© 2015 Pearson Education, Inc.

The Digestive System Functions
Ingestion—taking in food Digestion—breaking food into nutrient molecules Absorption—movement of nutrients into the bloodstream Defecation—elimination of indigestible waste © 2015 Pearson Education, Inc.

Organs of the Digestive System
Two main groups of organs Alimentary canal (gastrointestinal or GI tract)— continuous, coiled, hollow tube These organs ingest, digest, absorb, defecate Accessory digestive organs Includes teeth, tongue, and other large digestive organs © 2015 Pearson Education, Inc.

Mouth (oral cavity) Parotid gland Sublingual gland Tongue
Figure 14.1 The human digestive system: Alimentary canal and accessory organs. Mouth (oral cavity) Parotid gland Sublingual gland Tongue Salivary glands Submandibular gland Pharynx Esophagus Stomach Pancreas (Spleen) Liver Gallbladder Transverse colon Duodenum Descending colon Small intestine Jejunum Ascending colon Ileum Large intestine Cecum Sigmoid colon Rectum Appendix Anus Anal canal

Organs of the Alimentary Canal
The alimentary canal is a continuous, coiled, hollow tube that runs through the ventral cavity from stomach to anus: Mouth Pharynx Esophagus Stomach Small intestine Large intestine Anus © 2015 Pearson Education, Inc.

Mouth (Oral Cavity) Anatomy of the mouth
The mouth (oral cavity)—mucous membrane–lined cavity Lips (labia)—protect the anterior opening Cheeks—form the lateral walls Hard palate—forms the anterior roof Soft palate—forms the posterior roof Uvula—fleshy projection of the soft palate © 2015 Pearson Education, Inc.

Mouth (Oral Cavity) Anatomy of the mouth (continued)
Vestibule—space between lips externally and teeth and gums internally Oral cavity proper—area contained by the teeth Tongue—attached at hyoid bone and styloid processes of the skull, and by the lingual frenulum to the floor of the mouth © 2015 Pearson Education, Inc.

Mouth (Oral Cavity) Anatomy of the mouth (continued) Tonsils
Palatine—located at posterior end of oral cavity Lingual—located at the base of the tongue © 2015 Pearson Education, Inc.

Figure 14.2a Anatomy of the mouth (oral cavity).
Nasopharynx Hard palate Soft palate Oral cavity Uvula Lips (labia) Palatine tonsil Vestibule Lingual tonsil Oropharynx Lingual frenulum Epiglottis Tongue Laryngopharynx Hyoid bone Esophagus Trachea (a)

Figure 14.2b Anatomy of the mouth (oral cavity).
Upper lip Gingivae (gums) Hard palate Soft palate Uvula Palatine tonsil Oropharynx Tongue (b)

Mouth Functions of the mouth Mastication (chewing) of food
Tongue mixes masticated food with saliva Tongue initiates swallowing Taste buds on the tongue allow for taste © 2015 Pearson Education, Inc.

Pharynx Food passes from the mouth posteriorly into the:
Oropharynx—posterior to oral cavity Laryngopharynx—below the oropharynx and continuous with the esophagus © 2015 Pearson Education, Inc.

Figure 14.2a Anatomy of the mouth (oral cavity).
Nasopharynx Hard palate Soft palate Oral cavity Uvula Lips (labia) Palatine tonsil Vestibule Lingual tonsil Oropharynx Lingual frenulum Epiglottis Tongue Laryngopharynx Hyoid bone Esophagus Trachea (a)

Pharynx 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 Longitudinal inner layer Circular outer layer Alternating contractions of the muscle layers (peristalsis) propel the food © 2015 Pearson Education, Inc.

Esophagus (Gullet) Anatomy Physiology About 10 inches long
Runs from pharynx to stomach through the diaphragm Physiology Conducts food by peristalsis (slow rhythmic squeezing) to the stomach Passageway for food only (respiratory system branches off after the pharynx) © 2015 Pearson Education, Inc.

Layers of Tissue in the Alimentary Canal Organs
Summary of the four layers from innermost to outermost (detailed next): Mucosa Submucosa Muscularis externa Serosa © 2015 Pearson Education, Inc.

Mucosa 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 Lines the cavity (known as the lumen) © 2015 Pearson Education, Inc.

Muscularis externa—smooth muscle Inner circular layer Outer longitudinal layer Serosa—outermost layer of the wall contains fluid-producing cells Visceral peritoneum—innermost layer that is continuous with the outermost layer Parietal peritoneum—outermost layer that lines the abdominopelvic cavity by way of the mesentery © 2015 Pearson Education, Inc.

