Albia Dugger • Miami Dade College

• 10 trillion or more individual microorganisms reside in your gut and affect your health
• More than half of the human population has the bacterium
Helicobacter pylori, which can cause peptic ulcers
• Low numbers of the bacterium Bifidobacterium in the gut may be associated with irritable bowel syndrome (IBS)
• Researchers are studying the effectiveness of probiotics as a treatment for some disorders

• Digestive system
• A body cavity or tube that mechanically and chemically breaks food down to small particles, then to molecules that can be absorbed into the internal environment

Incomplete and Complete
Digestive Systems
• Incomplete digestive system
• A saclike gut with one opening in the body surface for food to enter and waste to leave
• Complete digestive system
• A tubular gut with an opening at both ends
• Includes mouth, pharynx, esophagus, stomach, small and large intestines, and anus

A Incomplete digestive system of a flatworm.
branching saclike gut pharynx single opening takes in food, expels waste

mouth with tongue liver gallbladder pancreas pharynx esophagus stomach small intestine large intestine cloaca (opening through which digestive wastes, urinary wastes, and gametes exit the body)
B Complete digestive system of a frog.

beak mouth esophagus crop glandular part of stomach gizzard intestines cloaca

ANIMATED FIGURE: Examples of digestive systems
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Five Functions of a Complete Digestive System
1. Mechanical processing and motility
2. Secretion of digestive enzymes into the lumen
3. Digestion of food into absorbable molecules
4. Absorption of nutrients into extracellular fluid
5. Elimination of solid residues

• Bird adaptations
• Size and shape of beaks adapted to different diets
• Crops and gizzards
• Mammal adaptations
• Teeth adapted to different diets
• Multiple stomach chambers in ruminants
• Carnivores have a shorter gut than grazers

gumline crown root antelope molar crown root human molar
Figure 39-3a p697

ingestion, regurgitation, reswallowing of food through esophagus stomach chamber 2 stomach chamber 3 stomach chamber 4 stomach chamber 1 to small intestine
Figure 39-3b p697

ANIMATED FIGURE: Antelope stomach function
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Take-Home Message: How does digestive system structure reflect function?
• Digestive systems mechanically and chemically degrade food into small molecules that can be absorbed, along with water, into the internal environment. These systems also expel the undigested residues from the body.
• Incomplete digestive systems are a saclike cavity with one opening. Complete digestive systems are a tube with two openings and regional specializations in between.
• Structural variations in bills, teeth, and regions of the gut are adaptations that allow an animal to exploit a particular type or types of foods.

39.3 Overview of the Human Digestive System
• Humans have a complete digestive system lined with mucuscovered epithelium
• Accessory organs such as salivary glands, pancreas, liver, and gallbladder secrete enzymes and other substances that aid in the breakdown of food and absorption of nutrients

• Food is partially digested in the mouth and forced into the pharynx by swallowing
• Food is moved through the esophagus by peristalsis through a sphincter to the stomach , which adds acids and enzymes to food and mixes them together

• The gastrointestinal tract starts at the stomach and extends through the intestines to the terminal opening
• In the small intestine , carbohydrates, lipids and proteins are digested by secretions from liver and pancreas; nutrients and water are absorbed
• The large intestine absorbs water and ions, and compacts wastes, which collect in the rectum , and are expelled from the anus

Major Organs
Mouth
Esophagus
Stomach
Small Intestine
Large Intestine
(colon)
Rectum
Anus
Accessory Organs
Salivary Glands
Pharynx (throat)
Liver
Gallbladder
Pancreas
Figure 39-5 p698

ANIMATED FIGURE: Human digestive system
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Take-Home Message: What type of digestive system do humans have?
• Humans have a complete digestive system with a tubular, mucosa-lined gut.
• Accessory organs positioned adjacent to the gut secrete substances into its interior. These substances aid in digestion of food or absorption of nutrients.

