The Digestive

System

Digestive TractMouthPharynxEsophagusStomach Small intestine Large intestineRectumAnus

TeethTongueSalivary GlandsPancreasLiverGallbladder

Accessory Organs/Structures

Nutrition is the process of taking in food and using it for growth, metabolism, and repair.

Nutritional Stages:IngestionDigestion AbsorptionTransport

AssimilationExcretion

Digestive Processes Ingestion – taking food/drink into mouth.

Mechanical Digestion – Physical breakdown of food (chewing, churning).

Chemical Digestion – Enzymatic breakdown of chemical bonds from complex to simple.

Polymers Monomers

Absorption – moving nutrients and digested products from lumen of G.I. tract to into the circulatory system (blood and lymph).

Excretion – elimination of indigestible and waste products from body (feces).

Transport – Distribution of absorbed nutrients around the body via the circulatory system (blood and lymph).

Assimilation – Incorporation of newly acquired nutrients into body’s cells and tissues.

inside body

The Journey of Ingested Food• The GI tract is 25 to 30 feet long muscular ‘tube’ that makes

the digestive tract.• 5 Organs (Regions) of GI Tract:

– Mouth – taste, chewing, getting food into solution

– Esophagus – bolus ready for transport down to stomach

– Stomach - acidic chyme liquefies everything

– Small Intestine – chemical breakdown and absorption

– Large Intestine – water and ions absorption, compaction

Sphincters are smooth muscle that open and close regions of the tract controlling food movement from one organ to the next.

Digestion Begins in the Mouth• Both Mechanical and Chemical Digestion begin in the mouth.

– Mechanical: chewing, grinding, squeezing, – Chemical : Breaking down food via enzymatic reactions

• Saliva– Dissolves small food particles.

– Contains the enzyme lingual amylase, which begins to break down carbohydrate. Plus lysozymes (antibacterial).

– *Lingual lipase in young, but in adults, no other chemical digestion of nutrients takes place in the mouth.

Tongue – contains taste buds on surface for 5 tastes:Sweet, Sour, Salt, Bitter and Umami

Nose – together with olfaction (sense of smell), the basic tastes in the mouth combine with the fragrances to finely discriminate very specific tastes.

The Role of the Epiglottis

Mouth• Once adequately chewed and moistened, the tongue

rolls food into a bolus and pushes into pharynx (throat) to be swallowed.

• Esophagus transports bolus and fluids to stomach.

• There are two sphincters in the esophagus:– Upper esophageal sphincter: Allows the bolus of food

to enter the esophagus– Lower esophageal sphincter (LES): Allows the bolus of

food to enter the stomach

The Stomach• The Stomach has more muscle layers than any other

region! Mixes food with gastric juices and churns contents = Mechanical Digestion

• HCl (pH of 2) released to assist Protein Digestion

• When empty stomach holds 1 cup…expands ~ 1 gallon!

• Chyme (partially digested food) leaves stomach and enters the Small Intestine.

Anatomy of the Stomach

Pyloric Sphincter - gateway for chyme to Small Intestine

Gastrointestinal HormonesActivate 2nd messenger system to carry out the task, usually in the form of ATP or Calcium

Gastrointestinal Sphincters (Valves)Sphincter - a circular muscle acting as valves to regulate passage or flow of material. Including:

• Upper Esophageal Sphincter

• Lower Esophageal Sphincter - prevents backflow (reflux) or ‘heartburn’ of stomach.

• Pyloric Sphincter controls movement of stomach contents into small intestine.

Sphincters at Work

Sphincters respond to stimuli from nerves, hormones and pressure

AKA cardiac sphincter

• Ileoceal Valve controls flow from the small intestine into the large intestine.

• Internal Anal Sphincter – composed of Smooth muscle (involuntary control).

• External Anal Sphincter – composed of skeletal muscle (voluntary control).

• Once toilet-trained, a child knows when to relax the external sphincter and when to keep it constricted.

Small Intestine• 3 Segments in the Small Intestine:

– Duodenum: 10 inches long

– Jejunum: 8 feet long

– Ileum: 12 feet long

• Both Mechanical and Chemical Digestion occur here.

• Pancreas squirts Pancreatic Juices into Duodenum for complete Digestion of everything!

• Surface of small intestine highly specialize for Absorption - lined with villi to maximize absorption.

• Contents may be in there from 3 to 10 hours, depending on the food eaten.

Large Intestine Absorbs Water & Electrolytes• About 5 feet long and 2.5 inches in diameter.

• The large Intestine has 3 main segments.

– Cecum: The beginning of the intestine

– Colon: The largest part of the intestine

– Rectum: Final 8-inch portion of the large intestine

• Chyme enters large intestine via ileocecal valve.

• Site of H2O, Na+, K+ and Cl- absorption.

• Bacteria make vitamin K, thiamin, riboflavin, biotin, and vitamin B12, only biotin vitamin K absorbed : (

Anatomy of the Large Intestine

The Large Intestine• Bacteria in colon ferment some undigested and

unabsorbed carbohydrates (fibers) into simpler compounds, methane gas (CH4), CO2, and H2.

