DIGESTIVE SYSTEM

•Gastroenterology - the study of the digestive sytem

Structures of Digestive System•1. Gastrointestinal tract (GI)▫Also called the alimentary canal▫Long continuous tube from the mouth to the anus (~ 9 meters long)▫Includes mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus

•2. Accessory Organs▫Teeth, tongue, salivary glands, liver, gall bladder, pancreas (most do not come in direct contact with the food)

Functions of Digestive System

•1. Ingestion - eating/drinking•2. Secretion - ~ 7L of H2O, acid, buffers, enzymes per day•3. Mixing/Propulsion - alternate contraction & relaxation of smooth muscle lining the GI tract▫Mixes food & secretions, propels them toward the anus

•4. Digestion▫Mechanical - begins in the mouth with teeth chewing & grinding food▫- continues in the stomach with churning & mixing food with digestive enzymes▫Chemical – carbohydrates, proteins and lipids are broken down ▫- digestive enzymes speed up this process

•5. Absorption – fluids and molecules from chemical digestion pass through the lining of the small intestine into the bloodstream•6. Defecation - elimination of waste material (feces…more commonly known as “poop”)

Walls of the Digestive Tract•1. mucosa – innermost mucus membrane layer•2. submucosa – surrounds mucosa•3. muscularis - outside the submucosa (2 layers of smooth muscle)•4. serosa – below the diaphragm (secretes serous fluid for lubrication)

Upper GI Tract

•Includes:▫mouth (salivary glands, tongue, teeth, hard & soft palate)▫Pharynx▫Esophagus▫Stomach

Salivary Glands

•3 main glands that produce and secrete saliva▫1. parotid gland – largest; anterior & inferior to the ear▫2. sublingual gland – under the tongue▫3. submandibular gland – floor of the mouth

Composition of Saliva•99.5% H2O•0.5% solutes, including Na+, K+, Cl-, HCO3

-, PO4-3, bacteria inhibitors, enzymes

and mucus•Salivary amylase – breaks down starches•Lingual lipase – breaks down lipids•HCO3

-, PO4-3 act as buffers to keep pH

between 6.35-6.85•Mucus – acts as lubrication for food

Upper GI Tract SecretionsSubstance

Secretion

Classif-ication

Secreted By

Secreted Into

Acts On Function

Saliva Salivary Amylase

Enzyme

Salivary glands

Mouth starch Breaks down starch to polysac-charides

Lingual Lipase

Enzyme

Salivary glands

Mouth Lipids (in stomach)

Breaks down lipids

Mucus ~~~~~~

Salivary glands

mouth Food Moistens food

Functions of Saliva

•1. cleansing the mouth•2. solvent for taste•3. lubricate food•4. breakdown starches•5. inhibits bacterial growth

Salivation

•Controlled by the nervous system•Touch, taste, smell, sight & sound of food cause increases in salivation•Normal conditions – moistens mouth & esophagus and cleanses teeth• stress – secretions stop (mouth gets dry)•Eating – secretions increase

Function of the Tongue in Digestion

•Move food in the mouth and to back of mouth for swallowing (degluttition)•Contains taste buds – found on the papillae (bumps) of the tongue•Gustation - taste

Mechanical Digestion in the Mouth

•Mastication – chewing▫Food is manipulated by the tongue, ground by the teeth and mixed with saliva to form a bolus (chewed up, soft mass)

Chemical Digestion in the Mouth

•Salivary amylase – breaks down polysaccharides (starch) into maltose▫Continues in the stomach ~ 1 hour after being swallowed

•Lingual lipase – breaks down (triglycerides) lipids into fatty acids▫Becomes activated in the acidic stomach environment, so happens after food is swallowed

Teeth

•Two sets▫Deciduous (baby teeth) – 20▫Permanent (including wisdom) -32•3 types of teeth▫Incisors - sharp edges (biting,cutting)▫Cuspids – pointed (grasp, tear)▫Molars - flat (grinding)

Parts of teeth•Crown - visible portion of tooth•Enamel - hardest substance in the

body; surrounds dentin in the crown•Root -portion that is in the sockets of

mandible and maxilla•Pulp - central core of tooth; blood

vessels & nerves•Dentin – surrounds pulp cavity; a living

cellular substance similar to bone•Neck –region between crown and root•Root canal - pulp cavity

Other parts of the Mouth

•Hard palate - roof of the mouth•Soft palate - skeletal muscle & connective tissue; ends in the uvula•Uvula - moves food away from the nasal cavity & into the oral pharynx

Pharynx

•3 parts of pharynx▫Nasopharynx – upper region; contains tonsils & adenoids; functions in respiration▫Oropharynx – behind oral cavity; first region where food enters when swallowed▫Laryngopharynx - behind larynx

Stages of Swallowing

•1. Voluntary Stage – bolus to back of mouth and into oropharynx•2. Pharyngeal Stage – bolus from oropharynx into esophagus

Involuntary Airway closes, breathing is interrupted Soft palate & uvula close off nasopharynx Epiglottis blocks respiratory tract

