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DIGESTIVE SYSTEM
Anatomy and Physiology of the
Digestive Tract
DIGESTIVE ANATOMY
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Mouth and tongue
Salivary Glands
Pharynx
Esophagus
Stomach
Liver
Pancreas
Small intestine
Large intestine
LAYERS OF THE GI TRACT
(1) SEROSA:OUTER TOUGH CONNECTIVE TISSUE
MEMBRANE FOR PROTECTION.
(2) MUSCULARIS EXTERNA:LONGITUDENAL AND CIRCULAR
MUSCLE LAYERS FOR CONTRACTION
(3) SUBMUCOSA:LOOSE CONNECTIVE TISSUE, BLOOD
VESSELS AND GLANDS.
LAYERS OF THE GI TRACT
(4) MUCOSA:- MADE OF THREE LAYERS
(A) MUSCULARIS MUCOSA
(B) LAMINA PROPRIA
(C) EPITHELIUM LINNING
FUNCTION FOR DIGESTION AND
ABSORPTION OF NUTRIENTS.
LAYERS OF THE GI TRACT
Muscularis
Mucosa
Submucosa
FUNCTIONS OF THE GI TRACT
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INGESTION
MECHANICAL DIGESTION
CHEMICAL DIGESTION
SECRETION
ABSORPTION
EXCRETION
DIGESTIVE ENZYMES
• Enzymes are protein catalysts
• Enzymes are not altered themselves
• Enzymes speed up reactions at body
temperatures
• Digestive enzymes are called hydrolytic
enzymes. Water is used to split food
molecules
DIGESTIVE ENZYMES
• Enzymes are sensitive to such things as
temperature and pH
• The names of enzymes usually end in “ase”
For example, sucrase is the enzyme that
catalyzes the hydrolysis of the sugar sucrose
DIGESTION IN THE MOUTH
• Mastication
– Another name for chewing
– Breaks-up and lubricates the food
• Swallowing is easier
• Increases surface area of food so enzymes
can work more efficiently
DIGESTION IN THE MOUTH
• Salivary amylase, secreted by salivary glands,
digests starch into smaller molecules, the
smallest being the disaccharides.
Starch
amylase
Disaccharides
SWALLOWING
Tongue pushes bolus of food from oral cavity into oropharynx
SWALLOWING
Soft palate closes nasopharynx and epiglottis closes glottis
SWALLOWING
ESOPHAGEAL PHASE
Upper esophageal sphincter open
Peristalsis propels bolus down esophagus toward stomach
SWALLOWING
Cardiac (lower esophageal) sphincter opens and bolus enters stomach
STOMACH HISTOLOGY
Rugae---------------------
Gastric Pit----------------Gastric Gland------------
GASTRIC GLANDS
• Gastric glands
– Mucous neck cells
secrete protective
mucus
– Parietal cells secrete
hydrochloric acid
and intrinsic factor
– Chief cells secrete
pepsinogen and
gastric lipase
– G cells secrete
hormone gastrin
which stimulates
gastric secretions
Mucous Neck cells-----------
Parietal cell------------------Chief cell---------------------G cell----------------------------
FUNCTIONS OF THE STOMACH
(1) Storage:- can eat lots of food at one sitting
(2) Mechanical Digestion
– Mixing waves every 15-20 seconds
• Reduce food to liquid acid chyme
• Force small amounts of chyme from stomach into
the small intestine
(3) Chemical digestion
– Protein digestion begins
– Inactive enzyme pepsinogen secreted by chief cells of
gastric glands
CHEMICAL DIGESTION
• Pepsinogen Converted to active enzyme pepsin in
stomach lumen (cavity) by hydrochloric acid (HCl)
• Pepsin digests proteins to smaller polypeptides
Pepsinogen
Protein
HCl
Pepsin
Pepsin
Polypeptides
FUNCTIONS OF THE STOMACH
(4) Limited absorption
– Aspirin and some other drugs
– Alcohol
– Some water
– Electrolytes
SMALL INTESTINE HISTOLOGY
• Lined with about 4.5
million villi (villus)
– Small finger like extensions
– Covered with a simple
columnar mucous
membrane
– Blood capillaries inside for
absorbing most substances
– Single lymph capillary
called a lacteal for
absorbing most fat
THE VILLUS
---------Absorptive Cell
Simple Columnar Cells-------
Blood Capillaries-------------
Lacteal--------------------------
---------Goblet Cell
---------Endocrine Cell
-----------Paneth Cell of intestinal
crypt
SMALL INTESTINE FUNCTIONS
(1) Mechanical digestion
– Peristalsis propels chyme along intestine
– Segmentation move chymes back and forth to mix it
thoroughly
(2) Chemical digestion
– Enzymes from pancreas and small intestine complete
digestion of protein, starch, disaccharide sugars and fat
– Gallbladder empties bile into small intestine to aid in
fat digestion
(3) Absorption of most substances
THE PANCREAS
• Head, neck ,body and
tail
• Head into duodenum
• Tail to spleen
• Pancreatic duct joins
bile duct and connect
to duodenum
HISTOLOGY OF PANCREAS
• Acini are exocrine
cells that secrete
digestive enzymes
into ducts
• Duct cells secrete
bicarbonate to buffer
the acid chyme from
stomach and raise its
pH from 2-3 to 7-8
SECRETION OF PANCREATIC JUICE
• Enzymes are secreted by Acinar cells
• Bicarbonate solution is secreted by Ductal
cells
• About 1 liter of pancreatic juice is secreted
by the pancreas per day
• Bicarbonate solution neutralize the acidity
• Enzymes digest proteins, starch and fat
DIGESTION BY PANCREATIC
ENZYMES
• Proteins Digestion
– Four proteolytic enzymes secreted as inactive
proenzymes
– Proenzymes sequentially activated in
duodenum
– These enzymes digest proteins & polypeptides
to smaller peptides.