– Fermented fiber produces short-chain fatty acids.

• In large intestine, 1 liter of fluid material is gradually reduced to 200 grams of brown fecal material.

• The intestinal matter passes through the large intestine in 12 to 70 hours, depending on the person's health, age, diet, and fiber intake.

Large Intestine

• Stool propelled through large intestine until it reaches the rectum, the last 8-inch portion gut.

• The anus is the opening of the rectum, or end of the GI tract.

• The final stage of defecation is under voluntary control. *External Anal Sphincter and abdominal muscle

Note: When bile is metabolized by your gut bacteria, it makes stercobilin = brown-ish.

Accessory Organs

• Extrinsic Salivary Glands

– Dissolve small food particles to lubricate food.

– In preparation for swallowing bolus

– The body produces 1 quart of saliva per day..

Saliva contains water, mucus, electrolytes, immunoglobbulins and a few enzymes

• Liver– Largest internal organ of body, wt = ~3 lbs.– Major role in digestion, absorption, and transport of

nutrients.– Essential in carbohydrate metabolism.– It makes Proteins.– Hepatocytes make bile used to Emulsify Fats.– Site of alcohol metabolism– Removes and Degrades Toxins & Excess Hormones.

Accessory Organs

• Gallbladder– Stores Bile from the liver via common hepatic duct– Concentrates Bile– Releases Bile to small intestine via common bile

duct

• Pancreas– Exocrine Gland: Secretes digestive enzymes into the

small intestine (pancreatic juices!)

– Endocrine Gland: Releases Hormones to maintain blood glucose levels: Insulin and Glucagon.

Accessory Organs

Propelling Food through the GI Tract

• Food propelled along GI tract by strong muscular contractions.

• Two primary contractions are:– Peristalsis: Squeezes food forward through the GI

tract (Mechanical Digestion)– Segmentation: Shifts food back and forth along

the GI tract in the intestines and adds chemical secretions

*Allows more contact with surface of small and large intestines and increases absorption.

Peristalsis and Segmentation

How Is Food Chemically Digested?Digestive Enzymes

Catalyze (speed up) Reactions:Hydrolysis

• Chemical rxn uses water (H2O) to split chemical bonds of digestible nutrients.

• The hydroxyl group (OH) from water joins one molecule while the hydrogen ions (H) joins the other molecule.

– Also Regulated by Hormones– e.g., Gastrin, CCK, Secretin, GIP, Ghrelin, Leptin, Chemical Digestion Completed by the time Food

reaches the Large Intestine

A Digestive Enzyme in Action

3 Conditions Required for Enzymes to Work

1. The compatible enzyme and nutrient are both present, e.g. Sucrase hydrolyzes sucrose.

2. pH of surroundings must be in optimal range.• Outside of that range of acidity and alkalinity,

the enzyme activity is decreased or halted.

3. Temperature of environment must be optimal.• Enzyme activity is slowed if temperature too

low and halted if temperature is too high.

How Are Digested Nutrients Absorbed?

Vast majority of absorption occurs in Small Intestine

Nutrients are Absorbed via:Passive Diffusion: Nutrients move from high concentration to low

concentration; no E required.

Facilitated Diffusion: Nutrients move from high concentration to low concentration with the help of a carrier protein; no E is required.

Active Transport: Nutrients move from low concentration to high concentration with the help of a carrier protein; E is required.

Endocytosis: Cell forms a vesicle to surround and engulf a nutrient. E is required

4 Methods of Nutrient Absorption in theSmall Intestine

Fluid Absorption Occurs in the Large Intestine

• Majority of nutrients already absorbed when chyme enters the large intestine.

• Water and Electrolytes are absorbed Here!

• Same mechanisms used in the small intestine are also used in the large intestine.

– Water: Passive Diffusion– Sodium: Active Transport

How Do Hormones and the Nervous SystemRegulate Digestion?

• The Endocrine and Nervous Systems together coordinate digestion, absorption, and excretion of waste products.

• Endocrine System communicates with the nervous systems built into the GI tract, called “Enteric” NS

• Enteric Nervous System controls Digestion with a network of nerve fibers that innervate the GI tract, pancreas, and gallbladder.

• Enteric Nerves monitor stomach contractions after eating and the secretions of the cells in the GI tract.

Hormones in the GI Tract Regulate Digestion

• Hormones regulate digestion by controlling: – Release of gastric and pancreatic secretions– Peristalsis– Enzyme Activity

• Enterogastrones are produced and secreted by the cells lining the stomach and small intestine.– Influence GI motility, stomach emptying, gallbladder

contraction, intestinal absorption & hunger.

– Release of hormones is stimulated by the types of food passing through the digestive tract.

– Gastrin: Triggered by food in stomach - stimulates acid (HCl) release and gastric enzymes (commence!)

– Secretin: Triggered by acidic chyme & partially digested proteins – stimulates pancreas to release bicarbonate (base) to small intestine; pH is raised

– Cholecystokinin (CCK): Triggered by fat and proteins, stimulates the pancreas to release lipase and the gallbladder to release bile; (slows down gastric motility).