Figure 14.3 Basic structure of the alimentary canal wall.
Visceral peritoneum Intrinsic nerve plexuses Myenteric nerve plexus Submucosal nerve plexus Submucosal glands Mucosa Surface epithelium Lamina propria Muscle layer Submucosa Muscularis externa Longitudinal muscle layer Circular muscle layer Serosa (visceral peritoneum) Nerve Gland in mucosa Lumen Artery Mesentery Vein Duct of gland outside alimentary canal Lymphoid tissue

Figure 14.5 Peritoneal attachments of the abdominal organs.
Diaphragm Falciform ligament Lesser omentum Liver Spleen Pancreas Gallbladder Stomach Duodenum Visceral peritoneum Transverse colon Greater omentum Mesenteries Parietal peritoneum Small intestine Peritoneal cavity Uterus Large intestine Cecum Rectum Anus Urinary bladder (a) (b)

Alimentary Canal Nerve Plexuses
Two important nerve plexuses serve the alimentary canal Both are part of the autonomic nervous system Submucosal nerve plexus Myenteric nerve plexus Function is to regulate mobility and secretory activity of the GI tract organs © 2015 Pearson Education, Inc.

Stomach 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) © 2015 Pearson Education, Inc.

Stomach Regions of the stomach Cardial part (cardia)—near the heart
Fundus—expanded portion lateral to the cardiac region Body—midportion Pylorus—funnel-shaped terminal end © 2015 Pearson Education, Inc.

Stomach Stomach can stretch and hold 4 L (1 gallon) of food when full
Rugae—internal folds of the mucosa present when the stomach is empty External regions Lesser curvature—concave medial surface Greater curvature—convex lateral surface © 2015 Pearson Education, Inc.

Figure 14.4a Anatomy of the stomach.
Cardia Fundus Esophagus Muscularis externa Serosa Longitudinal layer Circular layer Body Oblique layer Lesser curvature Rugae of mucosa Pylorus Greater curvature Duodenum Pyloric sphincter (valve) Pyloric antrum (a)

Figure 14.4b Anatomy of the stomach.
Fundus Body Rugae of mucosa (b) Pyloric sphincter Pyloric antrum

Stomach Layers of peritoneum attached to the stomach
Lesser omentum—attaches the liver to the lesser curvature 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 Oblique layer helps to churn, mix, and pummel the food © 2015 Pearson Education, Inc.

Figure 14.5a Peritoneal attachments of the abdominal organs.
Diaphragm Falciform ligament Liver Spleen Gallbladder Stomach Greater omentum Small intestine Large intestine Cecum (a)

Figure 14.5b Peritoneal attachments of the abdominal organs.
Diaphragm Lesser omentum Liver Pancreas Duodenum Stomach Visceral peritoneum Transverse colon Greater omentum Mesenteries Parietal peritoneum Small intestine Peritoneal cavity Uterus Rectum Anus Urinary bladder (b)

Stomach Functions of the stomach Temporary storage tank for food
Site of food breakdown Chemical breakdown of protein begins Delivers chyme (processed food) to the small intestine © 2015 Pearson Education, Inc.

Stomach Structure of the stomach mucosa:
Simple columnar epithelium dotted by gastric pits that lead to gastric glands Mucous cells produce bicarbonate-rich alkaline mucus Gastric glands—situated in gastric pits and secrete gastric juice, including: Intrinsic factor, which is needed for vitamin B12 absorption in the small intestine © 2015 Pearson Education, Inc.

Stomach Structure of the stomach mucosa (continued)
Chief cells—produce protein-digesting enzymes (pepsinogens) Parietal cells—produce hydrochloric acid Mucous neck cells—produce thin acidic mucus (different from the mucus produced by cells of the mucosa) Enteroendocrine cells—produce a hormone called gastrin © 2015 Pearson Education, Inc.

Figure 14.4c Anatomy of the stomach.
Gastric pits Surface epithelium Gastric pit Pyloric sphincter Mucous neck cells Parietal cells Gastric gland Gastric glands Chief cells (c)

Figure 14.4d Anatomy of the stomach.
Pepsinogen Pepsin HCI Parietal cells Chief cells Enteroendocrine cell (d)

Small Intestine 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 © 2015 Pearson Education, Inc.