•3. Esophageal Stage - bolus goes from esophagus into stomach▫Peristalsis - rhythmic contractions that move the bolus along the digestive tract▫When bolus is at the end of the esophagus, the cardiac sphincter relaxes and food enters the stomach

Functions of the Stomach

•1. mix saliva, food & gastric juices to form chyme (semisolid mixture)•2. hold food until it moves into the small intestine•3. secretes gastric juice into stomach•4. secretes gastrin into blood

Mechanical Digestion in the Stomach

•Food enters stomach through a valve called the gastroesophageal sphincter, which keeps food from going back into the esophagus•Mixing waves in the body of the stomach & the pylorus move the chyme into the duodenum

Chemical Digestion in the Stomach

•Gastric Juice consists of:▫1. Mucus – protects stomach lining▫2. Pepsinogen - breaks down proteins▫3. Gastric lipase – breaks down fats▫4. HCl – kills bacteria; stimulates secretion of gastrin (a hormone)▫5. Intrinsic factor - necessary to absorb Vitamin B12 in the small intestine

Upper GI Tract SecretionsSubstance

Secretion

Classif-ication

Secreted By

Secreted Into

Acts On Function

Gastric Juice

Pepsin (Pepsinogen)

Enzyme Gastric Glands

stomach Proteins Breaks proteins into peptides

Gastric Lipase

Enzyme Gastric Glands

Stomach Lipids Breaks down lipids

HCl Acid Gastric Glands

Stomach Contents of stomach

Decreases pH; kills bacteria

Mucus ~~~~~~

Gastric Glands

Stomach Walls of stomach

Protects from acid

Gastrin Hormone Gastric Glands

Blood Gastric glands; sphincters

Stimulates gastric glands & cardiac sphincter; inhibits pyloric & ileocecal

Intrinsic Factor

~~~~~~

Gastric Glands

Stomach Vitamin B12

Lets B12 get absorbed

3 Phases of Stomach Digestion•1. Cephalic Phase▫Controlled by nerve stimuli▫Sight, smell, sound or taste of food causes increased nerve impulses▫Stomach secretes pepsinogen, HCl and mucus into the chyme▫Gastrin is secreted into the bloodstream▫Stomach churning increases

•2. Gastric Phase▫Stomach stretches to accommodate food

▫pH in stomach increases▫Gastric juices are secreted▫Gastrin – hormone secreted by the stomach Stimulates gastric glands Strengthens the contraction of the cardiac sphincter

Relaxes pyloric & ileocecal sphincters▫pH below 2 - secretions inhibited▫pH above 2 – secretions stimulated

•3. Intestinal Phase▫Activated by the small intestine▫Inhibitory effect on the stomach▫Slowing of stomach emptying▫2 hormones secretedSecretin – decreases gastric secretionsCCK (cholecystokinin) - inhibits stomach emptying

▫Stomach empties in 2-4 hours…first carbs, then proteins, lastly fats

Lower GI Tract

•Duodenum through Anus

Small Intestine•Duodenum (approx 10 inches)▫Recieves chyme from the stomach▫Pancreatic juice & bile enter here

•Jejunum (approx 3 feet)▫Begins where duodenum turns downward

•Ileum (approx 6 feet)▫Ends at ileocecal sphincter, which prevents feces from backing into small intestine

mesentary – thin tissue that prevents small intestines from twisting or tangling

Functions of Small Intestine

1. Digestion2. Absorption

- food remains in intestine for 3-5 hours- Intestinal glands secrete intestinal juice, which contains enzymes:- sucrase, lactase, maltase, peptidase & enterokinase

Digestive Secretions – Lower GISubstance

Secretion

Classif-ication

Secreted By

Secreted Into

Acts On Function

Intestinal Juice

Sucrase Enzyme Small Intestine

Small Intestine

Sucrose (breaks into - )

Glucose & fructose

Intestinal Juice

Lactase Enzyme Small Intestine

Small Intestine

Lactose (breaks into-)

Glucose & galactose

Intestinal Juice

Maltase Enzyme Small Intestine

Small Intestine

Maltose (breaks into -)

2 or 3 molecules of glucose

Intestinal Juice

Peptidase

Enzyme Small Intestine

Small Intestine

Peptides (breaks into -)

Amino acids

Intestinal Juice

Entero-kinase

Enzyme Small Intestine

Small Intestine

Trypsin-ogen (breaks into -)

trypsin

Characteristics of Small Intestine that increase surface area for absorption

•1. Length - ~ 10 feet long•2. Villi – finger-like folds in mucosa•3. Microvilli – hair-like projections in villi•4. Circular folds – causes chyme to “spiral”

instead of moving straight through

- 90% of absorption occurs in small intestine- nutrients, electrolytes, water, Vitamins A, B, C,

D, E and K are absorbed (Vitamin B12 must be bound to intrinsic factor in order to be absorbed)

Hormones of the Small Intestine•These secretions act to stimulate the pancreas▫1. Secretin – tells the pancreas to secrete sodium bicarbonate ions to increase pH

▫2. CCK (Cholecystokinen) – tells the pancreas to secrete digestive enzymes & gall bladder to secrete bile