• Digestion begins when teeth mechanically break down food into smaller bits
• Teeth consist mostly of bonelike dentin ; the crown is covered by a hard layer of enamel
• Salivary amylase secreted by salivary glands begins the breakdown of starch

lower jaw molars
(12) premolars
(8) canines (4) incisors
(8) upper jaw
Figure 39-6a p699

enamel dentin pulp cavity
(contains nerves and blood vessels) ligaments root canal periodontal membrane crown gingiva
(gum) root bone
Figure 39-6b p699

Take-Home Message:
What happens to food in the mouth?
• Teeth mechanically break food into smaller particles.
Enzymes in saliva begin the chemical digestion of carbohydrates.

3D ANIMATION: Swallowing

39.5 Food Storage and
Digestion in the Stomach
• The stomach is a muscular, stretchable sac with a sphincter at either end
• The stomach has three digestive functions:
• Stores food and controls the rate of passage to the small intestine
• Mechanically mixes and breaks down food
• Secretes substances used in chemical digestion

esophagus pyloric sphincter small intestine gastroesophageal sphincter serosa longitudinal muscle circular muscle oblique muscle submucosa mucosa
Figure 39-7 p700

• Stomach mucosa secretes gastric fluid containing hydrochloric acid and enzymes that begin protein digestion
• Gastrin signals secretion of acid and pepsinogens
• Acid unfolds proteins
• Pepsin breaks proteins into peptides
• Chyme passes into the small intestine

Take-Home Message:
What are the functions of the stomach?
• The stomach receives food from the esophagus and stretches to store it.
• Contractions of stomach muscles break up food and mix it with acidic gastric fluid. They also move the resulting mixture
(the chyme) into the small intestine.
• Chemical digestion of proteins begins in the stomach.

ANIMATION: Digestion in mammalian stomachs

• The small intestine has a highly folded lining with many projections that make its surface area enormous
• Most digestion and absorption take place at the surface of the small intestine

• Many folds and projections of the small intestinal lining increase its surface area for absorption
• Multicelled, fingerlike absorptive structures ( villi ) with lymph and blood vessels extend from folds
• Brush border cells at the surface have membrane extensions ( microvilli ) that project into the lumen

• Brush border cells function in both digestion and absorption
• Digestive enzymes at the surface of a microvillus break down sugars, protein fragments, and nucleotides
• Many transport proteins facilitate the movement of nutrients into the microvillus

A Longitudinal cross section through the small intestine showing its folded lining.
Figure 39-8a p701

villi blood vessels lymph vessel
B Intestinal fold with villi at its surface.
C One villus with brush border cells at its surface.
Figure 39-8bcde p701

time
Figure 39-9 p701

Take-Home Message:
Small intestine structure and function
• The surface of the small intestine is highly folded and each fold has many projections (villi). Brush border cells at the surface of a villus have tiny projections (microvilli) at their surface.
• The many folds and projections greatly increase the surface area for the two functions of the small intestine —digestion and absorption.

• In the small intestine, chyme mixes with secretions from the pancreas and liver
• Pancreatic enzymes break down larger molecules into units that can be absorbed
• Monosaccharides, monoglycerides, fatty acids, amino acids, nucleotides, nucleotide bases
• Bicarbonate from the pancreas buffers acids so enzymes can work

• In the mouth, salivary amylase broke polysaccharides into disaccharides
• In the small intestine, pancreatic amylase breaks down polysaccharides
• Enzymes embedded in the plasma membrane of the brush border cells split disaccharides into monosaccharide
• Monosaccharides are actively transported into a brush border cell, then into interstitial fluid inside a villus, then enter blood

• Protein digestion began in the stomach, where pepsin broke proteins into polypeptides
• The small intestine releases cholecystokinin (CCK) into the blood, which causes the pancreas to secrete proteases such as trypsin and chymotrypsin into the small intestine
• Enzymes at the surface of brush border cell break peptide fragments into amino acids, which are actively transported into brush border cells, interstitial fluid, and blood

• Lipid (fat) digestion in the small intestine requires enzymes and bile, which is produced by the liver and stored in the gallbladder
• Bile is a mixture of salts, pigments, cholesterol and lipids that emulsifies fats into small drops that enzymes can break down into fatty acids and monoglycerides

• Fatty acids and monoglycerides combine with bile salts to form micelles, which aid diffusion into brush border cells (bile salts stay in lumen)
• In brush border cells, fatty acids and monoglycerides combine with proteins to form lipoproteins, which enter the villus by exocytosis
• From interstitial fluid, triglycerides enter lymph vessels, which empty into the bloodstream

