Download The Digestive System Functions Organs of the

The Digestive System Functions
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Ingestion—taking in food
Digestion—breaking food into nutrient molecules
Absorption—movement of nutrients into the bloodstream
Defecation—elimination of indigestible waste
Organs of the Digestive System
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Two main groups of organs
o Alimentary canal (gastrointestinal or GI tract)—continuous, coiled, hollow tube
! These organs ingest, digest, absorb, defecate
o Accessory digestive organs
! Includes teeth, tongue, and other large digestive organs
Organs of the Alimentary Canal
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The alimentary canal is a continuous, coiled, hollow tube that runs through the ventral
cavity from stomach to anus:
o Mouth
o Pharynx
o Esophagus
o Stomach
o Small intestine
o Large intestine
o Anus
Mouth (Oral Cavity)
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Anatomy of the mouth
o The mouth (oral cavity)—mucous membrane–lined cavity
o Lips (labia)—protect the anterior opening
o Cheeks—form the lateral walls
o Hard palate—forms the anterior roof
o Soft palate—forms the posterior roof
o Uvula—fleshy projection of the soft palate
Mouth (Oral Cavity)
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Anatomy of the mouth (continued)
o Vestibule—space between lips externally and teeth and gums internally
o Oral cavity proper—area contained by the teeth
o Tongue—attached at hyoid bone and styloid processes of the skull, and by the
lingual frenulum to the floor of the mouth
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Mouth (Oral Cavity)
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Anatomy of the mouth (continued)
o Tonsils
! Palatine—located at posterior end of oral cavity
! Lingual—located at the base of the tongue
Mouth
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Functions of the mouth
o Mastication (chewing) of food
o Tongue mixes masticated food with saliva
o Tongue initiates swallowing
o Taste buds on the tongue allow for taste
Pharynx
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Food passes from the mouth posteriorly into the:
o Oropharynx—posterior to oral cavity
o Laryngopharynx—below the oropharynx and continuous with the esophagus
Pharynx
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The pharynx serves as a passageway for food, fluids, and air
Food is propelled to the esophagus by two skeletal muscle layers in the pharynx
o Longitudinal inner layer
o Circular outer layer
Alternating contractions of the muscle layers (peristalsis) propel the food
Esophagus (Gullet)
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Anatomy
o About 10 inches long
o Runs from pharynx to stomach through the diaphragm
Physiology
o Conducts food by peristalsis (slow rhythmic squeezing) to the stomach
o Passageway for food only (respiratory system branches off after the pharynx)
Layers of Tissue in the Alimentary Canal Organs
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Summary of the four layers from innermost to outermost (detailed next):
1. Mucosa
2. Submucosa
3. Muscularis externa
4. Serosa
Layers of Tissue in the Alimentary Canal Organs
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1. Mucosa
o Innermost, moist membrane consisting of:
! Surface epithelium that is mostly simple columnar tissue (except for
esophagus)
! Small amount of connective tissue
(lamina propria)
! Small smooth muscle layer
o Lines the cavity (known as the lumen)
Layers of Tissue in the Alimentary Canal Organs
2. Submucosa
o Just beneath the mucosa
o Soft connective tissue with blood vessels, nerve endings, mucosa-associated
lymphoid tissue, and lymphatics
Layers of Tissue in the Alimentary Canal Organs
3. Muscularis externa—smooth muscle
o Inner circular layer
o Outer longitudinal layer
4. Serosa—outermost layer of the wall contains
fluid-producing cells
o Visceral peritoneum—innermost layer that is continuous with the outermost
layer
o Parietal peritoneum—outermost layer that lines the abdominopelvic cavity by
way of the mesentery
Alimentary Canal Nerve Plexuses
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Two important nerve plexuses serve the alimentary canal
Both are part of the autonomic nervous system
o Submucosal nerve plexus
o Myenteric nerve plexus
Function is to regulate mobility and secretory activity of the GI tract organs
Stomach
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The stomach is a C-shaped organ located on the left side of the abdominal cavity
Food enters at the cardioesophageal sphincter from the esophagus
Food empties into the small intestine at the pyloric sphincter (valve)
Stomach
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Regions of the stomach
o Cardial part (cardia)—near the heart
o Fundus—expanded portion lateral to the cardiac region
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o Body—midportion
