Transcript Digestive System

Chapter
24
The Digestive
System
Pages 874-928
PowerPoint® Lecture Slides
prepared by John L. Wilson
Copyright © 2009 Pearson Education, Inc.,
publishing as Pearson Benjamin Cummings
Functions of the Digestive Tract
Ingestion – intake of food (eating)
 Mechanical Digestion – chewing, swallowing,
peristalsis and other muscular actions of GI
tract
 Chemical Digestion – chemical break-down of
food molecules catalyzed by digestive enzymes
 Secretion – release of useful chemicals such as
enzymes and mucus by cells and glands
 Absorption – uptake of substances, such as
nutrients, into circulation
 Excretion – elimination of digestive wastes

Digestive Enzymes
Enzymes are protein catalysts
 Enzymes are not altered themselves
 Enzymes speed up chemical reactions,
and chemical digestion involves a series
of chemical reactions

Digestive Enzymes Continued
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 Mouth
 Mastication
 Another
name for chewing
 Breaks-up and lubricates food
 Swallowing
 Increases
surface area of food so
enzymes can work more efficiently
Digestion in Mouth

Salivary amylase, secreted by salivary
glands, digests starch into smaller
molecules, the smallest being the
disaccharide sugar maltose
Starch
amylase
Maltose
Swallowing (Deglutition) – Buccal Phase
Tongue pushes bolus of food from oral cavity into oropharynx
Swallowing – Pharyngeal Phase
Soft palate closes nasopharynx and epiglottis closes glottis
Swallowing – Esophageal Phase
Peristalsis propels bolus down esophagus toward stomach
Swallowing – Esophageal Phase
Cardiac (lower esophageal) sphincter opens and bolus enters stomach
Stomach Histology
Stomach Lumen
Gastric Pit----------------Gastric Gland------------
Muscularis - 3 layers
Mucosa – pits with
gastric glands
Submucosa with CT,
glands and vessels
Serosa – visceral
peritoneum
Gastric Glands

Gastric glands

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
Mucous Neck Cells
Mucous neck cells
secrete protective
mucus
Chief cells secrete
pepsinogen
Parietal cells Parietal Cell
secrete
Chief Cell
hydrochloric acid
G cells secrete G Cell
hormone gastrin
which stimulates
gastric secretions
Functions of the Stomach

Storage-can eat lots of food at one
sitting

Limited absorption in stomach
Aspirin and some other drugs
 Alcohol
 Some water


Electrolytes
Functions of the Stomach

Protection
Acid retards bacterial growth
 Some detoxification of alcohol


Chemical digestion
Protein digestion begins, but not completed
 Inactive enzyme pepsinogen secreted by
chief cells of gastric glands. Why must it be
inactive when secreted?

Chemical Digestion Details

Pepsinogen is converted to active enzyme
pepsin in stomach lumen (cavity) by pepsin
and hydrochloric acid (HCl)

Pepsin digests proteins to smaller chains of
amino acids called peptides
Pepsinogen HCl & pepsin
Protein
Pepsin
Pepsin
Peptides
Functions of Stomach
 Mechanical
 Mixing
Digestion in Stomach
waves every 15-20 seconds
 Reduce
food to liquid acid chyme
 Force small amounts of acid chyme from
stomach into the small intestine
Small Intestine Histology

Lined with about 4.5
million villi (villus)

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
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
Small Intestine Histology - Villus
Blood Capillaries
Simple Columnar Cells-------
Absorptive Cells –
digestion &
absorption
Goblet Cells – secretes
mucus
Lacteal – lymph
capillary crypt-------Intestinal
Endocrine Cell – secretes
intestinal hormones such
as gastrin,
cholecystokinin and
secretin
Small Intestine Physiology

Responsible for most 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


Absorbs most substances
Small Intestine Physiology

Mechanical digestion
Peristalsis propels chyme along intestine
 Segmentation moves chyme back and
forth to mix it thoroughly

