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Chapter 8

Digestive System and Nutrition

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Outline

• Overview of Digestion• The First Part of the Digestive Tract• The Stomach and Small Intestine• Three Accessory Organs and Regulation of

Secretions• The Large Intestine and Defecation• Nutrition and Weight Control

(Should review Ch.2, Molecules of Life, pp. 22-30)

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Overview of Digestion

• Also called the alimentary tract, gastrointestinal tract (G.I. tract), “gut”.

• From the mouth to the anus.• There are two types of digestive processes:

– 1. Mechanical. . . .– 2. Chemical. . . .

• Digestion of food is an extracellular process due to enzymes secreted into the gut by glands that reside in the lining or lie nearby.

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Overview of Digestion

• Functions of this system include: – Ingest food.– Digest it into small molecules that can

cross the cell membrane.– Movement of contents along tract.– Absorb nutrient molecules.– Eliminate nondigestible wastes.

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The Wall of the Digestive Tract• Layers from inside, out: (MSMS)

– Mucosa - Mucus membrane layer.– Submucosa - Submucosal layer.– Muscularis – Longitudinal and circular

smooth muscle layer.– Serosa - Serous membrane layer.

• Disorders:– Diverticulosis, diverticulitis, inflammatory

bowel disease (IBD), irritable bowel syndrome (IBS), appendicitis, peritonitis.

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The Digestive TractMouth : “Food Receiver”

• Functions: ingest, break down (mechanical), begin digestion (chemical), taste, form into bolus for swallowing.

• Roof of the mouth composed of two parts.Anterior (front) hard palate.Posterior (back) soft palate.

• Tonsils help protect against infection.• Three pairs of salivary glands (exocrine glands)

send juices through ducts to the mouth.– Saliva contains digestive enzymes

(bicarbonate and salivary amylase).

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The Digestive TractMouth : “Food Receiver”

• Teeth used to chew food into pieces small enough to swallow.– Each tooth has two main divisions.

Crown.Root.

• Tongue mixes chewed food with saliva and forms a bolus in preparation for swallowing.

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The Pharynx : “A Crossroad”

• The pharynx (throat) receives air from the nasal cavities and food from the mouth.– Swallowing is a reflex action that occurs

in the pharynx.Soft palate moves to close off

nasopharynx, and the trachea moves under the epiglottis to cover the glottis.

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Swallowing

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The Esophagus : “A Food Conductor”• Function?• The esophagus is a muscular tube that passes

from the pharynx through the thoracic cavity and diaphragm into the abdominal cavity.– Rhythmic peristalsis pushes food along the

digestive tract. – Sphincters encircle tubes and act as valves.– Heartburn vs. Acid reflux disease

( gastroesophageal reflux disease, GERD)

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HEALTH FOCUS

Heartburn (GERD)

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The Stomach : “Food Storer and Grinder”

• Functions: stores food, mixes food (mechanical), kills bacteria, and aids in digestion (chemical).– Columnar epithelial lining contains gastric

pits leading into gastric glands.Produce gastric juice containing HCl (pH

~2) and enzyme pepsin.– Walls of stomach have folds which help

churn and mix food and gastric juices.Chyme leaves the stomach and enters the

small intestine.

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The Stomach : “Food Storer and Grinder”

• Ulcers?• Form when HCl penetrates mucus barrier

and disintegrates tissue.• Called a peptic ulcer.• However, recently discovered that may be

caused by bacteria (Helicobacter pylori ) that inhibit mucus production!

• What is the significance of this discovery?• William Beaumont and Alexis St. Martin !?

HISTORICAL FOCUS

A Window to the Stomach: Alexis St. Martin and Dr. William

Beaumont

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The Small Intestine : “Food Processor”• Functions:

– Receives secretions from liver, pancreas.– Mechanical, chemical digestion of food.– Absorption of nutrients.– Transports undigested material to large

intestine.• Thus, is the primary site for chemical

breakdown of food and nutrient absorption.• ~ 1 inch wide, ~ 18 feet long (6 meters).

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The Small Intestine : “Food Processor”• The duodenum (first ~10”) is where the chemical

breakdown of food gets under way.– Receives bile from liver (gall bladder) to

“emulsify” fat.– Receives digestive enzymes and sodium

bicarbonate from the pancreas for chemical breakdown of food and to neutralize the chyme.

– Duodenal ulcers?• Along the small intestine are the villi (s. villus) –

the structural unit of the small intestine.

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The Small Intestine : “Food Processor”• Are covered by microvilli to increase the

surface area for absorption of nutrients.• Surface area ~ that of NFL football field!!!• Blood vessels absorb sugars (from CHO)

and amino acids (from proteins).• Lacteal – small lymph vessels which absorb

“fats” and transport / return them and fluids to the blood stream.

• Lactose intolerance?• Link between obesity, type 2 diabetes, and

cardiovascular disease?

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Three Accessory Organs• Pancreas

– Exocrine part produces digestive enzymes– Endocrine part secretes insulin and glucagon that

regulate blood glucose levels.• Liver maintains homeostasis by. . . .

