Digestion
Chapter 23
Biology 2122
Introduction
 Alimentary canal and the accessory digestive organs
 General processes of digestion
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Ingestion
Propulsion
Mechanical and chemical digestion
Absorption and defecation
The Digestive Process – An Overview
Digestive Processes
Peristalsis and
Segmentation
Animation
Functional concepts and Controls
Initiated by both mechanical
and chemical stimuli (23.4)
•Sensors located in the G.I tract
respond to stretching,
osmolarity (solute
concentration) and pH levels
Extrinsic control (external
stimulus); intrinsic (nerve plexuses
in the walls of the digestive tract
Digestive Cavities
The peritoneum is the
serous membrane digestive
cavity
 1. Visceral peritoneum
 2. Parietal peritoneum
 3. Peritoneal cavity
Mesentary
 It is a double serous
membrane
 Retroperitoneal organs
 Intraperitoneal organs
Histology
1. Mucosa
 produces mucus, digestive
enzymes, hormones
 absorption of nutrients
 Sublayers of the mucosa
▪ (a). Epithelium
▪ (b). Lamina propria
▪ (c). Muscularis mucosae
2. Submucosa
 Dense irregular CT
Histology
3. Muscularis Externa or
“muscularis”
– promotes peristalsis
4. Serosa
– “viseral peritoneum”
– areolar CT
– mesothelium
Anatomy of the Digestive System
1. Mouth (buccal cavity)
 Stratified squamous epithelium (walls); slight keratin build up on
tongue
 Defensins
2. Palate
 hard palate (anterior) and the soft palate (posterior) formed
from skeletal muscle
 Uvula is attached to the soft palate
 Animation
3. Tongue -skeletal muscles
– Formation of the Bolus
– Intrinsic muscles allow the tongue to change
shape
– Extrinsic muscles allow the tongue to protrude,
retract and sideways movement.
– Contains both fungiform and
circumvallate papillae
• taste buds
• 4. Salivary glands
– Submandibular, sublingual glands
– Largest - parotid gland
• Mumps- inflammation of the parotid glands
– Contains 99% water; 7 pH; electrolytes, muscin,
lysozyme proteins
– Controlled by facial VII and glossopharyngeal IX
nerves
Anatomy- Mouth
Anatomy
5. Classification of teeth on page 893
– 6 mo. teeth appear; 24 mo. All 20 milk teeth
emerge
– Permanent and deciduous or baby teeth (6-12 yrs)
– Wisdom teeth complete the full set (17-25 yrs)
– Incisors (cutting of food); canines (tear); premolars
or bicuspids and molars (grinding and crushing)
– Structure
• Crown is the enamel cover that covers the
gingiva or gum
• Root – most teeth have one root (except for
the first two upper molars have 3); outer part
covered by cementum (CT) which attaches to
periodontal ligament which anchors tooth in
the bony alveolus of jaw
• Crown and root connected by neck
• Dentin forms bulk of tooth and surrounds the
pulp cavity which contains CT, BV, NF;
collectively called the pulp.
