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Anatomy & Physiology
Lecture 24: The Digestive
System
Lecturer: Dr. Barjis
Room P307
Phone: (718) 260-5285
E-Mail: [email protected]
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Frederic H. Martini
Fundamentals of
Learning Objectives
• Identify the organs of the digestive system and
their major functions
• Outline the mechanisms that regulate digestion
• Describe the anatomy of the organs and accessory
organs of the digestive system
• Discuss the functions of the major structures and
regions of the digestive system and discuss the
regulation of their activities
Learning Objectives
• Explain the significance of the large intestine in
the absorption of nutrients
• Describe the events involved in the digestion of
organic and inorganic nutrients
• Summarize the effects of the aging process on the
digestive system
The Digestive system includes:
• The muscular digestive tract
• Esophagus
• Stomach
• Intestine
• Various accessory organs
• Liver
• Pancreas
• Oral Cavity e.g. tongue
The Components of the Digestive System
Functions of the digestive system
• Functions of digestive system include:
• Ingestion
• Mechanical processing
• Digestion
• Secretion
• Absorption
• Excretion
• Storage
The digestive system organs and the peritoneum
• Mesenteries
• Mesenteries are layers of serous membranes
that support portions of the digestive tract
• Provides padding, protection, insulation,
and energy reserves
Mesenteries
Histological organization of the digestive tract
• Digestive tract consist of four layers: Mocusa,
Submocusa, Muscularies and Serosa.
• Mucosa lines digestive tract (mucous
epithelium)
• Moistened by glandular secretions
• Lamina propria and epithelium form
mucosa
• Submucosa
• Layer of dense irregular connective tissue
Histological organization of the digestive tract
• Muscularis
• Smooth muscle arranged in circular and
longitudinal layers
• Serosa
• Serous membrane covering most of the
muscularis
The Structure of the Digestive Tract
Movement of digestive materials
• Smooth Muscles show rhythmic cycles of activity
that generates contraction that spreads
• Peristalsis is waves or contractions that move and
propel food (bolus) along the GI tract
• Segmentation
• Segmentation is the contraction that is
produced in small intestine and some portion
of large intestine.
• This contraction leads to mixing of food and
fragment a bolus
Peristalsis
Control of the digestive system
• Movement of materials along the digestive tract is
controlled by:
• Neural mechanisms
• Parasympathetic and local reflexes
• Hormonal mechanisms
• Enhance or inhibit smooth muscle
contraction
• Local mechanisms
• Coordinate response to changes in pH or
chemical stimuli
The Regulation of Digestive Activities
The Oral Cavity
The mouth opens into the oral or buccal cavity
• Its functions include:
• Analysis of material before swallowing
• Mechanical processing by the teeth, tongue,
and palatal surfaces
• Lubrication
• Limited digestion e.g. salivary amylase initiates
digestion of complex carbohydrates
The Oral Cavity
The tongue
• primary functions include:
• Mechanical processing
• Assistance in chewing and swallowing
• Sensory analysis by touch, temperature, and
taste receptors
Salivary glands (three pairs)
• There are 3 pairs of salivary glands: Parotid,
sublingual, and submandibular
• Saliva
• Saliva is a watery solution that contain:
• 99.4 % water and 0.6% electrolytes, buffers,
glycoproteins, antibodies, and enzymes
• Functions include:
• Lubrication, moistening, and dissolving
• Initiation of digestion of complex carbohydrates
• Help to control bacterial population in the oral cavity
The Salivary Glands
The esophagus
• Carries solids and liquids from the pharynx to the
stomach
• Passes through esophageal hiatus in
diaphragm
• The wall of the esophagus contains mucosal,
submucosal, and muscularis layers
stomach
Functions of the stomach
• Storage - bulk storage of undigested food
• Mechanical processing of the food
• Chemical processing of the food – e.g. disruption
of chemical bonds via acids and enzymes
• Secretion – e.g. production of intrinsic factor,
HCl, pepsinogen and gastrin
Anatomy of the stomach
• Cardia – superior, medial portion (portion of the
stomach that connects to the esofagous)
• Fundus – portion superior to stomachesophageal junction
• Body – area between the fundus and the curve of
the J
• Pylorus – antrum and pyloric canal adjacent to
the duodenum (portion of the stomach that
connects to the duadenum)
Stomach anatomy
• Pyloric Sphincter
• Guards exit from stomach
• Pylorc sphincter regulates entry of food to the
small intestine (duadenum) from the stomach
• Rugae
• Ridges and folds in relaxed stomach
• Allow the stomach to stretch
The Stomach
Histology of the stomach
• In Fundus and body of the stomach there are gastric pits
• Each gastric pit communicate with several gastric gland
• Gastric glands are dominated with two types of secretary
cells:
• Parietal cells
• Secrete Intrinsic factor, and HCl
• Intrinsic factors are needed for vitamin B12 absorption
• HCl kill most acteria, denature proteins, inactivate food enzymes,
breakdown plant cells and connective tissue in meat, activate
pepsinogen to pepsin.
