1. School of Nursing & Midwifery Studies UWCM©
GI Tract Overview
Aim of the lectures
To provide a basic understanding of the structure and function of the gastrointestinal tract and its normal function
Margaret John
School of Nursing & Midwifery Studies UWCM©

2. Functions of the GI tract
1. Ingestion
2. Digestion
3. Absorption
CONCEPTS to ENGAGE
Motility - mechanical breakdown, propulsion
Secretion - enzymes, water & ions
Control -
of motility and secretion by nervous system and hormones
Mastication, salivary secretion and swallowing
Nutrient Transport mechanisms
Diffusion, facilitated and Active Transport
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Objectives from Fox
Describe the functions of the digestive system and lists its structures and regions.
Describe the layers of the gastrointestinal tract and the function(s) of each.
Describe the structure of the gastric mucosa, list the secretions of the mucosa and their functions, and identify the cells that produce each of these secretions.
Explain the roles of HCl and pepsin in digestion and explain why the stomach does not normally digest itself
Describe the structure and function of the villi, microvilli, and crypts of Lieberkuhn in the small intestine.
Describe the location and functions of the brush border enzymes of the intestine.
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Digestive System
GI tract divided into:
Alimentary canal.
Accessory digestive organs.
GI tract is 30 ft long and extends from mouth to anus.
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5. Functions of the GI Tract
Motility:
Movement of of food through the GI tract.
Ingestion:
Taking food into the mouth.
Mastication:
Chewing the food and mixing it with saliva.
Deglutition:
Swallowing the food.
Peristalsis:
Rhythmic wave-like contractions that move food through GI tract.

6. Functions of the GI Tract
Digestion:
Breakdown of food particles into subunits (chemical structure change).
Absorption:
Process of the passage of digestion (chemical subunits) into the blood or lymph.
Storage and elimination:
Temporary storage and elimination of indigestible food.
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7. Functions of the GI Tract
Secretion:
Includes both exocrine and endocrine secretions.
Exocrine:
HCl (Hydrochloric Acid),
H20 (water),
HC03- (Bicarbonate),
Bile-made by the liver, stored in gall bladder & secreted into duodenum,
lipase (breaks down fat),
pepsin (breaks down protein),,
amylase (breaks down carbohydrates),
trypsin (breaks down protein),and
histamine (stimulates production of HCl), etc. are secreted into the lumen of the GI tract.
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8. Functions of the GI Tract
Endocrine functions:
Stomach and small intestine secrete hormones to help regulate the GI system.
Examples
Gastrin,
Secretin,
Cholecystokinin (CCK),
Gastric Inhibitory Peptide (GIP),
Somatostatin, etc.
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9. Salivary Glands FUNCTIONS
Lubrication
Thins and dilutes food for swallowing
Anti acid function,
Buffers and neutralises acids, particularly important when there are acids of bacterial origin present
Bacteriostatic Bacteriolytic
Acts on endogenous bacteria
Digestive: Carbohydrates
(? fat)
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Oesophagus
A muscular tube that carries the food from the back of the throat to the stomach.
It must contract in a very co-ordinated fashion so we don't regurgitate our food or feel that our meals are sticking as we swallow.
It is inflammation within the oesophagus that gives the sensation of heartburn.
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Food chunks up here and amylase continues to work until food drops into the stomach
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Layers of GI Tract
Mucosa
Lining
Submucosa
(beneath the inner layer)
Muscularis
(Muscle Layer)
Serosa
(connective tissue outer layer)
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Mucosa
This lines the lumen of GI tract.
It consists of simple columnar epithelium.
The major absorptive and secretary layer
Lamina Propria: thin layer of columnar epithelium with many lymph nodes
Muscularis mucosae:Smooth muscle just inside the above layer -responsible for the folds.
Increase surface area
Goblet cells: Secrete mucus throughout the tract .
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Submucosa
Thick, highly vascular layer of connective tissue.
Absorbed molecules enter the blood and lymph vessels in this layer.
Submucosal plexuses (nerves):
Provide autonomic nerve supply to the muscularis mucosae.
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Muscularis
Responsible for segmental contractions and peristaltic movement through the GI tract.
Inner circular smooth muscle.
Outer longitudinal smooth muscle.
Contractions of these layers move food through the tract and pulverize the food.