o Pylorus—funnel-shaped terminal end
Stomach
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Stomach can stretch and hold 4 L (1 gallon) of food when full
o Rugae—internal folds of the mucosa present when the stomach is empty
External regions
o Lesser curvature—concave medial surface
o Greater curvature—convex lateral surface
Stomach
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Layers of peritoneum attached to the stomach
o Lesser omentum—attaches the liver to the lesser curvature
o Greater omentum—attaches the greater curvature to the posterior body wall
! Embedded fat insulates, cushions, and protects abdominal organs
! Lymph follicles contain macrophages
Muscularis externa has a third layer
o Oblique layer helps to churn, mix, and pummel the food
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Stomach
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Functions of the stomach
o Temporary storage tank for food
o Site of food breakdown
o Chemical breakdown of protein begins
o Delivers chyme (processed food) to the small intestine
Stomach
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Structure of the stomach mucosa:
o Simple columnar epithelium dotted by gastric pits that lead to gastric glands
o Mucous cells produce bicarbonate-rich alkaline mucus
o Gastric glands—situated in gastric pits and secrete gastric juice, including:
! Intrinsic factor, which is needed for vitamin B12 absorption in the small
intestine
Stomach
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Structure of the stomach mucosa (continued)
o Chief cells—produce protein-digesting enzymes (pepsinogens)
o Parietal cells—produce hydrochloric acid
! Mucous neck cells—produce thin acidic mucus (different from the
mucus produced by cells of the mucosa)
o Enteroendocrine cells—produce a hormone called gastrin
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Small Intestine
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The body’s major digestive organ
Longest portion of the alimentary tube (2–4 m or 7–13 feet in a living person)
Site of nutrient absorption into the blood
Muscular tube extending from the pyloric sphincter to the ileocecal valve
Suspended from the posterior abdominal wall by the mesentery
Small Intestine
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Subdivisions
o Duodenum
! Attached to the stomach
! Curves around the head of the pancreas
o Jejunum
! Attaches anteriorly to the duodenum
o Ileum
! Extends from jejunum to large intestine
! Meets the large intestine at the ileocecal valve
Small Intestine
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Chemical digestion begins in the small intestine
o Enzymes are produced by:
! Intestinal cells
! Pancreas
o Pancreatic ducts carry enzymes to the duodenum
o Bile, formed by the liver, enters the duodenum via the bile duct
Small Intestine
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Three structural modifications that increase surface area for food absorption
1. Microvilli—tiny projections of the plasma membrane (create a brush border
appearance)
2. Villi—fingerlike projections formed by the mucosa
! House a capillary bed and lacteal
3. Circular folds (plicae circulares)—deep folds of mucosa and submucosa
Large Intestine
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Larger in diameter, but shorter in length at 1.5 m, than the small intestine
Extends from the ileocecal valve to the anus
Subdivisions:
o Cecum
o Appendix
o Colon
o Rectum
o Anal canal
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Large Intestine Anatomy
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Cecum—saclike first part of the large intestine
Appendix
o Accumulation of lymphoid tissue that sometimes becomes inflamed
(appendicitis)
o Hangs from the cecum
Large Intestine Anatomy
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Colon
o Ascending—travels up right side of abdomen
o Transverse—travels across the abdominal cavity
o Descending—travels down the left side
o Sigmoid—S-shaped region; enters the pelvis
Sigmoid colon, rectum, and anal canal are located in the pelvis
Large Intestine Anatomy
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Anal canal ends at the anus
Anus—opening of the large intestine
o External anal sphincter—formed by skeletal muscle and under voluntary
control
o Internal anal sphincter—formed by smooth muscle and involuntarily controlled
o These sphincters are normally closed except during defecation
The large intestine delivers undigestible food residues to the body’s exterior
Large Intestine
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Goblet cells produce alkaline mucus to lubricate the passage of feces
Muscularis externa layer is reduced to three bands of muscle called teniae coli
These bands of muscle cause the wall to pucker into haustra (pocketlike sacs)
Accessory Digestive Organs
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Teeth
Salivary glands
Pancreas
Liver
Gallbladder