Small Intestine Subdivisions Duodenum Jejunum Ileum
Attached to the stomach Curves around the head of the pancreas Jejunum Attaches anteriorly to the duodenum Ileum Extends from jejunum to large intestine Meets the large intestine at the ileocecal valve © 2015 Pearson Education, Inc.

Small Intestine Chemical digestion begins in the small intestine
Enzymes are produced by: Intestinal cells Pancreas Pancreatic ducts carry enzymes to the duodenum Bile, formed by the liver, enters the duodenum via the bile duct © 2015 Pearson Education, Inc.

Figure 14.6 The duodenum of the small intestine and related organs.
Right and left hepatic ducts from liver Cystic duct Common hepatic duct Bile duct and sphincter Accessory pancreatic duct Pancreas Gallbladder Jejunum Duodenal papilla Main pancreatic duct and sphincter Hepatopancreatic ampulla and sphincter Duodenum

Small Intestine Three structural modifications that increase surface area for food absorption Microvilli—tiny projections of the plasma membrane (create a brush border appearance) Villi—fingerlike projections formed by the mucosa House a capillary bed and lacteal Circular folds (plicae circulares)—deep folds of mucosa and submucosa © 2015 Pearson Education, Inc.

Figure 14.7a Structural modifications of the small intestine.
Blood vessels serving the small intestine Muscle layers Lumen Circular folds (plicae circulares) Villi (a) Small intestine

Figure 14.7b Structural modifications of the small intestine.
Absorptive cells Lacteal Villus Blood capillaries Lymphoid tissue Intestinal crypt Venule Muscularis mucosae Lymphatic vessel Submucosa (b) Villi

Figure 14.7c Structural modifications of the small intestine.
Microvilli (brush border) (c) Absorptive cells

Large Intestine Larger in diameter, but shorter in length at 1.5 m, than the small intestine Extends from the ileocecal valve to the anus Subdivisions: Cecum Appendix Colon Rectum Anal canal © 2015 Pearson Education, Inc.

Large Intestine Anatomy
Cecum—saclike first part of the large intestine Appendix Accumulation of lymphoid tissue that sometimes becomes inflamed (appendicitis) Hangs from the cecum © 2015 Pearson Education, Inc.

Colon Ascending—travels up right side of abdomen Transverse—travels across the abdominal cavity Descending—travels down the left side Sigmoid—S-shaped region; enters the pelvis Sigmoid colon, rectum, and anal canal are located in the pelvis © 2015 Pearson Education, Inc.

Anal canal ends at the anus Anus—opening of the large intestine External anal sphincter—formed by skeletal muscle and under voluntary control Internal anal sphincter—formed by smooth muscle and involuntarily controlled These sphincters are normally closed except during defecation The large intestine delivers undigestible food residues to the body’s exterior © 2015 Pearson Education, Inc.

Figure 14.8 The large intestine.
Left colic (splenic) flexure Transverse mesocolon Right colic (hepatic) flexure Transverse colon Haustrum Descending colon Ascending colon Cut edge of mesentery Ileum (cut) Ileocecal valve Teniae coli Sigmoid colon Cecum Appendix Rectum Anal canal External anal sphincter

Large Intestine 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) © 2015 Pearson Education, Inc.

Accessory Digestive Organs
Teeth Salivary glands Pancreas Liver Gallbladder © 2015 Pearson Education, Inc.

Teeth Teeth masticate (chew) food into smaller fragments
Humans have two sets of teeth during a lifetime: Deciduous (baby or “milk”) teeth A baby has 20 teeth by age 2 First teeth to appear are the lower central incisors © 2015 Pearson Education, Inc.

Teeth Permanent teeth Replace deciduous teeth between the ages of 6 and 12 A full set is 32 teeth, but some people do not have wisdom teeth (third molars) If they do emerge, the wisdom teeth appear between ages of 17 and 25 © 2015 Pearson Education, Inc.

Classification of Teeth
Incisors—cutting Canines (eyeteeth)—tearing or piercing Premolars (bicuspids)—grinding Molars—grinding © 2015 Pearson Education, Inc.

Figure 14.9 Human deciduous and permanent teeth.
Incisors Central (6–8 mo) Lateral (8–10 mo) Canine (eyetooth) (16–20 mo) Molars First molar (10–15 mo) Second molar (about 2 yr) Deciduous (milk) teeth Incisors Central (7 yr) Lateral (8 yr) Canine (eyetooth) (11 yr) Premolars (bicuspids) First premolar (11 yr) Second premolar (12–13 yr) Molars First molar (6–7 yr) Permanent teeth Second molar (12–13 yr) Third molar (wisdom tooth) (17–25 yr)

Regions of a Tooth Two major regions of a tooth Crown Root
© 2015 Pearson Education, Inc.

