The Digestive System

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Objectives

• Name and locate the digestive organs and the accessory organs of digestion.

• List the general function of the digestive system and the function of each organ.

• Define mechanical and chemical digestion and tell where each takes place.

• Define terms related to the digestive system. • Name the origins and functions of the various

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Objectives

• Correctly label a diagram of the digestive

system

• State the purposes of HCl

• Name and locate the sphincters of the

digestive tract and describe the function of

each.

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Digestion

• The process by which food is broken down

mechanically and chemically and

converted into absorbable forms.

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• The main goal of digestion is to break

down macronutrients into molecules that

are readily absorbed.

• This is accomplished through mechanical

and chemical means

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Mechanical Digestion

• GI motility: Beginning in the mouth,

muscles and nerves in the tract coordinate

their actions to provide motility, an

automatic response to the presence of

food.

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Mechanical Digestion, cont’d

• Muscles

– Muscle tone/tonic contraction: Ensures

continuous passage of the food mass and valve control along the way

– Periodic muscle contraction and relaxation: Rhythmic waves that mix the food mass and move it forward

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Mechanical Digestion, cont’d

• Nerves

– Specific nerves regulate muscle action along the GI tract

– The intramural nerve plexus is the network of nerves in the GI wall extending from the

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Chemical Digestion

• Digestive enzymes: Break down nutrients

• Hydrochloric acid and buffer ions: Produce the correct pH necessary for enzyme activity

• Mucus: Lubricates and protects the GI tract tissues and helps mix the food mass

• Water and electrolytes: Carry and circulate the products of digestion through the tract and into the tissues

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End Products of Digestion

• Carbohydrates

– Starches, disaccharides digested to

monosaccharides (e.g. glucose, fructose, galactose)

• Proteins digested to amino acids

• Fats digested to fatty acids and glycerol

• Vitamins, minerals and water

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Three essential processes

• Secretion: Delivery of enzymes, mucus,

ions and the like into the lumen, and

hormones into blood.

• Absorption: Transport of water, ions and

nutrients from the lumen, across the

epithelium and into blood.

• Motility: Contractions of smooth muscle in

the wall of the tube that crush, mix and

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The Alimentary Canal

• Long tube

• Begins at the mouth • Ends at the anus

• Food moved via peristalsis

• Mechanical digestion • Chemical digestion

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• Mouth: Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant. In some species, saliva contains amylase, an enzyme that digests starch.

• Esophagus: A simple conduit between the mouth and stomach - clearly important but only marginally interesting compared to other regions of the tube.

• Stomach: Where the real action begins - enzymatic digestion of proteins initiated and foodstuffs reduced to liquid form.

• Liver: The center of metabolic activity in the body - its major role in the digestive process is to provide bile salts to the small intestine, which are critical for digestion and absorption of fats.

• Pancreas: Important roles as both an endocrine and exocrine organ - provides a potent mixture of digestive enzymes to the small

intestine which are critical for digestion of fats, carbohydrates and protein.

• Small Intestine: this is where the final stages of chemical enzymatic digestion occur and where almost all nutrients are absorbed.

• Large Intestine: water is absorbed, bacterial fermentation takes place and feces are formed

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Digestion in the Mouth and

Esophagus

• Mechanical digestion

– Mastication breaks down food.

– Food is swallowed and passes down esophagus. – Muscles at tongue base facilitate process.

– Gastroesophageal sphincter at stomach entrance relaxes, allowing food to enter, then constricts to retain food.

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Digestion in the Mouth and

Esophagus, cont’d

• Chemical digestion

– Salivary glands secrete material containing salivary amylase or ptyalin.

– Ebner’s glands at the back of the tongue secrete a lingual lipase.

– Salivary glands also secrete a mucous material to lubricate and bind food particles, facilitating the swallowing of the food bolus.

– Secretions from the mucous glands in the esophagus help move food toward the stomach.

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Structural Layers of the Alimentary

Canal

• Four basic layers

– Mucosa

– Submucosa – Muscle

– Serosa

• Exact structure may vary

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Stomach

• Five major functions

– Temporary food storage

– Control the rate at which food enters the duodenum

– Acid secretion and antibacterial action – Fluidisation of stomach contents

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Stomach: Four Regions with

Different Functions

• Fundus • Body • Pylorus • Cardiac

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Digestion in the Stomach

• Mechanical digestion

– Under sphincter control, the food enters the upper portion of the stomach as individual bolus lumps.

– Stomach muscles knead, store, mix, and propel the food mass forward.

– By the time the food mass reaches the lower portion of the stomach, it is a semiliquid acid/food mix called chyme.

– Chyme is released slowly into the first section of the small intestine (duodenum) by the pyloric valve.

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Digestion in the Stomach, cont’d

• Chemical digestion: three types of gastric secretions

– Hydrochloric acid: Parietal cells in the stomach lining secrete acid to promote gastric enzyme activity.