• Transport proteins move salts, sugars, and amino acids from the intestinal lumen, into brush border cells, then into interstitial fluid in a villus
• Water follows the solutes by osmotic gradient
• Capillaries in the villus distribute water and solutes through the body

Table 39-1 p702

1 carbohydrates
3 proteins monosaccharides amino acids fat globules
(triglycerides)
+ bile salts
2
Lumen of Small Intestine
4
5 emulsification droplets
6 free fatty acids, monoglycerides
+ bile salts
7
Brush
Border
Cell
Internal Environment
(interstitial fluid inside a villus) triglycerides + proteins lipoproteins
8
Stepped Art
Figure 39-10a p703

ANIMATED FIGURE: Digestion and absorption in the small intestine
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Take-Home Message:
What are the roles of the small intestine?
• Chemical digestion is completed in the small intestine.
Enzymes from the pancreas and enzymes embedded in the membrane of brush border cells break large molecules into smaller, absorbable subunits.
• Small subunits (monosaccharides, amino acids, fatty acids, and monoglycerides) enter the internal environment when they are absorbed into the interstitial fluid in a villus.
• Most fluid that enters the gut is also absorbed across the wall of the small intestine.

• The large intestine is wider than the small intestine, but also much shorter —only about 1.5 meters (5 feet) long
• The ascending colon begins at the cecum, where the appendix is attached
• The descending colon attaches to the rectum

• The large intestine completes the process of absorption, then concentrates wastes, and compacts them as feces
• Bacteria in the colon make vitamin B
12
, which is absorbed through the colon lining
• Stretch receptors in the rectum trigger the defecation reflex

• Stress, a diet low in fiber, minimal exercise, dehydration, and some medications can lead to constipation
• Diarrhea may result from a bacterial infection, and cause dehydration
• Appendicitis must be treated to prevent rupture and infection of the abdominal cavity
• Colon polyps leading to cancer can be detected and removed by colonoscopy

ascending colon cecum appendix last portion of small intestine
Figure 39-11 p704

colon polyp transverse colon descending colon
Figure 39-11 p704

Take-Home Message:
What is the function of the colon?
• The colon completes the process of absorption, then concentrates, stores, and eliminates wastes.

39.9 Metabolism of
Absorbed Organic Compounds
• Absorbed compounds are carried by the blood to the liver , which plays a central role in metabolism
• Most absorbed compounds are broken down for energy, stored, or used to build larger compounds
• Excess carbohydrates and proteins are converted to fat and stored in adipose tissue

FOOD INTAKE dietary carbohydrates, lipids dietary proteins, amino acids
Cytoplasmic Pool of Carbohydrates, Fats
(interconvertible forms)
Cytoplasmic Pool of Amino Acids ammonia storage forms
(e.g., glycogen) building blocks for cell structures specialized derivatives
(e.g., steroids, acetylcholine) instant energy sources for cells urea excreted in urine nitrogencontaining derivatives
(e.g., hormones, nucleotides) building blocks for structural proteins, enzymes
Stepped Art
Figure 39-12 p705

• The liver detoxifies dangerous substances (alcohol, NH
3
), and stores fat-soluble vitamins (A, D) and glucose (as glycogen)
• Between meals, the liver provides the brain with glucose by breaking down stored glycogen

Take-Home Message: What happens to compounds absorbed from the gut?
• Blood carries the absorbed compounds to the liver, which detoxifies dangerous substances and stores vitamins and glucose. The glucose is stored as glycogen.
• Adipose tissue takes up absorbed carbohydrates and proteins and converts them to fats.
• In between meals, the liver breaks down stored glycogen, and releases its glucose subunits into the blood. This ensures that the brain, which can only use carbohydrates as fuel, always has an adequate supply of energy.

