Digestive System
Objectives
At the end of this
lecture, student will be able to
• List the organs of the digestive system
• Differentiate the GIT and accessory digestive organs
• Explain the basic processes performed by the digestive
system
• Describe the parts of oral cavity
• Describe salivary glands and describe their functions
• Explain the anatomy of tongue
• Describe the structure of tooth
• Explain digestion in mouth
• Describe the structure of esophagus
• Explain the deglutition process
• Describe the anatomy of stomach
• Explain the histology of stomach
• Explain the digestion process in stomach
• Describe the structure of pancreas
• Explain the secretions of pancreas
• Describe the anatomy of liver and gall bladder
• Describe the histology of liver
• Explain the functions of liver
• Describe the anatomy and histology of small intestine
• Explain the functions of small intestine
• Describe the special features of small intestine
• List the brush border enzymes
• Explain the digestion process in small intestine
• Explain the absorption process in small intestine
• Explain the anatomy and functions of large intestine
• Describe the histology of large intestine
• Explain digestion in large intestine
• Outline defecation reflex
• List the phases of digestion
• Explain the phages of digestion
• Describe the regulation process of digestion
• Outline the disorders of digestive system
Content
Digestive system
• Organs of digestive system
• Accessory digestive organs
• Peritoneum
• Oral cavity
• Salivary glands
• Composition and functions of saliva
• Tongue
• Tooth
• Digestion in mouth
• Esophagus
• Deglutition
• Anatomy of Stomach
• Physiology of Stomach
• Pancreas
• Liver
• Gall bladder
• Small intestine
• Digestion process in small intestine
• Absorption in small intestine
• Anatomy and functions of large intestine
• Large intestine
• Defecation reflex
• Phases of digestion
• Regulation of digestion of process
• Disorders
Digestive System
• Contributes to homeostasis by:
– breaking down food into forms that can be absorbed and
used by body cells
• Also absorbs
– Water
– Vitamins
– Minerals
• Eliminates
– Wastes from the body
• Gastroenterology: Specialty that deals with the structure,
function, diagnosis, and treatment of diseases of the stomach and intestines
• Proctology: specialty that deals with the diagnosis and
treatment of disorders of the rectum & anus
The GI
Tract
• Continuous tube extending from the mouth to the anus
The
Accessory Digestive Organs
Includes:
• Teeth
• Tongue
• Salivary glands
• Liver
• Gallbladder
• Pancreas
Role of
Accessory Digestive Organs
• Teeth: Aid in the physical breakdown of food, and the
tongue assists in chewing and swallowing.
• The other accessory digestive organs – No direct contact
with food
• Produce or store secretions that flow into the GI tract
through ducts
• The secretions aid in the chemical breakdown of food
Functions
of Digestive System
• Ingestion: Taking food into the mouth
• Secretion: Water, acid, buffers, and enzymes
• Mixing and propulsion: Churning and propulsion of food
• Digestion: Mechanical and chemical breakdown of food
• Absorption: into the blood and lymph
• Defecation: Elimination of feces
Digestion
• Mechanical digestion
– Consists of mastication and movements of the GIT
• Chemical digestion
– Series of hydrolysis reactions
Layers of
GIT
• The basic arrangement:
– Mucosa
– Submucosa
– Muscularis
– Serosa
Innervation
of GIT Tract
Peritoneum
Divided into:
• Parietal peritoneum
– Lines the wall of the abdominopelvic cavity
• Visceral peritoneum
– Covers some of the organs in the cavity
Peritoneal cavity
• The slim space between the parietal and visceral portion
of peritoneum containing lubricating serous fluid
• Folds of the
peritoneum include
– Mesentery
– Mesocolon
– Falciform ligament
– Lesser omentum
– Greater omentum
Retroperitoneal
• Some organs lie on the posterior abdominal wall and are
covered by peritoneum only on their anterior surfaces
• They are not in the peritoneal cavity
• Includes:
– Kidneys
– Ascending and descending colons
– The large intestine
– Duodenum of the small intestine
– Pancreas
Mouth
The Stomach
The Pancreas
The Liver and Gall
Bladder
The Small Intestine
The Large Intestine
Common disorders of digestive system
Summary
• Digestion: Breaking
down of larger food molecules into smaller molecules is called
• The organs involved in the breakdown of food are
collectively known as the digestive system
• Composed of two main groups: the GIT and accessory
digestive organs
• The GIT is a continuous tube extending from the mouth to
the anus
• The accessory digestive organs: Teeth, tongue, salivary
glands, liver, gallbladder, and pancreas
• Digestion includes six basic processes: Ingestion,
secretion, mixing and propulsion,
mechanical and chemical digestion, absorption, and defecation
• Arrangement of layers in GIT: Mucosa, submucosa,
muscularis, and serosa
• The GIT is regulated by ENS and ANS
• Three pairs of major salivary glands: parotid,
submandibular, and sublingual glands
• Saliva
lubricates food and
starts the chemical
digestion of carbohydrates
• Chemically, saliva is 99.5% water and 0.5% solutes
• Provides a medium for dissolving food, activate salivary
amylase, buffer acidic foods that enter the mouth, lubricates food, prevents
attachment of microbes, and kills bacteria
• Tongue: Accessory
digestive organ composed of skeletal muscle covered with mucous membrane
• The teeth (dentes) project into the mouth and are adapted
for mechanical digestion
• A typical tooth consists of three principal regions: crown,
root and nec