– Mucus: Secretions protect the stomach lining from the erosive effect of the acid and also bind and mix the food mass and help move it along.

– Enzymes: Pepsinogen is secreted by stomach cells and activated by acid to become pepsin, a protein-splitting enzyme.

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The Stomach: Secretions

• Mucous cells secrete mucous

• Chief cells secrete pepsinogen

• Parietal cells produce HCl and secrete

intrinsic factor

• HCl converts pepsinogen to pepsin

• Acid environment kills microorganisms

• G cells secrete gastrin

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Proton Pumps

• Parietal cells

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Small Intestine

• 1 inch diameter • 20 feet long • Duodenum • Jejunum • Ileum • Peyer’s patches

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Digestion in the Small Intestine

• Mechanical digestion

– Peristaltic waves slowly push food mass forward. – Pendular movements sweep back and forth.

– Segmentation rings chop food mass into successive soft lumps and mix them with secretions.

– Longitudinal rotation rolls food in a spiral motion, exposing new surfaces for absorption.

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Digestion in Small Intestine,

cont’d

• Pancreatic enzymes

– Carbohydrate: Pancreatic amylase converts starch to maltose and sucrose.

– Protein: Trypsin and chymotrypsin split large protein molecules into small peptide fragments and

eventually into single amino acids.

– Fat: Pancreatic lipase converts fat to glycerides and fatty acids.

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Absorption in the Small Intestine

• Three absorbing structures

– Mucosal folds: Surface of small intestine piles into folds

– Villi: Small, finger-like projections cover the

mucosal folds, increasing the area of exposed intestinal surface

– Microvilli: Smaller projections cover each villi (look like bristles on a brush)

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Absorption Processes

• Simple diffusion: The force by which particles move

outward in all directions—from areas of greater to lesser concentration.

• Facilitated diffusion: Similar to simple diffusion but uses a protein channel to carry larger items.

• Active transport: The force by which particles move from areas of greater to lesser concentration using a carrier to “ferry” particles.

• Pinocytosis: Penetration of larger materials by attaching to the cell membrane and being engulfed by the cell.

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Large Intestine

• AKA: the colon

• 2.5 inches in diameter

• 5 feet long

• Joined to the ileum at the ileocecal valve

• Functions: absorption of water, minerals,

vitamins and elimination of waste

• Appendix

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Absorption in Large Intestine

• Water is taken up by the large intestine

– Most water in chyme is absorbed in the first half of the colon

– Only a small amount remains to form and eliminate feces

• Dietary fiber is not digested

– Contributes bulk to food mass – Helps form feces

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Absorption and Transport

• Carbohydrates: Reduced to simple sugars (glucose, fructose, galactose)

• Fats: Changed into fatty acids and glycerides • Proteins: Changed into single amino acids

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Transport

• Nutrients must be transported to cells • Vascular (blood circulatory) system

– Veins and arteries

– Transports waste, such as carbon dioxide and nitrogen, to lungs and kidneys for removal

• Lymphatic system

– Route for fatty materials, which are not water soluble – Fat molecules pass into lymph vessels in villi

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Accessory Organs of Digestion

• Liver

• Gall bladder

• Pancreas

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Liver Functions

• Carbohydrate metabolism • Amino acid metabolism • Lipid metabolism

• Synthesis of plasma proteins • Formation of biliruben

• Phagocytosis • Storage

• Detoxification

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Gall Bladder

• Sac 3 to 4 inches long • Undersurface of the

right liver lobe

• Concentrates bile by absorbing water

• Not a gland, just a storage sac

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Pancreas

• Upper left abdominal quadrant

• Endocrine and exocrine functions

• Exocrine glands: acini

– Lipase – Amylase

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Pancreatic Juice

• pancreatic amylase: This enzyme digests carbohydrates.

• pancreatic lipase: This enzyme digests a variety of lipids. It works especially well on

triglycerides, which are the fats and lards that we normally think about in our diets.

• trypsinogen/trypsin: Trypsinogen is what is actually secreted by the pancreatic acinar

cells, but then an enzyme in the small intestines changes trypsinogen into its active enzymatic form, trypsin. The enzyme that converts trypsinogen into trypsin is called

enterokinase, and I'll describe this more on the regulation webpage. But trypsin is an

enzyme that digests proteins.

• chymotrypsin: This is an enzyme that digests proteins.

• carboxypeptidase: This is an enzyme that digests proteins.

– Why do we need three enzymes to digest proteins? Because proteins are made from a pool of 20 amino acid building blocks that can come together in any order. And each enzyme only cuts through the connection between specific amino acids. So the more enzymes for breaking down protein, the more places the protein can be split and the smaller the pieces we can derive from the breakdown.