Myenteric plexus located between 2 muscle layers.
It is the major nerve supply to GI tract.
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Serosa
Binding, protective outer layer.
connective tissue.
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18. Regulation of the GI Tract
Nerve Supply
Parasympathetic Nervous System:
Stimulate motility.
Sympathetic Nervous System:
Reduce peristalsis and secretory activity.
The GI system Nervous Supply
Submucosal plexus and myenteric plexus:
Local regulation of the GI tract.
Endocrine secretion:
Molecules acting locally.
Hormonal secretion:
Secreted by the mucosa.
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Oesophagus
Concept: Deglutition (swallowing)
Oesophagus Connects pharynx to the stomach.
Upper third contain skeletal muscle.
Middle third contains a mixture of skeletal and smooth muscle.
Terminal portion contains only smooth muscle.
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Oesophagus
Peristalsis:
Local reflexes in response to distention of the wall of the Oesophagus by bolus. (food ball)
Wave-like contractions:
Circular smooth muscle contract behind, relaxes in front of the bolus.
Followed by longitudinal contraction (shortening) of smooth muscle.
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Stomach
Most distensible part of GI tract.
Empties into the duodenum.
Functions of the stomach:
Store food.
Initiate digestion of proteins.
Kills bacteria.
Moves food (chyme) into intestine.
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Stomach
Contractions of the stomach churn chyme.
Mix chyme with gastric secretions.
Push food into intestine.
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Stomach
Gastric mucosa has gastric pits in the folds.
Cells that line the folds deeper in the mucosa are gastric glands.
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Gastric Glands
Gastric Juice:
Goblet cells: mucous.
Parietal cells: HCl and intrinsic factor.
Chief cells: pepsinogen.
Enterochramaffin-like cells (ECL): histamine and serotonin.
G cells: gastrin.
D cells: somatostatin.
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25. Digestion and Absorption in the Stomach
Proteins partially digested by pepsin.
Hydrochloric acid changes the structure of proteins to kick start the digestive rocess
Carbohydrate digestion by salivary amylase is soon inactivated by acidity.
Alcohol and aspirin are the only commonly ingested substances absorbed.
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26. Protective Mechanisms of Stomach
Alkaline mucus contains HC03- (bicarbonate which bufers HCl and protects mucosa).
Tight junctions between adjacent cells.
Rapid rate of cell division (3 days).
Prostaglandins which inhibit gastric secretions.
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Small Intestine
In folds - villi.
Covered with columnar epithelial cells and goblet cells.
Epithelial cells are rubbed off by passing food and replaced by replaced
Lamina propria contain lymphocytes, capillaries, and a central lacteal (Lymph duct).
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28. Absorption in Small Intestine
Duodenum and jejunum:
Carbohydrates, Amino acids, Lipids, Ca++ (Calcium), and Fe++ (Iron),
Ileum:
Bile salts, vitamin B12, electrolytes, and H20.
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Intestinal Enzymes
Microvilli contain ‘brush border’ enzymes
Brush border enzymes remain attached to the cell membrane & react with chyme.
Absorption of nutrients requires both brush border enzymes and pancreatic enzymes.
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30. Intestinal Contractions and Motility
Contraction in the small intestine:
Peristalsis:
Slow movement.
Segmentation:
Major contractile activity of the small intestine.
Contraction of circular smooth muscle
This will also mix chyme
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Large Intestine
Absorption of H20, electrolytes,
vitamin B complex vitamins, vitamin K, and folic acid.
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32. Circulation between the liver & the intestine
compounds are secreted by the liver into the bile ducts.
these compounds flow into the intestine with the bile.
Eliminated in the faeces.Or reabsorbed back into the liver
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Gall bladder
Sac-like organ attached to the inferior surface of the liver.
Stores and concentrates bile.
Contraction of the muscle layer of the gallbladder, ejects bile into the common bile duct.
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Pancreas
Exocrine:
Secretes pancreatic juice.
Endocrine:
Secrete insulin and glucagon.
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35. Regulation of Gastric Function
Gastric motility and secretion are automatic.
Waves of contraction are initiated spontaneously by pacesetter cells.
Extrinsic control of gastric function is divided into 3 phases:
Cephalic phase.
Gastric phase.
Intestinal phase.
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