1. Introduction to the
Gastrointestinal System
Chapter 56
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2. GI System
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3. GI System (cont.)
• The GI system is the only system in the body open to the
external environment
• Composed of one continuous tube
– Begins at the mouth
– Progresses through the esophagus, stomach, and
small and large intestines
– Ends at the anus
• Accessory Organs
– Pancreas
– Liver
– Gallbladder
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4. Major Activities of the GI System
• Secretion: Of enzymes, acid, bicarbonate, and mucus
• Absorption: Of water and almost all of the essential
nutrients needed by the body
• Digestion: Of food into usable and absorbable component
• Motility: Movement of food and secretions through the
system
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5. Layers of the GI Tube
• Mucosa
• Muscularis Mucosa
• Nerve Plexus
• Adventitia
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6. Layers of Nerves in the Nerve Plexus
• Submucosal Layer
• Myenteric Layer
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7. Layers of the Gastrointestinal Tract
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8. Control of the GI System
• GI system is controlled by the nerve plexus
– Maintains basic electrical rhythm
– Responds to local stimuli to increase or decrease
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activity
• Activity of GI tract can be influenced by the autonomic
system
• Initiation of activity depends on local reflexes

9. Types of Secretions of the GI Tract
• Saliva
• Mucus
• Acid and Digestive Enzymes (Gastrin)
• Secretin
• Sodium Bicarbonate
• Pancreatic Enzymes, Other Lipases, and Amylases
• Bile
• Endocrine Hormones
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10. Question
Please answer the following statement as true or false.
The GI system is controlled by the nerve plexus which
maintains basic electrical rhythm.
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11. Answer
True
Rationale: GI system is controlled by the nerve plexus,
which maintains basic electrical rhythm and responds to
local stimuli to increase or decrease activity.
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12. Digestion
• Is the process of breaking food into usable, absorbable
nutrients
• Begins in the mouth with enzymes in the salvia
• The stomach continues the digestion process
• In the small intestines, the food mixes with bile which
breaks down the fat molecules
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13. Absorption
• Is the active process of removing water, nutrients, and
other elements from the GI tract
• Once removed from the GI tract, they are delivered to
the bloodstream for use in the body
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14. Motility
• GI tract depends on an inherent motility to keep things
moving through the system.
• In the esophagus, the basic movement is peristalsis.
• The stomach uses its three muscle layers to produce a
churning action.
• Small intestines use a process of segmentation.
• Large intestines use a process of mass movement.
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15. Local Gastrointestinal Reflexes
• Gastroenteric Reflex: Stimulation of the stomach by
stretching
• Gastrocolic Reflex: Stimulation of the stomach causing
increased activity in the colon
• Duodenal–Colic Reflex: Stimulation of colon activity and
mass movement by the presence of food or stretch in the
duodenum
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16. Other Local GI Reflexes
• Ileogastric Reflex
• Intestinal–Intestinal Reflex
• Peritoneointestinal Reflex
• Renointestinal Reflex
• Vesicointestinal Reflex
• Somatointestinal Reflex
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17. Centrally Mediated Reflexes of the GI
Tract
• Swallowing Reflex
– Stimulated when a food bolus stimulates pressure
receptors in the back of the throat and pharynx
• Vomiting Reflex
– Protects the system from unwanted irritants
– Stimulated by two centers in the medulla
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18. Swallowing Reflex
• Stimulated whenever a food bolus stimulates pressure
receptors in the back of the throat and pharynx.
• This send impulses to the medulla, which stimulates a
series of nerves.
• Involves more than 25 pairs of muscles
• This reflex can be facilitated in a number of ways if
swallowing is a problem.
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19. Stimulation of the Chemoreceptor Trigger
Zone (CTZ)
• Tactile stimulation of the back of the throat
• Excessive stomach distention
• Increasing intracranial pressure by direct stimulation
• Stimulation of the vestibular receptors in the inner ear
• Stimulation of stretch receptors in the uterus and bladder
• Intense pain fiber stimulation
• Direct stimulation by various chemicals
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20. Maintaining Homeostasis of the GI Tract
• A series of local reflexes within the GI tract helps
maintain homeostasis within the system
• Overstimulation can result in:
– Constipation (underactivity)
– Diarrhea (overactivity)
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21. Question
Please answer the following statement as true or false.
When the renal capsule becomes irritated the
gastrointestinal tract shuts down.
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22. Answer
True
Rationale: Renointestinal Reflex: Irritation or swelling of
the renal capsule causes a cessation of movement in the
GI tract, again to prevent further irritation to the capsule.
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