Teeth
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Teeth masticate (chew) food into smaller fragments
Humans have two sets of teeth during a lifetime:
1. Deciduous (baby or “milk”) teeth
! A baby has 20 teeth by age 2
! First teeth to appear are the lower central incisors
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Teeth
2. Permanent teeth
o Replace deciduous teeth between the ages of 6 and 12
o A full set is 32 teeth, but some people do not have wisdom teeth (third molars)
o If they do emerge, the wisdom teeth appear between ages of 17 and 25
Classification of Teeth
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Incisors—cutting
Canines (eyeteeth)—tearing or piercing
Premolars (bicuspids)—grinding
Molars—grinding
Regions of a Tooth
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Two major regions of a tooth
1. Crown
2. Root
Regions of a Tooth
1. Crown—exposed part of tooth above the gingiva (gum)
o Enamel—covers the crown
o Dentin—found deep to the enamel and forms the bulk of the tooth, surrounds
the pulp cavity
o Pulp cavity—contains connective tissue, blood vessels, and nerve fibers (pulp)
o Root canal—where the pulp cavity extends into the root
Regions of a Tooth
Note: The neck is a connector between the crown and root.
o Region in contact with the gum
o Connects crown to root
2. Root
o Cement—covers outer surface and attaches the tooth to the periodontal
membrane (ligament)
o Periodontal membrane holds tooth in place in the bony jaw
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Figure 14.10 Longitudinal section of a molar.
Salivary Glands
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Three pairs of salivary glands empty secretions into the mouth
1. Parotid glands
! Found anterior to the ears
2. Submandibular glands
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3. Sublingual glands
! Both submandibular and sublingual glands empty saliva into the floor of
the mouth through small ducts
Salivary Glands
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Saliva
o Mixture of mucus and serous fluids
o Helps to moisten and bind food together into a mass called a bolus
o Contains salivary amylase to begin starch digestion
o Dissolves chemicals so they can be tasted
Pancreas
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Found posterior to the parietal peritoneum
o Mostly retroperitoneal
Extends across the abdomen from spleen to duodenum
Pancreas
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Produces a wide spectrum of digestive enzymes that break down all categories of
food
Secretes enzymes into the duodenum
Alkaline fluid introduced with enzymes neutralizes acidic chyme coming from
stomach
Hormones produced by the pancreas
o Insulin
o Glucagon
Liver
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Largest gland in the body
Located on the right side of the body under the diaphragm
Consists of four lobes suspended from the diaphragm and abdominal wall by the
falciform ligament
Connected to the gallbladder via the common hepatic duct
Liver
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Bile is produced by cells in the liver
Bile leaves the liver through the common hepatic duct and enters duodenum through
the bile duct
Bile is a yellow-green, watery solution containing:
o Bile salts and bile pigments (mostly bilirubin from the breakdown of
hemoglobin)
o Cholesterol, phospholipids, and electrolytes
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Liver
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Function of bile
o Emulsify fats by physically breaking large fat globules into smaller ones
Gallbladder
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Sac found in shallow fossa of liver
When no digestion is occurring, bile backs up the cystic duct for storage in the
gallbladder
During digestion of fatty food, bile is introduced into the duodenum from the
gallbladder
Gallstones are crystallized cholesterol, which can cause blockages
Functions of the Digestive System
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Major functions of the digestive system are summarized as:
o Digestion
o Absorption
We will cover 6 more specific processes next
Functions of the Digestive System
1. Ingestion—placing of food into the mouth
2. Propulsion—movement of foods from one region of the digestive system to another
o Peristalsis—alternating waves of contraction and relaxation that squeezes
food along the GI tract
o Segmentation—movement of materials back and forth to foster mixing in the
small intestine
Functions of the Digestive System
3. Food breakdown: mechanical breakdown
o Examples:
! Mixing of food in the mouth by the tongue
! Churning of food in the stomach
! Segmentation in the small intestine
o Mechanical digestion prepares food for further degradation by enzymes
Functions of the Digestive System
4. Food breakdown: digestion
o Digestion occurs when enzymes chemically break down large molecules into
their building blocks
