The Digestive System

1
The Digestive System
2
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.

3
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

4
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.

5
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

6
Digestive Processes
7
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.

8
Organs of the Alimentary Canal

• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus

9
Accessory Digestive Organs

• Salivary glands
• Teeth
• Pancreas
• Liver
• Gall Bladder

10
Digestive Organs
11
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

12
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

13
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

14
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

15
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)

16
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)

17
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

18
Stomach (Alimentary Canal)

• Located on the left side of the abdominal cavity
• Food enters at the cardioesophageal sphincter
• Regions of the stomach
• 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

19
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

20
Stomach Mucosal Layer

• Simple columnar epithelium
• Mucous neck cells (goblet cells) produce a
  sticky alkaline mucus
• Gastric glands secrete gastric juice
• Chief cells produce protein-digesting 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

21
Small Intestine (Alimentary Canal)

• The bodys 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

22
Duodenum and Accessory Glands Connected
23
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 Peyers patches (collections of
  lymphatic tissue)

24
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.

25
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

26
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
• 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

27
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
• Ascending
• Transverse
• Descending
• S-shaped sigmoidal
• Rectum
• Anus external body opening

28
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.

29
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
• 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!

30
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

31
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.

32
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.

33
Diseases and Disorders of the Digestive System

• Crohns 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.

34
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.