– Produces bile (from breakdown of RBC’s).– Detoxifies blood.– Stores iron, fat-soluble vitamins: A, D, E, K– Maintains blood glucose levels (< 0.1%)– Produces urea.– Makes blood proteins (clotting, etc.)– Helps regulate blood cholesterol level (bile salts)

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Three Accessory Organs• Liver disorders:

– Jaundice. Hemolytic - RBC broken down. Obstructive - bile duct blocked due to

gallstones. Hepatitis – viral infection causing inflammation.

– Cirrhosis. Common among alcoholics. Liver becomes fatty and tissue is replaced by

inactive fibrous scar tissue . . . . liver cancer!• Gallbaldder stores excess bile. . . . Gallstones?

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Regulation of Digestive Secretions

• Secretion of digestive juices is promoted by the nervous system and by hormones.– Hormones are produced by one set of

cells and affect a different set of cells.Gastrin.Gastric inhibitory peptide (GIP).Secretin.Cholecystokinin (CCK).

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Hormonal Control of Digestive Secretions

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Hepatic Portal Circuit

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The Large Intestine : “H2O, salt Processor”• Functions:

– Absorbs water and salts.– Prepares and stores nondigestible material

until “defecation”.– Absorbs vitamins produced from bacterium E.

coli.• Is ~ 2 ½ in. wide, ~ 5 feet long.• Parts:

– Cecum is blind end of large intestine below entrance from small intestine.

Contains the vermiform (“worm-like”) appendix – plays role in immunity.

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The Large Intestine : “H2O, salt Processor”Appendicitis? peritonitis?

– Colon.Ascending, transverse, descending,

sigmoid.May develop polyps – small growths

arising from the epithelial lining.May be benign or malignant.May increase likelihood of colon cancer.Can be removed surgically.Risk reduced by eating a low-fat, high-

fiber diet.

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The Large Intestine : “H2O, salt Processor”– Rectum.

Muscular, last 8 inches.– Anus: “defecation” of “feces”.

• Diarrhea is caused by infection of the lower intestinal tract and nervous stimulation, and results in increased peristalsis – too much water. . . .– Dehydration.

• Constipation can be caused by a lack of H2O and fiber, and leads to dry, hard feces. . . . – Hemorrhoids (“piles”).– Laxatives!?

Disorders of the Colon and Rectum - continued

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• Hemorrhoids (“piles”) ?• Diverticulosis ?• Irritable Bowel Syndrome (IBS) or spastic colon ?• Inflammatory Bowel Disease (IBD) or colitis ?• Polyps and cancer ?

HEALTH FOCUS

Swallowing a Camera

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Digestive Enzymes / Nutrition• Dehydration synthesis – “building”, (+).

• also called condensation reaction.

• Hydrolysis – “tearing down”, (-).

• Prefixes: mono- = one, single / di- = two / poly- = many (> 3).

• Words for Food Components:

• peptide = protein (from peptide bond).

• saccharide = sugar (from latin for sugar).

• glyceride = lipids (fats, oils, etc.).

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Digestive Enzymes

• Digestive enzymes are hydrolytic enzymes that break down substances into nutrient molecules.– Glucose and amino acids are absorbed

into blood capillaries of the villi.– Fatty acids and glycerol reform within

epithelial cells and enter lacteals as lipoprotein droplets.

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Digestive Enzymes

• Are proteins.• Are specific.• Have a preferred pH.• Have a preferred temperature.• Are found (produced and released) in specific

places:– Mouth.– Stomach.– Duodenum (pancreatic juice, epithelial cells

of villus).

Nutrition and Weight Control

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Classes of Nutrients

• Nutrition?• Involves interaction between “food” and the

organism.• Food is an organic, energy containing

substance.• Nutrient is a substance found in food that is

used by the body to maintain health.• Balanced diet is obtained by eating a variety

of food from four food groups.• Represented by the Food Guide Pyramid,

old followed by the new.

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Food Components• The body requires three major classes of

micronutrients to supply energy and optimum cellular metabolism.– Carbohydrates.– Protein.– Fat.

• In addition, the body requires.– Vitamins.– Minerals.

• Human diet has changed substantially with the development of civilization. . . .

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Food Components - Carbohydrates• Carbohydrates (CHO) are digested to simple

sugars, which are, or can be, converted to glucose.– Quickest, most readily absorbed source of

energy for the body.– Complex CHO (breads, cereals) should make

up the bulk of the diet, not simple sugars (candy, “sweets”, soft drinks).

– Simple sugars are labeled “empty calories” because. . ? Glycemic index (GI)

– Blood glucose is kept at ~0.1% by breakdown of glycogen or conversion of amino acids.

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Food Components - Proteins• Found in meat, fish, poultry, dairy products,

legumes, nuts, cereals.• Incorporated into structural proteins in muscles,

skin, hair, nails.• Foods richest in protein are apt to be richest in fat!• Adequate protein formation requires twenty different

amino acids, including 8 (adult), 9 (children) essential amino acids.