DIGESTION BY PANCREATIC
ENZYMES
• Starch digestion
– Remaining starch is digested in intestine by
pancreatic amylase
– Digestion same as in the mouth
STARCH > AMYLASE > DISACCHARIDES
DIGESTION BY PANCREATIC
ENZYMES
• Fats digestion
– Triglycerides (fat molecules made of glycerol
and three fatty acids) digested in small intestine
by pancreatic lipase
– Digestion of each triglyceride yields a
monoglyceride molecule and two fatty acid
molecules
Triglyceride
lipase
Monoglyceride + Two fatty acids
ROLE OF BILE
• Bile from the gallbladder is required for lipase to
digest fat more efficiently.
• Bile flows from gallbladder down the bile duct
into duodenum to mix with and emulsify the fat.
• Emulsification is breaking fat drops into very
small droplets
THE LIVER – Anterior View
THE LIVER
– Posterior View
LIVER HISTOLOGY
• Lobes contain
microscopic
lobules
• Lobules consist
of rows (plates)
of liver cells
(hepatocytes) and
rich blood supply
• Blood supplied
by branches of
the hepatic artery
and portal vein at
six corners of
lobule
LIVER HISTOLOGY
• Blood flows toward the center of each
lobule through liver capillaries (sinusoids)
• Rows of liver cells surround the capillaries
• Blood flows from the capillaries into the
central vein in the center of the lobule
• Liver macrophages called Kupffer cells are
in the capillaries
LIVER HISTOLOGY
central vein----------------------------------------Sinusoid---------------------------(liver capillary)
Hepatocyte------------------Kupffer cell-----------------------portal vein---------------------bile duct-----------------hepatic artery-------------------------
THE BILIARY SYSTEM
• Liver secretes bile
• Bile flows from liver
through hepatic ducts into
the ballbladder
• Gallbladder stores and
concentrates bile
• Common hepatic duct and
cystic duct from GB unite
to form common bile duct
• Common bile duct unites
with pancreatic duct
• Bile and pancreatic juices
enter the duodenum
THE BILIARY SYSTEM
• One-half to one liter of bile secreted by
the liver each day
• Functions of bile
– Emulsification of fat in small intestine
– Aids in fat absorption
– Excretion of bilirubin and cholesterol
FUNCTIONS OF THE LIVER
(1) Carbohydrate, lipid and amino acid metabolism
(2) Removal of waste products
(3) Storage of glycogen, vitamins and iron
(4) Phagocytosis by Kupffer cells
(5) Detoxification
(6) Bile secretion
(7) Plasma protein synthesis
DIGESTION BY INTESTINAL
ENZYMES
• Called brush-border enzymes
– Located in microvilli of intestinal absorptive cells
Peptidases digest peptides to amino acids
Intestinal lipase digest fats to glycerol & fatty acids
DIGESTION BY INTESTINAL
ENZYMES
• Disaccharidases digest disaccharides to
monosaccharides
Sucrose
sucrase
Glucose + Fructose
Maltose
maltase
Glucose + Glucose
Lactose
lactase
Glucose + Galactose
ABSORPTION IN THE SMALL
INTESTINE
• Absorption is the transfer (uptake) of substances
into absorptive cells then into blood or lymph
• Villi and microvilli of absorptive cells provide a
very large surface area for absorption
• Most digested foods, water, electrolytes, vitamins
are absorbed in the small intestine
ABSORPTION IN THE SMALL
INTESTINE
• Absorption into the blood
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Monosaccharides
Amino acids
Water
Electrolytes
Water soluble vitamins
ABSORPTION IN THE SMALL
INTESTINE
• Absorption into the lacteals (lymph
capillaries in villi)
– Fat soluble vitamins
– Fats (triglycerides)
• Bile aids in absorption of fat digestion
products (fatty acids and monoglycerides)
• Fat digestion products converted back to
triglycerides in absorptive cells
ABSORPTION IN THE SMALL
INTESTINE
• Triglycerides and other lipids combine with protein
to form small water soluble particles called
chylomicrons
• Chylomicrons absorbed into lymph of lacteals
INTESTINAL ABSORPTION
LARGE INTESTINE HISTOLOGY
• Simple columnar
mucosa
• Deep crypts with
intestinal glands
• Glands secrete
lots of mucus
• No villi
FUNCTIONS OF THE LARGE
INTESTINE
(1) Feces formation by bacterial action
(2) Limited digestion of undigested food by
bacteria
(3) Formation of vitamin K and some B
vitamins by bacteria
(4) Absorption of some water, electrolytes,
vitamins and bile salts
CLINICAL TERMS
• Gingivitis:- Inflammation of the gums
• Periodontal disease:- Inflammations of the
teeth, ligaments and alveolar bones
• Stomatitis:- Inflammation of the mouth
• Esophagitis:- Inflammation of esophagus
CLINICAL TERMS
• Gastritis:- Inflammation of the stomach
• Enteritis:- Inflammation of small intestine
• Diverticulitis:- inflammation of the colon
• Hepatitis:- inflammation of the liver
• Pancreatitis:- Inflammation of the pancreas