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Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Introduction to the Gastrointestinal System
Introduction to the Gastrointestinal System
Chapter 56
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
GI System GI System
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
GI System (cont.)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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Major Activities of the GI SystemMajor 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Layers of the GI TubeLayers of the GI Tube• Mucosa
• Muscularis Mucosa
• Nerve Plexus
• Adventitia
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Layers of Nerves in the Nerve PlexusLayers of Nerves in the Nerve Plexus
• Submucosal Layer
• Myenteric Layer
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Layers of the Gastrointestinal TractLayers of the Gastrointestinal Tract
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Control of the GI SystemControl of the GI System
• GI system is controlled by the nerve plexus
– Maintains basic electrical rhythm
– Responds to local stimuli to increase or decrease activity
• Activity of GI tract can be influenced by the autonomic system
• Initiation of activity depends on local reflexes
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Types of Secretions of the GI TractTypes 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
QuestionQuestion
Please answer the following statement as true or false.
The GI system is controlled by the nerve plexus which maintains basic electrical rhythm.
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
AnswerAnswer
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.
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
DigestionDigestion
• 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
AbsorptionAbsorption
• 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
MotilityMotility
• 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.
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Local Gastrointestinal ReflexesLocal 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Other Local GI ReflexesOther Local GI Reflexes• Ileogastric Reflex
• Intestinal–Intestinal Reflex
• Peritoneointestinal Reflex
• Renointestinal Reflex
• Vesicointestinal Reflex
• Somatointestinal Reflex
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Centrally Mediated Reflexes of the GI Tract
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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Swallowing ReflexSwallowing 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.
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Stimulation of the Chemoreceptor Trigger Zone (CTZ)
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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Maintaining Homeostasis of the GI Tract 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)
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
QuestionQuestion
Please answer the following statement as true or false.
When the renal capsule becomes irritated the gastrointestinal tract shuts down.
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
AnswerAnswer
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.