Chapter 15
Lecture Outline*
The Digestion and Absorption
of Food
Eric P. Widmaier
Boston University
Hershel Raff
Medical College of Wisconsin
Kevin T. Strang
University of Wisconsin - Madison
*See PowerPoint Image Slides for all
figures and tables pre-inserted into
PowerPoint without notes.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
Anatomy of the Digestive System
Fig. 15-1
2
4 Major Functions of the Digestive System
Fig. 15-2
3
Ingestion
Fig. 15-4
4
Structure of the Gastrointestinal Tract Wall
Fig. 15-5
5
Absorption in the Small Intestine
• The small intestine is anatomically arranged for a large
surface area, which enhances the absorption of
nutrients.
• One of the specialized anatomical structures is the
Villi. Villi increase surface area and contain blood
vessels and lacteal, which play a role in the absorption
of nutrients.
• Another specialized structure is the microvilli.
Microvilli increase surface area and form the brush
border.
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Villi
Fig. 15-6
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Carbohydrates
• Most carbohydrates in the diet are consumed as disaccharides
or polysaccharides:
–
–
–
–
–
–
Sucrose
Lactose
Maltose
Starch
Glycogen
Cellulose
• Only monosaccharides are absorbed by the intestinal cells for
use in the body.
• Disaccharides and polysaccharides must be digested to
monosaccharides before they can be absorbed for use in the
body.
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Carbohydrate Absorption and
Digestion
Fig. 15-8
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Carbohydrates
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Proteins
• Proteins are broken down to peptide fragments in the stomach
by pepsin, and in the small intestine by trypsin and
chymotrypsin, the major proteases secreted by the pancreas.
• These fragments are further digested to free amino acids by
carboxypeptidase from the pancreas and aminopeptidase, located
on the luminal membranes of the small intestine epithelial cells.
• The free amino acids then enter the epithelial cells by secondary
active transport coupled to Na+.
• Short chains of two or three amino acids are also absorbed by a
secondary active transport coupled to the hydrogen ion gradient.
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Protein Digestion and Absorption
Fig. 15-9
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Fats
Fig. 15-11
13
Fats
Fig. 15-12
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Fats
Fig. 15-13
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Vitamins
• Fat-soluble vitamins (A, D, E, and K) are
absorbed like other lipids.
• Water-soluble vitamins are absorbed by
diffusion or mediated transport, except for
vitamin B12, which must first bind to a
transport protein known as intrinsic factor.
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Water and Minerals
• Water and minerals are absorbed by diffusion
primarily within the small intestines.
• The absorption of Na+, Cl-, and K+ as well as
Ca2+ and iron are important to maintain
physiological processes.
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How are Gastrointestinal Processes
Regulated?
• Control mechanisms of the gastrointestinal
system are governed by the volume and
composition of the luminal contents, rather
than by the nutritional state of the body.
• This means that the body is designed to absorb
all the nutrients that are ingested, whether or
not the body really needs them to function.
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Basic Principles of Control
• Neural regulation comes through the CNS and
ENS:
– Enteric nervous system
• Submucosal plexus
• Myenteric plexus
– CNS contributions to neural control of the GI system
through regulation of the SNS and PSNS.
• Hormonal regulation
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Reflexes
Fig. 15-14
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Mouth, Pharynx, and Esophagus
Fig. 15-15
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Stomach
Fig. 15-17
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Gastric Gland
Fig. 15-18
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HCl Production
Fig. 15-19
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Regulation of HCl Production
Fig. 15-20
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Stomach
Fig. 15-21
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Stomach Motility
Fig. 15-23
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Gastric Emptying
Fig. 15-25
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Pancreas
• The pancreas has both exocrine and endocrine
functions.
• The exocrine portion produces “pancreatic
juice.” This is rich in bicarbonate as well as
digesting enzymes.
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Pancreatic Secretions
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Pancreatic Secretion Control
Fig. 15-28
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Pancreatic Secretion Control
Fig. 15-29
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Liver
• The liver serves as a secretory organ. One of its major
functions is to secrete bile.
• The liver also processes and stores nutrients.
• The liver also serves as a filter and functions in the
removal of old red blood cells which leads to
hemoglobin processing and the generation of
bilirubin.
• The liver is also responsible for the synthesis of
plasma proteins (albumin, clotting proteins,
angiotensinogen, steroid binding proteins).
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Hepatic Portal System
• The Hepatic Portal System is a specialized
vasculature that delivers absorbed nutrients to the
liver for processing before they enter the general
systemic circulation.
• Nutrients are absorbed from the small intestine into
mesenteric veins which then carry them to the liver
via the hepatic portal vein.
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Liver Structure
Fig. 15-30
35
Bile Secretion and Liver Function
Fig. 15-31
Fig. 15-32
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Small Intestine
Fig. 15-33
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Large Intestine
Fig. 15-34
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Pathophysiology of the Gastrointestinal Tract
•
•
•
•
•
•
Ulcers
Vomiting
Gallstones
Lactose Intolerance
Inflammatory Bowel Disease
Constipation and Diarrhea
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Ulcers
• Ulcers affects approximately 10% of the population in the USA.
• An ulcer is an erosion of the lining of the GI wall. This is usually due to
pepsin and acid. This is called a peptic ulcer.
• They are most common in the lower part of the stomach and the initial part of
the small intestine.
• Symptoms include a chronic rhythmic and periodic gnawing or burning pain.
This can be alleviated by drinking milk or by using antacids.
• If the ulcer is deep enough it can affect the blood vessels and result in
bleeding.
• In severe cases the hole can go all the way through the wall and allow the
contents to leak out into the abdominal cavity. This is called a perforated
ulcer. This is very serious and can lead to an infection of the peritoneum
(peritonitis) and can be fatal.
• Ulcers can be caused by stress, chronic use of aspirin and other non-steroidal
anti-inflammatory medications (they decrease mucus and bicarbonate
production), chronic alcohol use, the bacteria helicobacter pylori.
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