o Each major food group uses different enzymes
! Carbohydrates are broken to monosaccharides (simple sugars)
! Proteins are broken to amino acids
! Fats are broken to fatty acids and glycerol
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Functions of the Digestive System
5. Absorption
o End products of digestion are absorbed in the blood or lymph
o Food must enter mucosal cells and then into blood or lymph capillaries
6. Defecation
o Elimination of indigestible substances from the GI tract in the form of feces
Activities Occurring in the Mouth, Pharynx, and Esophagus
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Food ingestion and breakdown
o Food is placed into the mouth
! Physically broken down by chewing
! Mixed with saliva, which is released in response to mechanical
pressure and psychic stimuli
! Salivary amylase begins starch digestion
o Essentially, no food absorption occurs in the mouth
Activities Occurring in the Mouth, Pharynx, and Esophagus
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Food propulsion—swallowing and peristalsis
o Pharynx and esophagus have no digestive function
! Serve as passageways to the stomach
o Pharynx functions in swallowing (deglutition)
! Two phases of swallowing:
1. Buccal phase
2. Pharyngeal-esophgeal phase
Activities Occurring in the Mouth, Pharynx, and Esophagus
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Food propulsion—swallowing and peristalsis (continued)
1. Buccal phase
! Voluntary
! Occurs in the mouth
! Food is formed into a bolus
! The bolus is forced into the pharynx by the tongue
Activities Occurring in the Mouth, Pharynx, and Esophagus
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Food propulsion—swallowing and peristalsis (continued)
2. Pharyngeal-esophageal phase
! Involuntary transport of the bolus by peristalsis
! Nasal and respiratory passageways are blocked
Activities Occurring in the Mouth, Pharynx, and Esophagus
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Food propulsion—swallowing and peristalsis (continued)
2. Pharyngeal-esophogeal phase (continued)
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Peristalsis moves the bolus toward the stomach
The cardioesophageal sphincter is opened when food presses against
it
Activities in the Stomach
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Food breakdown
o Gastric juice is regulated by neural and hormonal factors
o Presence of food or rising pH causes the release of the hormone gastrin
o Gastrin causes stomach glands to produce:
! Protein-digesting enzymes
! Mucus
! Hydrochloric acid
Activities in the Stomach
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Food breakdown (continued)
o Hydrochloric acid makes the stomach contents very acidic
o Acidic pH
! Activates pepsinogen to pepsin for protein digestion
! Provides a hostile environment for microorganisms
Activities in the Stomach
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Food breakdown (continued)
o Protein digestion enzymes
! Pepsin—an active protein-digesting enzyme
! Rennin—works on digesting milk protein in infants, not adults
o Alcohol and aspirin are virtually the only items absorbed in the stomach
Activities in the Stomach
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Food propulsion
1. Peristalsis: Waves of peristalsis occur from the fundus to the pylorus, forcing
food past the pyloric sphincter
2. Grinding: The pylorus meters out chyme into the small intestine (3 ml at a
time)
3. Retropulsion: Peristaltic waves close the pyloric sphincter, forcing content
back into the stomach. The stomach empties in 4–6 hours
Activities of the Small Intestine
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Food breakdown and absorption
o Intestinal enzymes from the brush border function to:
! Break double sugars into simple sugars
! Complete some protein digestion
o Intestinal enzymes and pancreatic enzymes help to complete digestion of all
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food groups
Activities of the Small Intestine
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Food breakdown and absorption (continued)
o Pancreatic enzymes play the major role in the digestion of fats, proteins, and
carbohydrates
o Alkaline content neutralizes acidic chyme and provides the proper
environment for the pancreatic enzymes to operate
Activities of the Small Intestine
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Food breakdown and absorption (continued)
o Release of pancreatic juice from the pancreas into the duodenum is stimulated
by:
! Vagus nerves
! Local hormones that travel via the blood to influence the release of
pancreatic juice (and bile):
o Secretin
o Cholecystokinin (CCK)
Activities of the Small Intestine
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Food breakdown and absorption (continued)
o Hormones (secretin and CCK) also target the liver and gallbladder to release
bile
! Bile
o Acts as a fat emulsifier