• Chief cells
• Secrete Pepsinogen (inactive porenzyme) that converts to pepsin (active
form) in the presence of acid.
• Pepsin breaks down proteins
• In kids chief cells also produce lipase and rennin (help milk digestion)
Histology of the stomach
• Pyloric glands
• Primarily produces mucous containing several
hormones
• Enteroendocrine cells
• G cells secrete gastrin
• Gastrin stimulates paraietal cells and chief
cells to secrete HCl and pepsinogen i.e. HCl
and pepsinogen is produced in response to
gastrin
• D cells secrete somatostatin
• Somatostatin inhibits gastrin secretion
The Stomach Lining
The Stomach Lining
The Secretions of Hydrochloric Acid
• H+ and Cl- ions are
transported
separately outside
the cell into the
lumen of the gastric
gland.
• Negative (Cl-) and
positive (H+) ions
attract each other
and HCl is
produced in the
lumen of gastric
gland.
Regulation of gastric activity
• Gastric control and production is done in 3
phase:
• Cephalic phase - prepares stomach to receive ingested
material
• Begins when one sees, smells, tastes or thinks of food
• Increases gastric juice production
• Gastric phase - begins with the arrival of food in the
stomach
• Neural, hormonal, and local responses further increase secretion
• Intestinal phase - controls the rate of gastric emptying
i.e. control the rate of chime entry into duodenum
• Inhibit gastric acid and pepsinogen production
The Phases of Gastric Secretion
The Phases of Gastric Secretion
The Phases of Gastric Secretion
Digestion and absorption in the stomach
• Preliminary digestion of proteins
• Pepsin –enzyme that digests proteins
• However due to time limit digestion of protein can not be completed
• Permits digestion of carbohydrates by enzyme
lipase until the pH falls to 4.5
• Absorption does not occur in stomach because:
• Epithelial cells are not exposed to chime (covered by alkaline
mucus)
• Epithelial cells lack transport mechanism
• Gastric lining is impermeable to water
• Digestion is not completed
• Some drugs, however, are absorbed
Small Intestine (SI)
• Plays Important role in digestion and absorption
• Mucosa of SI produce few enzymes, and buffer to
neutralize chime
• Divided into three part:
• Duodenum – receives chime from stomach, bile from
gall bladder, and digestive secretion from pancreas
• Digestion continues in duodenum
• Jejunum – digestion and absorption takes place here
• Ileum
• Ileocecal sphincter
• Transition between small and large intestine
Regions of the Small Intestine
The Intestinal Wall
The Intestinal Wall
Intestinal movements
• Peristalsis
• Segmentation
• Gastroenteric reflexes
• Initiated by stretch receptors in stomach
• Gastroileal reflex
• Triggers relaxation of ileocecal valve
The pancreas
• Pancreatic duct penetrates duodenal wall
• Pancreas is both an endocrine and exocrine gland
• Endocrine functions
• Secretes Insulin and glucagons
• Exocrine functions
• Produces majority of pancreatic secretions
• Pancreatic juice secreted into small intestine contain:
• Carbohydrases- to digest carbohydrates
• Lipases – to digest lipids
• Nucleases
• Proteolytic enzymes
The Pancreas
The liver
• Performs metabolic and hematological regulation
and produces bile
• Histological organization
• Consist of two lobes
• Each lobe is divided into about 100K lobule
• Lobule is the functional unit of liver
• Lobules unite to form common hepatic duct
• Duct meets cystic duct to form common
bile duct
The Anatomy of the Liver
Liver lobule is the basic functional unit of the
