bleeding if a
blood vessel of the stomach is
involved. It is now known that
ulcers are closely associated
with infection by a bacteria
called Helicobacter pylori.
Diseases and Disorders of the
Digestive System
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Crohn’s Disease: Crohn's Disease
is an inflammatory disease of the
bowel. It can cause fever, pain,
diarrhoea and significant loss of
weight. Crohn's Disease can affect
any part of the bowel, but most
typically affects the lower end of
the small intestine, where it joins
the large intestine. The intestinal
wall becomes thick and inflamed,
producing ulcers and fissures, and
can in addition cause abnormal
passageways to form between
adjacent portions of the intestine.
The intestinal space becomes so
narrow that the passage of food can
become obstructed.
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Diseases and Disorders of the
Digestive System
Colon Cancer: Cancer starts in the inner
layer and can grow through some or all of
the other layers. Knowing a little about
these layers is helpful because the stage
(extent of spread) of a cancer depends to a
great degree on which of these layers it
affects.
Cancer that starts in the different areas
may cause different symptoms. Colon and
rectum cancers probably develop slowly
over a period of several years. We now
know that most of these cancers begin as a
polyp--a growth of tissue into the center of
the colon or rectum. Polyps are also known
as adenomas. Removing the polyp early
may prevent it from becoming cancer.
Over 95% of colon and rectal cancers are
adenocarcinomas. These are cancers of the
cells that line the inside of the colon and
rectum. There are some other, more rare,
types of tumors of the colon and rectum,
but the facts given here refer only to
adenocarcinomas.
Colon and rectal cancer have many features
in common and are often referred to
together as colorectal cancer.