– Gastric Inhibitory Peptide (GIP): Inhibits gastric motility and stomach secretions.

Gastrointestinal Hormones

Hormones Communicate Hunger and Satiety

• Ghrelin - triggers hunger; released by stomach, pancreas goes to feeding center in Hypothalamus. Inhibited by glucose but not fructose

• Leptin – Triggers satiety center in hypothalamus signals satiety center which turns OFF the feeding center. Released by adipocytes in fat tissue.

The Nervous System in GI Tract• Nervous System sends information about

when to eat and drink, and when to stop.• Extrinsic Nerves:

– Communicate changes in the GI tract and stimulate motility

– Originate in the brain or spinal cord

• Intrinsic Nerves: more neurons than brain!– Receive message from extrinsic nerves & respond

by stimulating the release of digestive juices– Interwoven in the linings of the esophagus,

stomach, and small and large intestines

• Fat-soluble nutrients are absorbed into the lymphatic system first, then delivered to blood.

• Fat-soluble vitamins, long-chain fatty acids, proteins too large to be transported via the capillaries.

• The waste products that remain after nutrient absorption are removed by the excretory system.

• Kidneys filter the blood, allowing waste products to be concentrated in the urine and excreted.

Blood and Lymph Receive Nutrients

Large Intestine and KidneyEliminate Wastes

What Are Some Common DigestiveDisorders?

• Digestive Disorders From Minor to Serious.Include:

• Ulcers• Gallbladder disease• Celiac disease• Flatulence• Diarrhea/constipation• Belching• Gastroenteritis

• Hemorrhoids• Irritable bowel syndrome• Ulcerative colitis• Diverticulitis • Crohn's disease• Colon cancer• Heartburn

An Ulcer and Gallstones

A hole in the lining of the tract!Gastric and duodenal.

Bile, billirubin or cholesterol become too concentrated in gallbladder!

Celiac Disease

• A genetic autoimmune disorder damages small intestine when foods containing gluten are consumed

• Causes the villi of small intestinal to flatten out, causing nutrient malabsorption

• Caused by an inflammatory response to the protein gluten, found in many grains

(wheat, rye, barley, oats).• Symptoms

– Reoccurring abdominal bloating– Cramping and/or gas– Diarrhea– Foul-smelling stools– Weight loss– Anemia– Bone or joint pain

Increased risk for osteoporosis, stunted growth, seizures

Treatment - Follow a gluten-free diet. Eliminate all foods that contain wheat.Include milk, meat, eggs, fruits, vegetables, rice, potatoes, beans, millet, amaranth.Moderate amounts of oats ( ¼ c) may be tolerated.

• Flatulence Results from the formation of intestinal gas. Most

adults release 10 to 20 times a day. – Caused by:

• Foods high in fiber (*fermentable) and starch• Eating too quickly• Drinking carbonated beverages • Lack of Exercise• Smoking?• Poor Food Combining (e.g. fruits w/Pro-s)

• Diarrhea: Passage of watery, loose stools more than 3 times/day.

– From bacterial, viral, or parasitic infections - food and fluids to pass too quickly through colon.

– Chronic diarrhea sign of more serious problems?

– Untreated diarrhea can lead to malnutrition.

– Can lead to dehydration and potentially death, (esply children and elderly).

– Treated with fluid and electrolyte replacement

• Constipation: Infrequent passage of dry, hardened stools.

– Often due to insufficient fiber or water intake– Other causes include stress, inactivity, quitting

smoking! And various illnesses.

– Treatments: • Exercise, improved eating patterns, and proper rest.•Laxatives… sparingly, they cause dehydration, salt imbalances, and laxative dependency.• Colon cleansing (enema) as a treatment?

• Hemorrhoids: Inflamed, swollen and herniated veins of the

rectum and anus – Can lead to bleeding, itching, and/or pain. – May be from straining to pass dry stools, pregnancy,

constant constipation or diarrhea.

• Hemorrhoids: – Treatments:Increased dietary fiber and

fluid intake.

Itching and pain may be relieved with ointments, ice packs, and soaking in a warm bath.

Severe cases may require surgery.

Hematochezia - passage of fresh (bright red) blood through the anus, usually in or with stools. Commonly associated with lower gastrointestinal bleeding.

Diverticulitis results when one of these diverticula becomes inflamed (or infected).

Treatments: –Increase Dietary Fiber–Ingest Less Toxins

• Diverticulitis: Typically occurring in the colon, Diverticulitis

involves the formation of pouches called “diverticula” within the bowel wall.

• This is called Diverticulosis

• Crohn's Disease: Similar to ulcerative colitis, but ulcers can occur

throughout entire the GI tract– There is no known cause or cure.– Treatments?

Ulcerative Colitis Crohn’s Disease Polyp

• Colon Cancer: The 2nd leading cause of cancer death, but one of

the most curable cancers if detected early – begins with polyps on the lining of the colon that are

often small, benign, and can be surgically removed.• Polyps… cancerous tumors if not detected early.

Treatment includes …– Survival rates vary depending on age, treatment

response, and stage of cancer diagnosis.