– Body is unable to produce essential amino acids.– Complete protein contains all essential amino

acids.– Amino acids are not stored in the body, and thus

a daily supply is needed.

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Food Components - Lipids• Found in butter, oils, high-protein foods.• Fats, oils, and cholesterol are all lipids.• Why essential to diet?• Body cannot make essential fatty acids, i.e.

linoleic acid, which must be obtained from diet.

• Fats have highest energy content / gram, . . . – Intake of fat increases weight gain, and

increases the risk of cardiovascular disease and cancer (colon, breast, prostate, pancreas).

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Food Components - Lipids– Better to get from plant sources rather than from

animal sources. Plaque deposits contain saturated fats and

cholesterol. Saturated fatty acid (transformed - fatty acids),

monounsaturated fatty acid, polyunsaturated fatty acid?

Which is best and why? Avoid trans fats, reduce partially

hydrogenated ingredients. For optimal cardiovascular health, less than

30% of calories should come from fat and less than 10% from saturated fat.

Olestra, “fake fat”, side effects?

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Food Components - Lipids• Cholesterol

– High blood cholesterol levels associated with cardiovascular disease.

– Low density lipoproteins (LDL) carry cholesterol from the liver to cells.

– High density lipoproteins (HDL) carry cholesterol from the cells to the liver.

– LDL is bad, contributes to arterial plaques.– HDL is good, protects against clogged arteries.– A low saturated fat, low cholesterol diet with

soluble fiber reduces blood cholesterol and LDL’s.– Exercise can increase HDL’s.

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Minerals• The body also requires minerals.

– Macrominerals (Major) are present in the body at a minimal level of 5 grams each.

> 100 mg / day.Calcium, Potassium, Sodium, Magnesium,

Phosphorus, Sulfur, Chloride.– Microminerals (Trace) are present in the

body at levels under 5 grams each.< 20 mg / day.Iodine, Copper, Mangenese, Zinc, Iron,

Selenium.

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Iron, Calcium and Sodium

• Iron – adult females need more per day due to hemoglobin loss during menstruation.

• Calcium counteracts osteoporosis and generally strengthens bones along with estrogen replacement and exercise.

• Sodium helps maintain osmotic balance, but too much sodium can intensify hypertension.

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HEALTH FOCUS

When Zero Is More Than Nothing

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Vitamins and Antioxidants

• Vitamins are organic compounds used by the body for metabolic purposes, but cannot be produced in adequate quantities.– Many are coenzymes.– Antioxidants are vitamins (A, E, C) that

defend the body against free radicals. . . .Molecules that carry an extra electron.

• If lacking in the diet, various symptoms develop. . . .

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For the fat-soluble vitamins, remember the word “ADEK”

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67Rickets (bowing of bones) – Vitamin D deficiency.

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Dermatitis (Pellagra) of areas exposed to light due to niacin (Vitamin B3) deficiency.

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• Bleeding of gums (Scurvy) – Vitamin C deficiency.

• “Limeys”?

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HEALTH FOCUS

Searching for the Magic Weight-Loss Bullet

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Weight Loss the Healthy Way• Basal metabolic rate (BMR) is best measured 14 hours after the last meal with subject lying down at rest.

• Is affected by sex, size, shape, weight, age, activity, etc.

• To lose weight. . . .

• Decrease calorics (> 2100 Cal/day), increase exercise.

• Pills? Liquid Diets? Low-carbohydrate Diets? Single-category Diets?

• Eat variety of foods, more fruits/vegetables, less in saturated fats, be physically active each day.

• The key is “balance”.

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Eating Disorders• Obesity.

– 1/3 adults are obese !– Most often defined as body weight 20% or more

above ideal weight.– Moderate obesity: 41-100% above. – Severe obesity: 100% or more above.

• Also measured by Body Mass Index (BMI) = wt. of person in Kg / square of height in meters.

Most likely caused by a combination of hormonal, metabolic, and social factors.

Increases risk of heart disease, type 2 diabetes, cancer.

Treatment depends on degree of obesity.

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Eating Disorders

• Bulimia and anorexia are serious disorders which require competent medical attention.

• Bulimia Nervosa.– Habit of eating to excess and then purging by

artificial means. Alters blood composition leading to heart

rhythm problems and kidney damage.• Anorexia Nervosa.

– Morbid fear of gaining weight (perceptional). Self-induced starvation. Low blood pressure, irregular heartbeat,

constipation, and constant chilliness.

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Eating Disorders

• Binge-eating disorder ?• Condition characterized by episodes of

overeating that are not followed by purging.• Stress, anxiety, anger, and depression can

trigger food binges.• Muscle dysmorphia –• Individual thinks, perceives, that their body

is underdeveloped when it is not.• Unlike anorexia nervosa and bulimia, it

affects more men than women.

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Outline

• Overview of Digestion• The First Part of the Digestive Tract• The Stomach and Small Intestine• Three Accessory Organs and Regulation of

Secretions• The Large Intestine and Defecation• Nutrition and Weight Control

(Should review Ch.2, Molecules of Life, pp. 22-30)

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