Regions of a Tooth Crown—exposed part of tooth above the gingiva (gum)
Enamel—covers the crown Dentin—found deep to the enamel and forms the bulk of the tooth, surrounds the pulp cavity Pulp cavity—contains connective tissue, blood vessels, and nerve fibers (pulp) Root canal—where the pulp cavity extends into the root © 2015 Pearson Education, Inc.

Regions of a Tooth Note: The neck is a connector between the crown and root. Region in contact with the gum Connects crown to root Root Cement—covers outer surface and attaches the tooth to the periodontal membrane (ligament) Periodontal membrane holds tooth in place in the bony jaw © 2015 Pearson Education, Inc.

Figure 14.10 Longitudinal section of a molar.
Enamel Dentin Crown Pulp cavity (contains blood vessels and nerves) Neck Gum (gingiva) Cement Root canal Root Periodontal membrane (ligament) Bone

Salivary Glands Three pairs of salivary glands empty secretions into the mouth Parotid glands Found anterior to the ears Submandibular glands Sublingual glands Both submandibular and sublingual glands empty saliva into the floor of the mouth through small ducts © 2015 Pearson Education, Inc.

Salivary Glands Saliva Mixture of mucus and serous fluids
Helps to moisten and bind food together into a mass called a bolus Contains salivary amylase to begin starch digestion Dissolves chemicals so they can be tasted © 2015 Pearson Education, Inc.

Pancreas Found posterior to the parietal peritoneum
Mostly retroperitoneal Extends across the abdomen from spleen to duodenum © 2015 Pearson Education, Inc.

Pancreas 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 Insulin Glucagon © 2015 Pearson Education, Inc.

Liver 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 © 2015 Pearson Education, Inc.

Liver 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: Bile salts and bile pigments (mostly bilirubin from the breakdown of hemoglobin) Cholesterol, phospholipids, and electrolytes © 2015 Pearson Education, Inc.

Gallbladder 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 © 2015 Pearson Education, Inc.

Functions of the Digestive System
Major functions of the digestive system are summarized as: Digestion Absorption We will cover 6 more specific processes next © 2015 Pearson Education, Inc.

Ingestion—placing of food into the mouth Propulsion—movement of foods from one region of the digestive system to another Peristalsis—alternating waves of contraction and relaxation that squeezes food along the GI tract Segmentation—movement of materials back and forth to foster mixing in the small intestine © 2015 Pearson Education, Inc.

Figure 14.12 Peristaltic and segmental movements of the digestive tract.
(b)

Food breakdown: mechanical breakdown Examples: Mixing of food in the mouth by the tongue Churning of food in the stomach Segmentation in the small intestine Mechanical digestion prepares food for further degradation by enzymes © 2015 Pearson Education, Inc.

Food breakdown: digestion Digestion occurs when enzymes chemically break down large molecules into their building blocks 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 © 2015 Pearson Education, Inc.

Starch and disaccharides
Figure Flowchart of digestion and absorption of foodstuffs. (1 of 3) Foodstuff Enzyme(s) and source Site of action Starch and disaccharides Digestion of carbohydrates Salivary amylase Mouth Pancreatic amylase Small intestine Oligosaccharides* and disaccharides Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, and sucrase) Small intestine Lactose Maltose Sucrose Galactose Glucose Fructose The monosaccharides glucose, galactose, and fructose enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein. Absorption of carbohydrates *Oligosaccharides consist of a few linked monosaccharides.

(trypsin, chymotrypsin, carboxypeptidase) Small intestine
Figure Flowchart of digestion and absorption of foodstuffs. (2 of 3) Foodstuff Enzyme(s) and source Site of action Protein Digestion of proteins Pepsin (stomach glands) in the presence of HCI Stomach Large polypeptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Small intestine Small polypeptides, small peptides Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Small intestine Amino acids (some dipeptides and tripeptides) Absorption of proteins Amino acids enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein.

bile salts from the liver Small intestine
Figure Flowchart of digestion and absorption of foodstuffs. (3 of 3) Foodstuff Enzyme(s) and source Site of action Unemulsified fats Digestion of fats Emulsified by the detergent action of bile salts from the liver Small intestine Pancreatic lipase Small intestine Monoglycerides and fatty acids Glycerol and fatty acids Fatty acids and monoglycerides enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. (Glycerol and short-chain fatty acids are absorbed into the capillary blood in the villi and transported to the liver via the hepatic portal vein.) Absorption of fats

Absorption End products of digestion are absorbed in the blood or lymph Food must enter mucosal cells and then into blood or lymph capillaries Defecation Elimination of indigestible substances from the GI tract in the form of feces © 2015 Pearson Education, Inc.