Digestive Secretions – Lower GISubstance Secretion Classifi-

cationSecreted by

Secreted Into

Acts On Function

Intestinal Juice

Secretin Hormone

Small intestine

Circulatory system (blood)

Stomach & pancreas

Inhibit gastric secretions, stimulate pancreatic buffers

Intestinal Juice

Chole-cystokinin (CCK)

Hormone

Small intestine

Circulatory system (blood)

Stomachpancreas & gall bladder

Inhibit stomach emptying, stimulate pancreatic enzymes, stimulate release of bile

Pancreas

•Produces and secretes insulin, which regulates blood sugar

•Produces pancreatic juice which is secreted into the small intestine

Pancreatic Juice

•Contains water, salts, sodium bicarbonate and enzymes▫sodium bicarbonate – keeps pH between 7.1-8.2 to buffer the acidic chyme from the stomach, stop the action of pepsin, and activate digestive enzymes in the small intestine

Pancreatic Enzymes•There are pancreatic enzymes that act to

break down carbohydrates, proteins and lipids.▫Pancreatic amylase - acts on carbohydrates▫Trypsin & chymotrypsin – act on proteins,

but are secreted in an inactive form so as to protect the pancreas from their effects (trypsinogen & chymotrypsinogen) Trypsinogen & entrokinase react to form

trypsin Trypsin and chymotrypsinogen react to form

chymotrypsin▫Pancreatic lipase - act on lipids

Digestive Secretions – Lower GISubstance

Secretion Class-ification

Secreted By

Secreted Into

Acts On Function

Pancreatic Juice

Pancreatic Amylase

Enzyme

Pancreas Small Intestine

Carbohy-drates

Break into disacch-arides

Pancreatic Juice

Trypsin (trypsino-gen)

Enzyme

Pancreas Small Intestine

Proteins Break into peptides

Pancreatic Juice

Chymotryp-sin (chymotry-psinogen)

Enzyme

Pancreas Small Intestine

Proteins Break into peptides

Pancreatic Juice

Pancreatic Lipase

Enzyme

Pancreas Small Intestine

Lipids Break into fatty acids

Liver•Heaviest gland, 2nd largest organ in the body•Multi-lobed organ•Hepatocytes – liver cells•Functions of liver▫1. produces bile - a yellow-greenish fluid consisting

of water, bile salts (helps break down fats), bile pigments (gives urine & feces their color) and cholesterol (a by-product of lipid metabolism)

▫2. stores glycogen▫3. filtration of blood absorbed from small intestine▫4. detoxification – alters chemical composition of

harmful substances to make them less harmful

Gall Bladder

•Function is to store and concentrate bile

•Fats in chyme cause CCK to be secreted into the blood…CCK, in turn, causes secretion of bile

Summary of Carbohydrate Digestion

•1. Salivary Amylase - secreted in mouth; breaks starches into maltose (a disaccharide)

•2. Pancreatic Amylase - secreted into small intestine; breaks polysaccharides into disaccharides

•3. Secreted into small intestine; breaks disaccharides into monosaccharides▫Sucrase – sucrose into glucose and fructose▫Lactase – lactose into glucose and galactose ▫Maltase - maltose into glucose (2-3

molecules)

Summary of Protein Digestion•1. Pepsin – produced & secreted in

stomach in inactive form of pepsinogen; breaks proteins into peptides

•2. Trypsin & Chymotrypsin - secreted from pancreas into small intestine in inactive form of trypsinogen and chymotrypsinogen; breaks proteins into peptides

•3. Peptidases – produced & secreted in small intestine; breaks peptides into amino acids

Summary of Lipid Digestion

•All three break lipids into fatty acids

•1. Lingual lipase – secreted in the mouth

•2. Gastric lipase - produced & secreted in the stomach

•3. Pancreatic lipase - secreted from pancreas into small intestine

Large Intestine•Approx. 5 ft long, 2 ½ inches in diameter•Begins at ileocecal sphincter (where

material passes from small intestine into large intestine)

•Cecum – inferior to ileocecal sphincter; basically a blind pouch

•Colon – longest portion; ascending, transverse, descending

•Sigmoid – S-shaped curve•Rectum - last 8 inches•Anal Canal - last inch of rectum; opens to

outside at the anus

•No villi present nor enzymes secreted•Goblet cells – secrete mucus for

lubrication•Haustra – a series of pouches that give the

colon a “puckered” appearance

•Functions of Large Intestine▫1. abosorb water▫2. produce and excrete solid waste

Formation and Expulsion of Feces

•Water and electrolytes are absorbed•Bacteria (normally present) - break down any

remaining carbs and proteins▫ - they also produce hydrogen, carbon dioxide

and methane gases, which contribute to flatus (gas in the colon)

• Feces – consists of water, salts, sloughed off cells, bacterial, unabsorbed or indigestible parts of food & mucus

•Defecation – discharge of feces from the rectum▫ - under voluntary control of external anal

sphincter