Pancreas

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
Head, neck ,body
and tail
Head into
duodenum
Tail to spleen
Pancreatic duct
joins bile duct and
connects to
duodenum
Histology of Pancreas


Acini are exocrine cells
that secrete digestive
enzymes into ducts
Ducts secrete
bicarbonate to buffer
acid chyme from
stomach and help to
raise its pH from 2-3 to
7-8

Why raise intestinal pH?
Secretion of Pancreatic Juices
Secreted by acini and ducts
 About 1 liter secreted per day
 Contains enzymes to digest protein,
starch fat and nucleic acids (DNA, RNA)

Digestion by Pancreatic Enzymes

Protein Digestion
Four proteolytic enzymes secreted as
inactive proenzymes. Why?
 Proenzymes sequentially activated in
duodenum (first part of small intestine)
 These enzymes digest protein and
polypeptides to smaller peptides and
amino acids

Pancreatic Digestion

Starch digestion
Starch not digested by salivary amylase in
the mouth and stomach is digested in
intestine by pancreatic amylase
 Digestion same as in mouth

Starch
Pancreatic Amylase
Maltose
Pancreatic Digestion

Fat Digestion
Bile from the gallbladder prepares fat
molecules for chemical digestion
 Bile flows down bile duct into duodenum to
mix with and emulsify the fat
 What does emulsify mean?

Chemical Digestion of Fats (triglycerides)

Fat (triglyceride) 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
Digestion by Intestinal Enzymes
 Called
brush-border enzymes
 Located
in microvilli of intestinal
absorptive cells
 Peptidases

Digest peptides to amino acids
Intestinal Enzymes

Disaccharidases digest disaccharides to
monosaccharides
Sucrose
Maltose
Lactose
sucrase
maltase
lactase
Glucose + Fructose
Glucose + Glucose
Glucose + Galactose
Absorption in the Small Intestine

Absorption is the uptake of substances
into absorptive cells then into blood and
lymph
 Villi and microvilli of absorptive cells
provide large surface area for absorption
 Most digested foods, water, electrolytes,
vitamins and other substances absorbed
in small intestine
Absorption into Blood
Monosaccharides
 Amino acids
 Water
 Electrolytes such as
sodium, potassium.
 Water soluble vitamins
such as vitamin C and
the B vitamins

Absorption Into Lacteals (lymph)

Lacteal
Fat soluble vitamins
 Fatty acids and monoglycerides
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Bile salts help with absorption into
intestinal cells (micelles)
Triglycerides (fat) reformed in intestinal
cells
Triglycerides combine with protein
and are transported to lymph of lacteal
(chylomicrons)
Triglycerides are then carried by
protein to the blood. Where?
Functions of Large Intestine
Feces formation by bacterial action
 Limited digestion of undigested food by
bacteria
 Formation of vitamin K and some B
vitamins by bacteria
 Absorption of some water, electrolytes,
vitamins and bile salts

Functions of the Liver

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Carbohydrate, lipid and protein metabolism
Removal of waste products such as bilirubin
Storage of glycogen, vitamins and iron
Phagocytosis by Kupffer cells
Bile secretion
Plasma protein synthesis
Detoxification of toxins such as alcohol
Bile
 One-half
liter to a liter of bile each
day
 Functions of bile
Emulsification of fat in small intestine
 Excretion of bilirubin and cholesterol
 Aid in fat absorption

Medical Terms

Gingivitis-inflammation of gums

Periodontal disease – plaque causes
destruction of periodontal membrane and
jaw bone with possible loss of teeth

Stomatitis – inflammation of mouth

Dental caries - cavities
Medical Terms

Esophagitis and GERDS – acid reflux from
stomach into esophagus

Gastritis – inflammation of stomach

GI neoplasms – cancers of GI tract

Hepatitis – inflammation of liver, several
types

Peptic ulcer – erosion of stomach/duodenal
lining, usually caused by a bacterium
Acid Blocking Drugs*

Histamine (H2) blockers
Tagamet
 Zantac
 Pepcid AC


Proton pump blockers
Prilosec
 Nexium
 Prevacid