o Needed for fat absorption and absorption of fat-soluble vitamins
(K, D, E, and A)
Activities of the Small Intestine
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Food breakdown and absorption (continued)
o A summary table of hormones is presented next
Activities of the Small Intestine
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Food breakdown and absorption (continued)
o Water is absorbed along the length of the small intestine
o End products of digestion
! Most substances are absorbed by active transport through cell
membranes
! Lipids are absorbed by diffusion
o Substances are transported to the liver by the hepatic portal vein or lymph
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Activities of the Small Intestine
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Food breakdown and absorption
o Peristalsis is the major means of moving food
o Segmental movements
! Mix chyme with digestive juices
! Aid in propelling food
Activities of the Large Intestine
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Food breakdown and absorption
o No digestive enzymes are produced
o Resident bacteria digest remaining nutrients
! Produce some vitamin K and B
! Release gases
o Water and vitamins K and B are absorbed
o Remaining materials are eliminated via feces
Activities of the Large Intestine
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Food breakdown and absorption (continued)
o Feces contains:
! Undigested food residues
! Mucus
! Bacteria
! Water
Activities of the Large Intestine
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Propulsion of the residue and defecation
o Sluggish peristalsis begins when food residue arrives
o Haustral contractions are most seen in the large intestine
o Mass movements are slow, powerful movements that occur 3 to 4 times per
day
Activities of the Large Intestine
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Propulsion of the residue and defecation (continued)
o Presence of feces in the rectum causes a defecation reflex
! Internal anal sphincter is relaxed
! Defecation occurs with relaxation of the voluntary (external) anal
sphincter
Nutrition and Metabolism
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Most foods are used as metabolic fuel
o Foods are oxidized and transformed into adenosine triphosphate (ATP)
o ATP is chemical energy that drives cellular activities
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Energy value of food is measured in kilocalories (kcal) or Calories (C)
Nutrition
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Nutrient—substance used by the body for growth, maintenance, and repair
Major nutrients
o Carbohydrates
o Lipids
o Proteins
o Water
Minor nutrients
o Vitamins
o Minerals
Nutrition
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A diet consisting of foods from the five food groups normally guarantees adequate
amounts of all the needed nutrients
The five food groups are summarized next in Table 14.2
Dietary Sources of Major Nutrients
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Carbohydrates
o Dietary carbohydrates are sugars and starches
o Most are derived from plants such as fruits and vegetables
o Exceptions: lactose from milk and small amounts of glycogens from meats
Dietary Sources of Major Nutrients
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Lipids
o Saturated fats from animal products (meats)
o Unsaturated fats from nuts, seeds, and vegetable oils
o Cholesterol from egg yolk, meats, and milk products (dairy products)
Dietary Sources of Major Nutrients
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Proteins
o Complete proteins—contain all essential amino acids
! Most are from animal products (eggs, milk, meat, poultry, and fish)
! Essential amino acids: those that the body cannot make and must be
obtained through diet
o Legumes and beans also have proteins, but the proteins are incomplete
Dietary Sources of Major Nutrients
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Vitamins
o Most vitamins are used as coenzymes
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o Found mainly in fruits and vegetables
Dietary Sources of Major Nutrients
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Minerals
o Mainly important for enzyme activity
o Foods richest in minerals: vegetables, legumes, milk, and some meats
o Iron is important for making hemoglobin
o Calcium is important for building bone, blood clotting, and secretory activities
Metabolism
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Metabolism is all of the chemical reactions necessary to maintain life
o Catabolism—substances are broken down to simpler substances; energy is
released
o Anabolism—larger molecules are built from smaller ones
Carbohydrate Metabolism
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Carbohydrates are the body’s preferred source to produce cellular energy (ATP)
Glucose (blood sugar)
o Major breakdown product of carbohydrate digestion
o Fuel used to make ATP
