Digestive System

Digestive System


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


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


• 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


• 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



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


• 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


The Stomach

The Pancreas

The Liver and Gall Bladder

The Small Intestine

The Large Intestine

Common disorders of digestive system


• 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

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