• Diet profoundly affects your body ’ s structure and function
• Eating provides your cells with a source of energy and a supply of essential building materials

• The USDA reviews research on human nutrition and issues a set of dietary guidelines
• Current guidelines recommend eating less refined grains, saturated fats, trans fatty acids, sugar or caloric sweeteners, and salt – and more vegetables and fruits, whole grains, and fat-free or lowfat milk products
• Guidelines based on age, sex, height, weight, and activity level can be generated online at www.ChoseMyPlate.gov

• Good (complex) carbohydrates provide energy, vitamins, and fiber (soluble and insoluble)
• Fresh fruits, whole grains, and vegetables
• Not so good (processed) carbohydrates have “ empty calories ”
• White flour, refined sugar, corn syrup

• Lipids are used in cell membranes (phospholipids and cholesterol), as energy reserves, insulation and cushioning, and to store fat-soluble vitamins
• Essential fatty acids (linoleic and alpha-linoleic acids) must be obtained from the diet

• Unsaturated fats are liquid at room temperature
• Polyunsaturated fats (such as omega-3 fatty acids) and monounsaturated fats (such as oleic acid) have specific health benefits
• Saturated fats (in meat and dairy products) can increase risk of heart disease, stroke, or cancer
• Trans fats are worse than saturated fats

Table 39-2 p706

• Proteins are the source of amino acids used to build all body proteins
• Meat provides all eight essential amino acids
• Most plant foods lack one or more amino acids, but can meet all human amino-acid needs when combined correctly

Take-Home Message: What are the main types of nutrients that humans require?
• A healthy diet provides energy and all necessary building blocks for assembling essential body components.
• Nutritional guidelines are periodically revised in light of new research. Current guidelines call for most calories to come from complex carbohydrates, rather than simple sugars. They also favor fat and protein sources that are low in saturated and trans fats.
• A person can obtain all the required nutrients from a vegetarian diet, but doing so requires combining plant foods so amino acids lacking in one are present in the other.

39.11 Vitamins, Minerals, and Phytochemicals
• Vitamins
• Organic substances that are essential in very small amounts in the diet (coenzymes)
• Minerals
• Inorganic substances with essential metabolic functions
(such as iron in hemoglobin)
• Phytochemicals
• Beneficial organic molecules found in plant foods

Table 39-3 p708

Table 39-4 p709

Take-Home Message: What roles do vitamins, minerals, and phytonutrients play?
• Vitamins are organic molecules with an essential role in metabolism.
• Minerals are inorganic substances with an essential role.
• Phytochemicals are plant molecules that are not essential but may reduce the risk of certain disorders.

• Body mass index (BMI) estimates health risks
• Overweight: 25 to 29.9
• Obese: 30 or more
BMI = [weight (lbs) x 703] ÷ height (in) 2

• To calculate how many kilocalories you need, multiply your weight in pounds by 10 (if you are not active), or by 15
(moderately active), or by 20 (highly active)
• Subtract one of the following amounts:
• Age: 25 –34; Subtract: 0
• 35 –44; 100
• 45 –54; 200
• 55 –64; 300
• Over 65; 400

• Being overweight increases health risks:
• Type 2 diabetes, high blood pressure, heart disease, breast and colon cancer, arthritis, gallstones
• An unhealthy overabundance of fat ( obesity ) stresses fat cells, triggers inflammatory response
• Fat cells do not increase in number after birth
• Excess weight overfills existing fat cells

• Genetic factors influence how difficult it is for a person to reach and maintain a healthy weight
• The ob gene encodes leptin, a hormone made by adipose cells, which acts in the brain and suppresses appetite
• About 16% of people of European ancestry are homozygous for an fto allele that predisposes them to obesity

• With anorexia nervosa, a person who has access to food routinely eats too little to maintain a normal weight
• Most affected people see themselves as fat, even when they are dangerously thin
• Anorexia damages organ systems throughout the body and can result in death from sudden cardiac arrest
• People with bulimia nervosa, “ binge and purge, ” which can shift pH, resulting in abnormal heart rhythm and convulsions

Take-Home Message:
How does weight affect health?
• A person who balances caloric intake with energy expenditures will maintain current weight.
• Obesity raises the risk of heart disease, type 2 diabetes, some cancers, and other disorders. These problems may arise because overstuffed adipose cells summon up inflammatory responses in organs throughout the body.
• Anorexia is an eating disorder that results in a lower than normal body weight. It harms organs throughout the body, especially the bones and the heart. Anorexia-induced changes in heart rhythms can be fatal.