• The pharynx has both respiratory and digestive functions
• The esophagus is a collapsible, muscular tube that connects
the pharynx to the stomach
• Deglutition, or swallowing, moves a bolus from the mouth
to the stomach
• The principal anatomic regions of the stomach are the
cardia, fundus, body and pylorus
• Adaptations of the stomach for digestion include rugae;
glands that produce mucus, hydrochloric acid, pepsin, gastric lipase, and
intrinsic factor; and a three-layered muscularis
• The stomach wall is impermeable to most substances
• Among the substances the stomach can absorb are water,
certain ions, drugs and alcohol
• The principal anatomic regions of the stomach are the
cardia, fundus, body and pylorus
• Adaptations of the stomach for digestion include rugae;
glands that produce mucus, hydrochloric acid, pepsin, gastric lipase, and
intrinsic factor; and a three-layered muscularis
• The stomach wall is impermeable to most substances
• Among the substances the stomach can absorb are water,
certain ions, drugs and alcohol
• Stomach – Mechanical digestion consists of mixing waves
• Chemical digestion consists mostly of the conversion of
proteins into peptides by pepsin
• The pancreas consists of a head, a body, and a tail and is
connected to the duodenum via the pancreatic duct and accessory duct
• Endocrine pancreatic islets (islets of Langerhans) secrete
hormones, and exocrine acini secrete pancreatic juice
• Pancreatic juice contains enzymes – pancreatic amylase,
trypsin, chymotrypsin, carboxypeptidase, elastase, triglycerides, ribonuclease
and deoxyribonuclease
• Liver functions in:
– Carbohydrate, lipid, and protein metabolism
– Processing of drugs and hormones
– Excretion of bilirubin
– Synthesis of bile salts
– Storage of vitamins and minerals
– Phagocytosis
– Activation of vitamin D
– Bile’s contribution to digestion is the emulsification of
dietary lipids
• The lobes of the liver are made up of lobules that contain
hepatocytes (liver cells), sinusoids, stellate reticuloendothelial (Kupffer)
cells, and a central vein
• Hepatocytes produce bile that is carried by a duct system
to the gallbladder for concentration and temporary storage
• The small intestine extends from the pyloric sphincter to
the ileocecal sphincter
• It is divided into duodenum, jejunum, and ileum
• Small intestine – The circular folds, villi, andmicrovilli
of its wall provide a large surface area for digestion and absorption
• Intestinal glands (crypts of Lieberkühn) and secrete
intestinal juice
• Special features of small intestine include circular
folds, villi and microvilli
• Brush-border enzymes digest -dextrins, maltose, sucrose,
lactose, peptides, and nucleotides at the surface of mucosal epithelial cells
• Carbohydrate
digesting enzyme
– Dextrinase – dextrins into glucose
– Maltase – Maltose to glucose
– Sucrase – Sucrose to glucose and fructose
– Lactase – Lactose to glucose and galactose
• Protein-digesting
enzymes – Peptidases
– Aminopeptidases
– Break off
amino acids at
the amino ends
of peptides
– Dipeptidases – Split dipeptides into aminoacids
• Nucleotide-digesting
enzymes
– Nucleosidases and Phosphatases – Nucleotides to pentoses
and nitrogenous bases
• Mechanical digestion in the small intestine involves
segmentation and migrating motility complexes
• Absorption occurs via diffusion, facilitated diffusion,
osmosis, and active transport; most absorption occurs in the small intestine
• Monosaccharides, amino acids, and short-chain fatty acids
pass into the blood capillaries
• Long-chain fatty acids and monoglycerides are absorbed
from micelles, resynthesized to triglycerides, and formed into chylomicrons
• Chylomicrons move into lymph in the lacteal of a villus
• The small intestine also absorbs electrolytes, vitamins,
and water
• Large intestine participates in the completion of
absorption and production of certain vitamins
• Haustral churning, peristalsis, and mass peristalsis are
the movements of large intestine
• Large intestine extends from the ileocecal sphincter to
the anus
• Regions include the cecum, colon, rectum and anal canal
• The mucosa contains many goblet cells, and the muscularis
consists of teniae coli and haustra
• Mechanical movements of the large intestine include
haustral churning, peristalsis, and mass peristalsis
• The last stages of chemical digestion occur in the large
intestine through bacterial action
• Substances are further broken down, and some vitamins are
synthesized
• The large intestine absorbs water, ions, and vitamins
• Feces consist of water, inorganic salts, epithelial cells,
bacteria and undigested foods
• The elimination of feces from the rectum is called
defecation
• Defecation is a reflex action aided by voluntary
contractions of the diaphragm and abdominal muscles and relaxation of the
external anal sphincter
• Digestive activities occur in three overlapping
phases: cephalic phase, gastric phase
and intestinal phase
• Cephalic phase of
digestion – Salivary glands secrete saliva and gastric glands secrete
gastric juice in order to prepare the mouth and stomach for food
• Gastric phase of
digestion – The presence of food in the stomach promotes gastric juice
secretion and gastric motility
• Intestinal phase of
digestion
– Food is digested in the small intestine
– In addition prevents the small intestine from being
overloaded
• The activities that occur during the various phases of
digestion are coordinated by neural pathways and by hormones
• Common disorders of
digestive system:
– Dental caries
– Periodontal disease
– Diverticular disease
– Anorexia nervosa
– Colorectal cancer
– Hepatitis
– Ulcer