Figure 14.11 Schematic summary of gastrointestinal tract activities.
Ingestion Food Mechanical breakdown Pharynx Chewing (mouth) Esophagus Churning (stomach) Propulsion Segmentation (small intestine) Swallowing (oropharynx) Digestion Peristalsis (esophagus, stomach, small intestine, large intestine) Stomach Absorption Lymph vessel Small intestine Blood vessel Large intestine Mainly H2O Feces Anus Defecation

Activities Occurring in the Mouth, Pharynx, and Esophagus
Food ingestion and breakdown 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 Essentially, no food absorption occurs in the mouth © 2015 Pearson Education, Inc.

Food propulsion—swallowing and peristalsis Pharynx and esophagus have no digestive function Serve as passageways to the stomach Pharynx functions in swallowing (deglutition) Two phases of swallowing: Buccal phase Pharyngeal-esophgeal phase © 2015 Pearson Education, Inc.

Food propulsion—swallowing and peristalsis (continued) Buccal phase Voluntary Occurs in the mouth Food is formed into a bolus The bolus is forced into the pharynx by the tongue © 2015 Pearson Education, Inc.

Food propulsion—swallowing and peristalsis (continued) Pharyngeal-esophageal phase Involuntary transport of the bolus by peristalsis Nasal and respiratory passageways are blocked © 2015 Pearson Education, Inc.

Food propulsion—swallowing and peristalsis (continued) Pharyngeal-esophogeal phase (continued) Peristalsis moves the bolus toward the stomach The cardioesophageal sphincter is opened when food presses against it © 2015 Pearson Education, Inc.

Bolus of food Tongue Pharynx Upper esophageal sphincter Epiglottis up
Figure 14.14a Swallowing. Bolus of food Tongue Pharynx Upper esophageal sphincter Epiglottis up Glottis (lumen) of larynx Trachea Esophagus (a) Upper esophageal sphincter contracted

Uvula Bolus Epiglottis down Larynx up Esophagus (b) Upper esophageal
Figure 14.14b Swallowing. Uvula Bolus Epiglottis down Larynx up Esophagus (b) Upper esophageal sphincter relaxed

Bolus (c) Upper esophageal sphincter contracted
Figure 14.14c Swallowing. Bolus (c) Upper esophageal sphincter contracted

Relaxed muscles Cardioesophageal sphincter open (d) Cardioesophageal
Figure 14.14d Swallowing. Relaxed muscles Cardioesophageal sphincter open (d) Cardioesophageal sphincter relaxed

Activities in the Stomach
Food breakdown Gastric juice is regulated by neural and hormonal factors Presence of food or rising pH causes the release of the hormone gastrin Gastrin causes stomach glands to produce: Protein-digesting enzymes Mucus Hydrochloric acid © 2015 Pearson Education, Inc.

Food breakdown (continued) Hydrochloric acid makes the stomach contents very acidic Acidic pH Activates pepsinogen to pepsin for protein digestion Provides a hostile environment for microorganisms © 2015 Pearson Education, Inc.

Food breakdown (continued) Protein digestion enzymes Pepsin—an active protein-digesting enzyme Rennin—works on digesting milk protein in infants, not adults Alcohol and aspirin are virtually the only items absorbed in the stomach © 2015 Pearson Education, Inc.

Food propulsion Peristalsis: Waves of peristalsis occur from the fundus to the pylorus, forcing food past the pyloric sphincter Grinding: The pylorus meters out chyme into the small intestine (3 ml at a time) Retropulsion: Peristaltic waves close the pyloric sphincter, forcing content back into the stomach. The stomach empties in 4–6 hours © 2015 Pearson Education, Inc.

Figure 14.15 Peristaltic waves in the stomach.
Pyloric valve closed Pyloric valve slightly opened Pyloric valve closed 1 Propulsion: Peristaltic waves move from the fundus toward the pylorus. 2 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum. 3 Retropulsion: The peristaltic wave closes the pyloric valve, forcing most of the contents of the pylorus backward into the stomach.