liver
• Hepatocytes form irregular plates arranged in
spoke-like fashion
• Bile canaliculi carry bile to the bile ductules
• Bile ductules lead to portal areas
Liver Histology
The gallbladder
• Functions of gall bladder are that it:
• Stores bile
• modifies bile and
• concentrates bile
• Bile slat break large droplet of fat for digestion
Animation: Accessory Organ (see tutorial)
The Gallbladder
Coordination secretion and absorption
• Neural and hormonal mechanisms coordinate
glands
• GI activity is stimulated by parasympathetic
innervation
• GI activity is inhibited by sympathetic
innervation
The Activities of Major Digestive Tract
Hormones
•
This diagram summarizes
the activities of major
hormones:
•
1) Chyme arrives in
Duodenum
•
2) Arrival of chime in the
intestine stimulates
secretion of GIP, CCK and
secretin by the small
intestine
•
3) secretin stimulate
release of biocarbonate
and pancreatic juice by the
pancreas, inhibits
secretion of acid and
pepsinogen by the stomach
(see red arrow and blue
line)
•
4) CCK stimulate release of
bile by the gall bladder,
relase of pancreatic juice
by the pancreas and
inhibits secretion of acid
and pepsinogen by the
stomach (see red arrow
and blue line)
Functions of the large intestine
• its major functions include:
1. absorption of water
2. absorption of vitamins produced by bacteria
3. storage of fecal material prior to defecation
Large intestine areas:
• Large intestine is divided into three areas:
• Cecum – collects and stores material from
ileum
• Colon – it consist of four regions: ascending
colon, transverse colon, descending colon and
sigmoid colon
• Rectum – it is expandable
The Large Intestine
Carbohydrate digestion and absorption
• Begins in the mouth
• Salivary and pancreatic enzymes
• Disaccharides and trisaccharides
• Brush border enzymes
• Monosaccharides
• Absorption of monosaccharides occurs across the
intestinal epithelia
Lipid digestion and absorption
• Lipid digestion utilizes lingual and pancreatic
lipases
• Bile salts improve chemical digestion by
emulsifying lipid drops
• Lipid-bile salt complexes called micelles are
formed
• Micelles diffuse into intestinal epithelia which
release lipids into the blood as chylomicrons
Protein digestion and absorption
• Low pH destroys tertiary and quaternary
structure
• Enzymes used include pepsin, trypsin,
chymotrypsin, and elastase
• Liberated amino acids are absorbed
Absorption
• Water
• Nearly all that is ingested is reabsorbed via
osmosis
• Ions
• Absorbed via diffusion, cotransport, and active
transport
• Vitamins
• Water soluble vitamins are absorbed by
diffusion
• Fat soluble vitamins are absorbed as part of
micelles
• Vitamin B12 requires intrinsic factor
Age related changes in the digestive system
include:
• Thinner, more fragile epithelium
• Reduction in epithelial stem cells
• Weaker peristaltic contractions
• Effects of cumulative damage
• Increased cancer rates
You should now be familiar with:
• The organs of the digestive system and their
major functions
• The mechanisms that regulate digestion
• The anatomy of the organs and accessory organs
of the digestive system
• The functions of the major structures and regions
of the digestive system and the regulation of their
activities
You should now be familiar with:
• The significance of the large intestine in the
absorption of nutrients
• The events involved in the digestion of organic
and inorganic nutrients
• The effects of the aging process on the digestive
system