gi tract overview aim of the lectures to provide a basic understanding of the structure and function...

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 JohnSchool of Nursing & Midwifery Studies UWCM©

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Functions of the GI tract1. Ingestion2. Digestion3. AbsorptionCONCEPTS to ENGAGEMotility - mechanical breakdown, propulsion • Secretion - enzymes, water & ionsControl - • of motility and secretion by nervous system and

hormones• Mastication, salivary secretion and swallowingNutrient 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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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.

Functions of the GI Tract

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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.

Functions of the GI Tract

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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.

Functions of the GI Tract

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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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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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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 .

Mucosa

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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.

Submucosa

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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.

Muscularis

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Serosa

Binding, protective outer layer.

connective tissue.

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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.

Regulation of the GI Tract

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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.

Oesophagus

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OesophagusPeristalsis: •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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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.

Stomach

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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 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.

Gastric Glands

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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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• 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.

Protective Mechanisms of Stomach

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Small IntestineIn 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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• Duodenum and jejunum:– Carbohydrates,

Amino acids, Lipids, Ca++ (Calcium), and Fe++ (Iron),

• Ileum:

– Bile salts, vitamin B12, electrolytes, and H20.

Absorption in Small Intestine

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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.

Intestinal Enzymes

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Intestinal Contractions and MotilityContraction 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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• Absorption of H20, electrolytes,

vitamin B complex vitamins, vitamin K, and folic acid.

Large Intestine

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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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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.

Gall bladder

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Pancreas

Exocrine:– Secretes

pancreatic juice.

Endocrine:– Secrete

insulin and glucagon.

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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.

Regulation of Gastric Function