Activities of the Small Intestine
Food breakdown and absorption Intestinal enzymes from the brush border function to: Break double sugars into simple sugars Complete some protein digestion Intestinal enzymes and pancreatic enzymes help to complete digestion of all food groups © 2015 Pearson Education, Inc.

Food breakdown and absorption (continued) Pancreatic enzymes play the major role in the digestion of fats, proteins, and carbohydrates Alkaline content neutralizes acidic chyme and provides the proper environment for the pancreatic enzymes to operate © 2015 Pearson Education, Inc.

Food breakdown and absorption (continued) 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): Secretin Cholecystokinin (CCK) © 2015 Pearson Education, Inc.

Food breakdown and absorption (continued) Hormones (secretin and CCK) also target the liver and gallbladder to release bile Bile Acts as a fat emulsifier Needed for fat absorption and absorption of fat-soluble vitamins (K, D, E, and A) © 2015 Pearson Education, Inc.

release cholecytokinin (CCK) and secretin.
Figure Regulation of pancreatic juice and bile secretion and release. 4 Secretin causes the liver to secrete more bile; CCK stimulates the gallbladder to release stored bile and the hepatopancreatic sphincter to relax (allows bile from both sources to enter the duodenum). 1 Chyme entering duodenum causes duodenal enteroendocrine cells to release cholecytokinin (CCK) and secretin. 2 CCK (red dots) and secretin (blue dots) enter the bloodstream. Stimulation by vagal nerve fibers cause release of pancreatic juice and weak contractions of the gallbladder. 5 3 Upon reaching the pancreas, CCK induces secretion of enzyme-rich pancreatic juice; secretin causes secretion of bicarbonate-rich pancreatic juice.

Table 14.1 Hormones and Hormonelike Products That Act in Digestion (1 of 2).

Table 14.1 Hormones and Hormonelike Products That Act in Digestion (2 of 2).

Food breakdown and absorption (continued) Water is absorbed along the length of the small intestine End products of digestion Most substances are absorbed by active transport through cell membranes Lipids are absorbed by diffusion Substances are transported to the liver by the hepatic portal vein or lymph © 2015 Pearson Education, Inc.

Figure 14.12b Peristaltic and segmental movements of the digestive tract.

Activities of the Large Intestine
Food breakdown and absorption No digestive enzymes are produced Resident bacteria digest remaining nutrients Produce some vitamin K and B Release gases Water and vitamins K and B are absorbed Remaining materials are eliminated via feces © 2015 Pearson Education, Inc.

Propulsion of the residue and defecation Sluggish peristalsis begins when food residue arrives Haustral contractions are most seen in the large intestine Mass movements are slow, powerful movements that occur 3 to 4 times per day © 2015 Pearson Education, Inc.

Propulsion of the residue and defecation (continued) 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 © 2015 Pearson Education, Inc.

Nutrition and Metabolism
Most foods are used as metabolic fuel Foods are oxidized and transformed into adenosine triphosphate (ATP) ATP is chemical energy that drives cellular activities Energy value of food is measured in kilocalories (kcal) or Calories (C) © 2015 Pearson Education, Inc.

Figure 14.17 Two visual food guides.
Red meat, butter: use sparingly White rice, white bread, potatoes, pasta, sweets: use sparingly Dairy or calcium supplement: 1–2 servings Fish, poultry, eggs: 0–2 servings Nuts, legumes: 1–3 servings Vegetables in abundance Fruits: 2–3 servings Whole-grain foods at most meals Plant oils at most meals Daily exercise and weight control (a) Healthy Eating Pyramid (b) USDA’s MyPlate

Nutrition 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 © 2015 Pearson Education, Inc.

Table 14.2 Five Basic Food Groups and Some of Their Major Nutrients (1 of 2).

Table 14.2 Five Basic Food Groups and Some of Their Major Nutrients (2 of 2).

Dietary Sources of Major Nutrients
Carbohydrates Dietary carbohydrates are sugars and starches Most are derived from plants such as fruits and vegetables Exceptions: lactose from milk and small amounts of glycogens from meats © 2015 Pearson Education, Inc.

Lipids Saturated fats from animal products (meats) Unsaturated fats from nuts, seeds, and vegetable oils Cholesterol from egg yolk, meats, and milk products (dairy products) © 2015 Pearson Education, Inc.

Proteins 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 Legumes and beans also have proteins, but the proteins are incomplete © 2015 Pearson Education, Inc.