Carbohydrate Metabolism
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Cellular respiration
o As glucose is oxidized, carbon dioxide, water, and ATP are formed
Carbohydrate Metabolism
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Events of three main metabolic pathways of cellular respiration
1. Glycolysis
! Occurs in the cytosol
! Energizes a glucose molecule so it can be split into two pyruvic acid
molecules and yield ATP
Carbohydrate Metabolism
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Events of three main metabolic pathways of cellular respiration (continued)
2. Krebs cycle
! Occurs in the mitochondrion
! Produces virtually all the carbon dioxide and water resulting from
cellular respiration
! Yields a small amount of ATP
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Carbohydrate Metabolism
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Events of three main metabolic pathways of cellular respiration (continued)
3. Electron transport chain
! Hydrogen atoms removed during glycolysis and the Krebs cycle are
delivered to protein carriers
! Hydrogen atoms are split into hydrogen ions and electrons in the
mitochondria
! Electrons give off energy in a series of steps to enable the production of
ATP
Carbohydrate Metabolism
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Hyperglycemia—excessively high levels of glucose in the blood
o Excess glucose is stored in body cells as glycogen or converted to fat
Hypoglycemia—low levels of glucose in the blood
o Glycogenolysis, gluconeogenesis, and fat breakdown occur to restore normal
blood glucose levels
Fat Metabolism
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Fats
o Insulate the body
o Protect organs
o Build some cell structures (membranes and myelin sheaths)
o Provide reserve energy
Excess dietary fat is stored in subcutaneous tissue and other fat depots
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Fat Metabolism
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When carbohydrates are in limited supply, more fats are oxidized to produce ATP
o Excessive fat breakdown causes blood to become acidic (acidosis or
ketoacidosis)
! Breath has a fruity odor
! Common with:
o “No carbohydrate” diets
o Uncontrolled diabetes mellitus
o Starvation
Protein Metabolism
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Proteins form the bulk of cell structure and most functional molecules
Proteins are carefully conserved by body cells
Amino acids are actively taken up from blood by body cells
Protein Metabolism
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Amino acids are oxidized to form ATP mainly when other fuel sources are not
available
Ammonia, released as amino acids are catabolized, is detoxified by liver cells that
combine it with carbon dioxide to form urea
The Central Role of the Liver in Metabolism
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Liver is the body’s key metabolic organ
Roles in digestion:
o Manufactures bile
o Detoxifies drugs and alcohol
o Degrades hormones
o Produces cholesterol, blood proteins (albumin and clotting proteins)
o Plays a central role in metabolism
Liver can regenerate if part of it is damaged or removed
The Central Role of the Liver in Metabolism
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To maintain homeostasis of blood glucose levels, the liver performs:
o Glycogenesis—“glycogen formation”
! Glucose molecules are converted to glycogen and stored in the liver
o Glycogenolysis—“glycogen splitting”
! Glucose is released from the liver after conversion from glycogen
o Gluconeogenesis—“formation of new sugar”
! Glucose is produced from fats and proteins
The Central Role of the Liver in Metabolism
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Fats and fatty acids are picked up by the liver
o Some are oxidized to provide energy for liver cells
o The rest are either stored or broken down into simpler compounds and
released into the blood
The Central Role of the Liver in Metabolism
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Cholesterol metabolism and transport
o Cholesterol is not used to make ATP
o Functions of cholesterol:
! Structural basis of steroid hormones and vitamin D
! Building block of plasma membranes
o Most cholesterol (85%) is produced in the liver; only 15% is from the diet
The Central Role of the Liver in Metabolism
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Cholesterol metabolism and transport (continued)
o Cholesterol and fatty acids cannot freely circulate in the bloodstream
o They are transported by lipoproteins (lipid-protein complexes) known as LDLs
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and HDLs
The Central Role of the Liver in Metabolism
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Cholesterol metabolism and transport (continued)
o Low-density lipoproteins (LDLs) transport cholesterol to body cells
! Rated “bad lipoproteins” since they can lead to artherosclerosis