• nucleases: These are enzymes that digest nucleic acids. The word "nuclease" represents

any nucleic acid breaker, and there are a few different ones that the pancreas secretes.

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Factors Influencing GI Tract

Secretions

• Nervous control

• Conditioned reflexes

• Oral reflexes

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Hormones involved in Digestion

• The presence of food in the stomach stimulates stretch receptors which relay this information to the medulla

oblongata. The medulla stimulates endocrine cells in the stomach to secrete the hormone gastrin into the

circulatory system. Gastrin stimulates the stomach to secrets gastric juice.

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Secretin

Secretin is produced by cells of the duodenum.

It’s production is stimulated by acid chyme from stomach. It stimulates the pancreas to produce sodium bicarbonate,

which neutralizes the acidic chyme. It also stimulates the liver to secrete bile.

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• CCK production is stimulated by the

presence of food in the duodenum.

• It stimulates the gallbladder to release bile

and the pancreas to produce pancreatic

enzymes.

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• GIP (Gastric Inhibitory Peptide)

• Food in the duodenum stimulates certain endocrine cells to produce GIP.

• It has the opposite effects of gastrin; it inhibits gastric glands in the stomach and it inhibits the mixing and churning movement of stomach

muscles. This slows the rate of stomach

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• Sympathetic impulses decrease

contractions and peristalsis

• Epinepherine is the neurotransmitter

hormone of the sympathetic system

• Parasympathetic increases

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Bioavailability

• Bioavailability refers to:

– Amount of nutrient present in the GI tract

– Competition between nutrients for absorption – Form in which the nutrient is present

• All nutrients present in a food are not

absorbed because of differing

bioavailability.

– This is considered when determining dietary intake standards

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Metabolism

• Nutrients are converted to energy or

stored in the body

• Metabolism: The sum of body processes

that change our food energy from the

three energy nutrients

– Chemical reactions within cell to maintain life – Occurs in mitochondrion of the cell

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Metabolism, cont’d

• Two metabolic processes

– Catabolism: Breaking down of large

substances into smaller units (e.g., breaking down a protein chain into amino acids)

– Anabolism: Building of larger substances from smaller particles (e.g., building a complex

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Metabolism, cont’d

• Metabolic processes ensure that the body has energy in the form of adenosine triphosphate (ATP).

• Metabolism of glucose from carbohydrates

yields less energy than metabolism of fat. Still, glucose is the body’s primary source of energy. • Protein can be an energy source, but it is

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Energy Storage: Glycogenesis

• Glycogenesis: Anabolic process of

converting extra glucose into glycogen

• Glycogen is stored in the liver and

muscles for quick energy to be used at a

later time

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Nursing Assessment

• Inspection

• Auscultation

• Percussion

• Palpation

• Questions

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GI Risk Factors

• Family history of GI disorders • Chronic laxative use

• Tobacco use

• Chronic alcohol consumption • Chronic high stress

• Chronic NSAID use

• Previous abdominal surgeries • Neurologic disorders

• Medication use that can lead to constipation • Cardiac, respiratory, endocrine disorders

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Genetic Disease

• Phenylketonuria

– Protein metabolism

• Galactosemia

– Carbohydrate metabolism

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Lactose Intolerance

• Most common disaccharidase deficiency

• Lactase in insufficient amounts, not absent

• Causes abdominal cramping and diarrhea

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Genetic Disease

• Phenylketonuria

– Protein metabolism

• Galactosemia

– Carbohydrate metabolism

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Lactose Intolerance

• Most common disaccharidase deficiency

• Lactase in insufficient amounts, not absent

• Causes abdominal cramping and diarrhea

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Summary

• Nutrients from food must be changed, released, regrouped, and rerouted into forms the body can use.

• The activities of digestion, absorption, and

transport ensure that key nutrients are delivered to the cells so metabolic tasks can be

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Summary, cont’d

• Mechanical digestion consists of spontaneous muscular activity responsible for initial

mechanical breakdown and the movement of the food mass along the GI tract by the motion of

peristalsis.

• Chemical digestion involves the enzymatic action that breaks food down into smaller components and releases nutrients for absorption.

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Summary, cont’d

• Absorption involves the passage of food

nutrients from the intestines into the mucosal lining of the intestinal wall.

• Nutrients absorbed are transported throughout the body by the circulatory system.

• Metabolism is the sum of the body processes

that change food energy taken in (carbohydrate, protein, and fat) into various forms of energy

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Summary, cont’d

• Metabolism is balanced by two types of

metabolic actions

– Catabolism – Anabolism

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Vocabulary for Digestion

• lysozyme • Borborygmi • Gastrectomy • Endoscopy • Esphagoplasty • choleylithiasis • choleycystectomy • Gastritis • Stomatitis • Peyer’s patches • Deamination • Lacteal • Peritoneum • sublingual