Figure 14.18 The eight essential amino acids.
Beans and other legumes Tryptophan Methionine Valine Threonine Phenylalanine Leucine Isoleucine Corn and other grains Lysine

Minerals Mainly important for enzyme activity Foods richest in minerals: vegetables, legumes, milk, and some meats Iron is important for making hemoglobin Calcium is important for building bone, blood clotting, and secretory activities © 2015 Pearson Education, Inc.

Metabolism Metabolism is all of the chemical reactions necessary to maintain life Catabolism—substances are broken down to simpler substances; energy is released Anabolism—larger molecules are built from smaller ones © 2015 Pearson Education, Inc.

Carbohydrate Metabolism
Carbohydrates are the body’s preferred source to produce cellular energy (ATP) Glucose (blood sugar) Major breakdown product of carbohydrate digestion Fuel used to make ATP © 2015 Pearson Education, Inc.

Figure 14.19 Summary equation for cellular respiration.
C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP Glucose Oxygen gas Carbon dioxide Water Energy

Events of three main metabolic pathways of cellular respiration Glycolysis Occurs in the cytosol Energizes a glucose molecule so it can be split into two pyruvic acid molecules and yield ATP © 2015 Pearson Education, Inc.

Events of three main metabolic pathways of cellular respiration (continued) Krebs cycle Occurs in the mitochondrion Produces virtually all the carbon dioxide and water resulting from cellular respiration Yields a small amount of ATP © 2015 Pearson Education, Inc.

Events of three main metabolic pathways of cellular respiration (continued) 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 © 2015 Pearson Education, Inc.

Chemical energy (high-energy electrons)
Figure During cellular respiration, ATP is formed in the cytosol and in the mitochondria. Chemical energy (high-energy electrons) CO2 Chemical energy CO2 Electron transport chain and oxidative phosphorylation Glycolysis Krebs cycle Pyruvic acid Glucose H2O Mitochondrion Cytosol of cell Mitochondrial cristae Via oxidative phosphorylation Via substrate-level phosphorylation 2 ATP 2 ATP 28 ATP 1 During glycolysis, each glucose molecule is broken down into two molecules of pyruvic acid as hydrogen atoms containing high-energy electrons are removed. 2 The pyruvic acid enters the mitochondrion, where Krebs cycle enzymes remove more hydrogen atoms and decompose it to CO2. During glycolysis and the Krebs cycle, small amounts of ATP are formed. 3 Energy-rich electrons picked up by coenzymes are transferred to the electron transport chain, built into the cristae membrane. The electron transport chain carries out oxidative phosphorylation, which accounts for most of the ATP generated by cellular respiration, and finally unites the removed hydrogen with oxygen to form water.

of the electron transport chain
Figure Electron transport chain versus one-step reduction of oxygen. NADH NAD+ + H+ Energy released as heat and light 2e− Energy released and now available for making ATP of the electron transport chain Protein carriers e− Electron flow O2 (a) (b)

Hyperglycemia—excessively high levels of glucose in the blood Excess glucose is stored in body cells as glycogen or converted to fat Hypoglycemia—low levels of glucose in the blood Glycogenolysis, gluconeogenesis, and fat breakdown occur to restore normal blood glucose levels © 2015 Pearson Education, Inc.

Figure 14.22a Metabolism by body cells.
Carbohydrates: polysaccharides, disaccharides; composed of simple sugars (monosaccharides) ATP Glycogen and fat broken down for ATP formation Polysaccharides Cellular uses GI digestion to simple sugars To capillary Excess stored as glycogen or fat Broken down to glucose and released to blood Monosaccharides

Fat Metabolism Fats Insulate the body Protect organs Build some cell structures (membranes and myelin sheaths) Provide reserve energy Excess dietary fat is stored in subcutaneous tissue and other fat depots © 2015 Pearson Education, Inc.

Fat Metabolism When carbohydrates are in limited supply, more fats are oxidized to produce ATP Excessive fat breakdown causes blood to become acidic (acidosis or ketoacidosis) Breath has a fruity odor Common with: “No carbohydrate” diets Uncontrolled diabetes mellitus Starvation © 2015 Pearson Education, Inc.

Figure 14.22b Metabolism by body cells.
Fats: composed of 1 glycerol molecule and 3 fatty acids; triglycerides Cellular uses ATP Fats are the primary fuels in many cells Lipid (fat) Metabolized by liver to acetic acid, etc. Fatty acids GI digestion to fatty acids and glycerol Insulation and fat cushions to protect body organs Fats build myelin sheaths and cell membranes Glycerol

Protein Metabolism 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 © 2015 Pearson Education, Inc.