• Root canal is where pulp cavity extends into
the root
Anatomy- Pharynx and Esophagus
6. Pharynx
 Mucosa contains stratified
squamous epithelium (with
goblet cells)
 Double external layer
7. Esophagus is about 25 cm
long (2.5 m)
 Passes through the diaphragm esophageal hiatus
 Meets the stomach at the
cardiac oriface
▪ Control of food – cardiac
sphincter
▪ Heartburn Animation
Digestion Begins in the Mouth –Salivary Glands
Parotid
Sublingual
Submandibular
Saliva
Saliva is produced by extrinsic and intrinsic salivary
glands
Saliva is a hypo-osmotic solution consisting of around
99% water
Other than it’s role in digestion, it also provides a
protective role (anti-microbial)
– IgA antibody
– Lysosyme- antibacterial effects
– Defensins
– Cyanide
Saliva
 Friendly bacteria on the back of the tongue – ‘NO’
 Secretion modes
 Intrinsic
 Extrinsic
 Salivary gland control
 1000 to 1500 ml/day
 Parasympathetic NS control
 Chemoreceptors pass afferent impulses to the salivatory nuclei in the
pons/medulla; efferent impulses returned via the facial (VII) and
glossopharyngeal nerves (IX)
Digestive Processes in the Mouth, Pharynx and
Esophagus
1. Mechanical digestion begins with
mastication
2. Deglutition -coordinated by 22
different muscles
– Two phases
• Buccal phase
• Pharyngeal phase
3. From the oropharynx to the stomach
takes 4-8 seconds and fluids only
around (2) seconds
4. A bolus moves down the esophagus to
the stomach
The stomach contains the 4 tunics
– Extra layer of smooth muscle
(oblique)
– Simple columnar epithelial
cells
– Rugae
The Cardia, Fundus, Body and
Pylorus
Glands of the fundus and body
contain the following secretory cells
– mucosal neck cells
– Parietal cells
Stomach anatomy
Glands of the Stomach
 Gland cells and secretions
 1. Chief cells
 2. Enteroendocrine Cells
▪ Functions in 23.1
 3. Gastrin
 Animation
 Stomach forms a ‘mucosal
barrier’ to protect itself
 Gastric Ulcers and H.Pylori
- Animation
Innervation – Blood Supply
 Automatic nervous system
 Sympathetic thoracic splanchnic nerves from the celiac plexus
▪ Reduces digestive activity
 Parasympathetic nerves (via Vagus X)
 Celiac trunk
 Provides arterial blood supply
 Liver, Stomach and Spleen
 Hepatic portal system veins drain into hepatic portal vein
Stomach and Omentum
• Omentum – mesenteries
– 1. Greater and Lesser Omentums
– 2. Greater and Lesser Curves
Digestive processes in the Stomach
1. Only protein digestion occurs in the stomach
2. Controlled by
 Nervous controls
 Hormonal controls
3. Phases of Gastric Secretions
 (a). Cephalic Reflex
 (b). Gastric
Digestive processes in the stomach
3.
Phases of Gastric Secretions
– (c). Intestinal
• Excitatory
• Inhibitory
• Animation
Neural and Hormonal mechanisms – Gastric Juice
Stomach Emptying
As more than 1 liter of
food enters the stomach
pressure begins to rise
This causes a reflexive
relaxation
 1. Receptive relaxation
 2. Adaptive
The stress-relaxation
response by the stomach
Peristalsis begins at the
cardiac sphincter and
increases near the pylorus
The Small intestine
Begins at the pyloric sphincter
(epigastric region)
 ends at the ileocecal valve – joins
the large intestine
 6-7 m long
 Three subdivisions
 1. Duodenum -25 cm long
 2. Jejunum - 2.5 meters long
 3. Ileum - 3.6 m long; ends at the
ileocecal valve
 The vagus nerve (parasympathetic)
and thoracic splanchnic nerves
(sympathetic)
 Arterial supply - superior mesenteric
artery
Microscopic Anatomy –
Small Intestine
Modifications for Absorption
1. Plicae circulares
2. Villi –include absorptive cells
3. Microvilli - “brush borders”
▪ Contain enzymes that complete
digestion
Epithelia of the mucosa
 Contains goblet cells; enteroendocrine
cells cells (CCK and secretin)
 T cells
Histology of the Small intestine
Pits - intestinal crypts or crypts of
Lieberkuhn
 Intestinal juice
 Paneth cells
Submucosa
 Areolar CT ; Peyer’s patches
 Brunner’s glands
Intestinal Juice
 1-2 L/day; 7.4-7.8 pH; mostly water with mucus with low
levels of enzymes
Digestive Processes in the small intestine
Most of the enzymes important for chemical digestion comes
from the liver and pancreas
Chyme slowly enters the duodenum - hypertonic
Controlled peristaltic movements mix juices and foods