o High-density lipoproteins (HDLs) transport cholesterol from body cells to the
liver
! Rated “good lipoproteins” since cholesterol is destined for breakdown
and elimination
Body Energy Balance
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Energy intake = Total energy output
(heat + work + energy storage)
o Energy intake is the energy liberated during food oxidation
! Energy produced during glycolysis, Krebs cycle, and the electron
transport chain
o Energy output
! Energy we lose as heat (60%)
! Energy stored as fat or glycogen
Body Energy Balance
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Interference with the body’s energy balance leads to:
o Obesity
o Malnutrition (leading to body wasting)
Regulation of Food Intake
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Body weight is usually relatively stable
o Energy intake and output remain about equal
Mechanisms that may regulate food intake
o Levels of nutrients in the blood
o Hormones
o Body temperature
o Psychological factors
Metabolic Rate and Body Heat Production
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Nutrients yield different amounts of energy
Energy value is measured in kilocalorie (kcal)
o Carbohydrates and proteins yield 4 kcal/gram
o Fats yield 9 kcal/gram
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Basal Metabolic Rate
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Basic metabolic rate (BMR)—amount of heat produced by the body per unit of time at
rest
Average BMR is about 60 to 72 kcal/hour for an average 70-kg (154-lb) adult
Basal Metabolic Rate
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Factors that influence BMR
o Surface area—a small body usually has a higher BMR
o Gender—males tend to have higher BMRs
o Age—children and adolescents have higher BMRs
o The amount of thyroxine produced is the most important control factor
! More thyroxine means a higher metabolic rate
Total Metabolic Rate (TMR)
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TMR—total amount of kilocalories the body must consume to fuel ongoing activities
TMR increases dramatically with an increase in muscle activity
TMR must equal calories consumed to maintain homeostasis and maintain a
constant weight
Body Temperature Regulation
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When foods are oxidized, more than 60% of energy escapes as heat, warming the
body
The body has a narrow range of homeostatic temperature
o Must remain between 35.6°C and 37.8°C (96°F and 100°F)
Body Temperature Regulation
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The body’s thermostat is in the hypothalamus
Hypothalamus initiates mechanisms to maintain body temperature
o Heat-loss mechanisms involve radiation of heat from skin and evaporation of
sweat
o Heat-promoting mechanisms involve vasoconstriction of skin blood vessels
and shivering
Body Temperature Regulation
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Fever—controlled hyperthermia
o Results from infection, cancer, allergic reactions, CNS injuries
o If the body thermostat is set too high, body proteins may be denatured, and
permanent brain damage may occur
Developmental Aspects of the Digestive System and Metabolism
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The alimentary canal is a continuous, hollow tube present by the fifth week of
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development
Digestive glands bud from the mucosa of the alimentary tube
The developing fetus receives all nutrients through the placenta
In newborns, feeding must be frequent, peristalsis is inefficient, and vomiting is
common
Developmental Aspects of the Digestive System and Metabolism
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Common congenital defects that interfere with normal nutrition:
o Cleft palate
o Cleft lip
o Tracheoesophageal fistula
Common inborn errors of metabolism:
o Phenylketonuria (PKU)
o Cystic fibrosis (CF)
Developmental Aspects of the Digestive System and Metabolism
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Newborn reflexes
o Rooting reflex helps the infant find the nipple
o Sucking reflex helps the infant hold on to the nipple and swallow
Teething begins around age 6 months
Developmental Aspects of the Digestive System and Metabolism
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Problems of the digestive system:
o Gastroenteritis—inflammation of the gastrointestinal tract; can occur at any
time
o Appendicitis—inflammation of the appendix; common in adolescents
Metabolism decreases with old age
Middle-age digestive problems
o Ulcers
o Gallbladder problems
Developmental Aspects of the Digestive System and Metabolism
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Later middle-age problems
o Obesity
o Diabetes mellitus
Activity of the digestive tract in old age
o Fewer digestive juices
o Peristalsis slows
o Diverticulosis and gastrointestinal cancers are more common
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