Transcript Document

PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 24
The Digestive System
Part D
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Digestion in the Stomach
• The stomach:
• Holds ingested food
• Degrades it both physically and chemically
• Delivers chyme to the small intestine
• Enzymatically digests proteins with pepsin
• Secretes intrinsic factor required for absorption of
vitamin B12
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Regulation of Gastric Secretion
• Neural and hormonal mechanisms regulate the release
of gastric juice
• Stimulatory and inhibitory events occur in three
phases
• Cephalic (reflex) phase: prior to food entry
• Gastric phase: once food enters the stomach
• Intestinal phase: as partially digested food enters the
duodenum
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Cephalic Phase
• Excitatory events include:
• Sight or thought of food
• Stimulation of taste or smell receptors
• Inhibitory events include:
• Loss of appetite or depression
• Decrease in stimulation of the parasympathetic
division
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Gastric Phase
• Excitatory events include:
• Stomach distension
• Activation of stretch receptors (neural activation)
• Activation of chemoreceptors by peptides, caffeine,
and rising pH
• Release of gastrin to the blood
• Inhibitory events include:
• A pH lower than 2
• Emotional upset which overrides the
parasympathetic division
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Intestinal Phase
• Excitatory phase – low pH and partially digested food
enters the duodenum
• Inhibitory phase – distension of duodenum, presence
of fatty, acidic, or hypertonic chyme, and/or irritants
in the duodenum
• Initiate inhibition of local reflexes and vagal nuclei
• Closes the pyloric sphincter
• Releases enterogastrones that inhibit gastric secretion
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Release of Gastric Juice
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Figure 24.16
Regulation and Mechanism of HCl Secretion
• HCl secretion is stimulated by ACh, histamine, and
gastrin
• All work through second messenger systems
• Release of hydrochloric acid:
• Is low if only one ligand binds to parietal cells
• Is prolific if all three ligands bind to parietal cells
• Antihistamines and cimetidine block H2 receptors and
decrease HCl release
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Regulation and Mechanism of HCl Secretion
Figure 24.17
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Response of the Stomach to Filling
• Stomach pressure remains constant until about 1L of
food is ingested
• Relative unchanging pressure results from reflexmediated relaxation and plasticity
• Reflex-mediated events include:
• Receptive relaxation – as food travels in the
esophagus, stomach muscles relax
• Adaptive relaxation – the stomach dilates in response
to gastric filling
• Plasticity – intrinsic ability of smooth muscle to
exhibit the stress-relaxation response
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Gastric Contractile Activity
• Peristaltic waves move toward the pylorus at the rate
of 3 per minute
• This basic electrical rhythm (BER) is initiated by
pacemaker cells (cells of Cajal)
• Most vigorous peristalsis and mixing occurs near the
pylorus
• Chyme is either:
• Delivered in small amounts to the duodenum or
• Forced backward into the stomach for further mixing
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Gastric Contractile Activity
Figure 24.18
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Regulation of Gastric Emptying
• Gastric emptying is regulated by:
• The neural enterogastric reflex
• Hormonal (enterogastrone) mechanisms
• These mechanisms inhibit gastric secretion and
duodenal filling
• Carbohydrate-rich chyme moves through the
duodenum quickly
• Fat-laden chyme is digested more slowly causing
food to remain in the stomach longer
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Regulation of Gastric Emptying
Figure 24.19
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Homeostatic Imbalance
• Vomiting (emesis) – the stomach empties via a
different route (oral)
• Causes include extreme stretching, irritants such as
bacterial toxins, excessive alcohol, spicy foods, and
certain drugs
• The emetic center of the medulla initiates a number of
motor responses
• Diaphragm and abdominal wall muscle contract
• Cardiac sphincter relaxes and soft palate closes off
the nasal passages
• Excessive vomiting can cause dehydration and upset
electrolyte and pH balance
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Small Intestine: Gross Anatomy
• Runs from pyloric sphincter to the ileocecal valve
• Has three subdivisions: duodenum, jejunum, and
ileum
• The bile duct and main pancreatic duct:
• Join the duodenum at the hepatopancreatic ampulla
• Are controlled by the sphincter of Oddi
• The jejunum extends from the duodenum to the ileum
• The ileum joins the large intestine at the ileocecal
valve
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Microscopic Anatomy of the Small Intestine
• Structural modifications of the small intestine wall
increase surface area
• Plicae circulares: deep circular folds of the mucosa
and submucosa
• Villi: fingerlike extensions of the mucosa
• Microvilli: tiny projections of absorptive mucosal
cells’ plasma membranes
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Microscopic Anatomy of the Small Intestine
Figure 24.21a-c
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Small Intestine: Histology of the Wall
• The epithelium of the mucosa is made up of:
• Absorptive cells and goblet cells
• Interspersed T cells (intraepithelial lymphocytes),
and
• Enteroendocrine cells
• Intestinal crypts cells secrete intestinal juice
• Peyer’s patches are found in the submucosa
• Brunner’s glands in the duodenum secrete alkaline
mucus
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Intestinal Juice
• Secreted by intestine glands in response to distension
or irritation of the mucosa
• It is slightly alkaline and isotonic with blood plasma
• Is largely water, enzyme-poor, but contains mucus
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Liver
• The largest gland in the body
• Superficially has four lobes – right, left, caudate, and
quadrate
• The falciform ligament:
• Separates the right and left lobes anteriorly
• Suspends the liver from the diaphragm and anterior
abdominal wall
• The ligamentum teres:
• Is a remnant of the fetal umbilical vein
• Runs along the free edge of the falciform ligament
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Liver: Associated Structures
• The lesser omentum anchors the liver to the stomach
• The hepatic blood vessels enter the liver at the porta
hepatis
• The gallbladder rests in a recess on the inferior
surface of the right lobe
• Bile leaves the liver via
• Bile ducts which fuse into the common hepatic duct
• The common hepatic duct fuses with the cystic duct
• These two ducts form the bile duct
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Liver: Associated Structures
Figure 24.20
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