Takes about 2 hrs for food to move from beginning to end
Mobility patterns are coordinated by enteric neurons of the GI
tract wall
Ileocecal sphincter and Valve
 Largest gland in the body (1.4 kg)
 four lobes
 main function is to filter and process
nutrients in the blood
 Gall bladder
 Bile leaves the liver through the common
hepatic duct which fuses with the cystic
duct which drains the gall bladder to form
the bile duct
 The lesser omentum (dorsal
mesentery) anchors the liver to the
lesser curvature of the stomach
 Hepatic artery and vein;
 Hepatic artery and vein enter the liver
through the porta hepatis
The Liver
Functional units
– called liver lobules which
is hexagonal containing
plates of liver cells
(hepatocytes)
Liver Sinusoids
Hepatic veins and arteries central vein
Kupffer cells
Bile flows through the bile
canaliculi that run between
hepatocytes
Histology of the
liver
Bile and the gall bladder
 The gallbladder stores bile
 The excretion of bile
 Regulation of bile release
 CCK
 Bile
 contains bile salts,
pigments, cholesterol,
neutral fats,
phospholipids and
electrolytes
Bile
• Bile salts emulsify fats
– Increases fat and cholesterol absorption
• Bile is not excreted via feces
– Enters enterohepatic circulation
– Reconstituted into new bile
• Bilirubin is the main bile pigment that is a waste
product of the heme of hemoglobin
Produces enzymes
– Enzymes – digest all major
macromolecules
Pancreas contains
– Acini secretory cells
– Pancreatic islets (endocrine)
Pancreatic Juice - water(most),
enzymes and electrolytes
• Proteases
– Trypsinogen
– Chymotrypsinogen
• Amylases
• Lipases
• Nucleases
The pancreas
Regulation of
Pancreatic
Secretions
–Parasympathet
ic nervous
system control
–CCK is released
when proteins
and fats enter
–Secretin
Pancreas
The large intestine
Extends from the ileocecal valve to the anus and
has a diameter of 7 cm
Function
Subdivisions are the cecum, appendix, colon,
rectum and anal canal
Colon is divided into the ascending colon,
transverse colon, and descending colon
Large intestine
Transverse
Descending
Haustrum
Ascending
Teniae coli
Sigmoid
Cecum
Appendix
Rectum
Large Intestine
 Muscularis contains three bands of smooth muscle
(teniae coli)
 Haustra
 Large intestine
 Bacterial flora
 Synthesize some B vitamins and vitamin K (clotting factors)
Mesenteries
2x peritoneum extends to the organs from the body wall.
Function is to provide blood and nerve supply; lymph; holds
organs in place and stores adipose tissue
Mostly dorsal and attaches to posterior abdominal wall; few
ventral mesenteries (omenta)
Parts of large intestine and pancreas not suspended by
mesentery (lie posterior to the peritoneum) “retroperitoneal
Large intestine Digestive Processes and elimination
 Main function is elimination not absorption
 Muscles here are inactive most of the time and at times move
sluggishly “haustral contractions” that occur every 30 minutes
 Mass peristalsis
 Stool contains undigestive foods, mucus, old epithelial cells,
bacteria and small amounts of water
 500 ml of food enter and only 150 ml exit as feces
 Defecation Reflex is activated by the stretching of the walls
 Parasympathetic reflex
 Animation
Mucosa and defecation
Columnar
Cells with
Goblet
Chemical digestion-carbohydrates
 Chemical digestion of starch begins in the mouth
 Salivary amylase in saliva begins the process of starch digestion
 Works best in pH of 6.75 to 7.00
 Oligosaccharides are larger than disaccharides but smaller than
polysaccharides
 Amylase is inactivated by stomach acid
 Pancreatic amylase finishes off what salivary amylase cannot digest
 Absorption
 Facilitated diffusion (fructose); cotransport (glucose and galactose)
Protein digestion
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Includes both enzyme and dietary forms
Digested into amino acid monomers
Begins in the stomach
Pepsin cleaves Polypeptides between tyrosine and phenylalanine
Absorption
 Coupled to active transport of sodium;
Protein digestion
lipids
 Almost all lipid digestion occurs in the small intestines
 Pancreas produces lipase enzymes
 Bile salts emulsify fats
 See next slide
 Absorption (next slide)
Fat digestion and absorption
Nucleic acids