Protein Metabolism 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 © 2015 Pearson Education, Inc.

Figure 14.22c Metabolism by body cells.
Proteins: polymers of amino acids ATP formation if inadequate glucose and fats or if essential amino acids are lacking ATP Normally infrequent Protein Functional proteins (enzymes, antibodies, hemoglobin, etc.) GI digestion to amino acids Cellular uses Structural proteins (connective tissue fibers, muscle proteins, etc.) Amino acids

Figure 14.22d Metabolism by body cells.
ATP formation (fueling the metabolic furnace): all categories of food can be oxidized to provide energy molecules (ATP) Carbon dioxide Cellular metabolic “furnace”: Monosaccharides Water Fatty acids Krebs cycle and electron transport chain Amino acids (amine first removed and combined with CO2 by the liver to form urea) ATP

The Central Role of the Liver in Metabolism
Liver is the body’s key metabolic organ Roles in digestion: Manufactures bile Detoxifies drugs and alcohol Degrades hormones Produces cholesterol, blood proteins (albumin and clotting proteins) Plays a central role in metabolism Liver can regenerate if part of it is damaged or removed © 2015 Pearson Education, Inc.

To maintain homeostasis of blood glucose levels, the liver performs: Glycogenesis—“glycogen formation” Glucose molecules are converted to glycogen and stored in the liver Glycogenolysis—“glycogen splitting” Glucose is released from the liver after conversion from glycogen Gluconeogenesis—“formation of new sugar” Glucose is produced from fats and proteins © 2015 Pearson Education, Inc.

HOMEOSTATIC BLOOD SUGAR
Figure Metabolic events occurring in the liver as blood glucose levels rise and fall. Glycogenesis: Glucose converted to glycogen and stored Stimulus: Rising blood glucose level IMBALANCE HOMEOSTATIC BLOOD SUGAR Stimulus: Falling blood glucose level IMBALANCE Glycogenolysis: Stored glycogen converted to glucose Gluconeogenesis: Amino acids and fats converted to glucose

Fats and fatty acids are picked up by the liver Some are oxidized to provide energy for liver cells The rest are either stored or broken down into simpler compounds and released into the blood © 2015 Pearson Education, Inc.

Cholesterol metabolism and transport Cholesterol is not used to make ATP Functions of cholesterol: Structural basis of steroid hormones and vitamin D Building block of plasma membranes Most cholesterol (85%) is produced in the liver; only 15% is from the diet © 2015 Pearson Education, Inc.

Cholesterol metabolism and transport (continued) Cholesterol and fatty acids cannot freely circulate in the bloodstream They are transported by lipoproteins (lipid-protein complexes) known as LDLs and HDLs © 2015 Pearson Education, Inc.

Cholesterol metabolism and transport (continued) Low-density lipoproteins (LDLs) transport cholesterol to body cells Rated “bad lipoproteins” since they can lead to artherosclerosis High-density lipoproteins (HDLs) transport cholesterol from body cells to the liver Rated “good lipoproteins” since cholesterol is destined for breakdown and elimination © 2015 Pearson Education, Inc.

(heat + work + energy storage)
Body Energy Balance Energy intake = Total energy output (heat + work + energy storage) Energy intake is the energy liberated during food oxidation Energy produced during glycolysis, Krebs cycle, and the electron transport chain Energy output Energy we lose as heat (60%) Energy stored as fat or glycogen © 2015 Pearson Education, Inc.

Regulation of Food Intake
Body weight is usually relatively stable Energy intake and output remain about equal Mechanisms that may regulate food intake Levels of nutrients in the blood Hormones Body temperature Psychological factors © 2015 Pearson Education, Inc.

Metabolic Rate and Body Heat Production
Nutrients yield different amounts of energy Energy value is measured in kilocalorie (kcal) Carbohydrates and proteins yield 4 kcal/gram Fats yield 9 kcal/gram © 2015 Pearson Education, Inc.

Basal Metabolic Rate 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 © 2015 Pearson Education, Inc.

Basal Metabolic Rate Factors that influence BMR
Surface area—a small body usually has a higher BMR Gender—males tend to have higher BMRs Age—children and adolescents have higher BMRs The amount of thyroxine produced is the most important control factor More thyroxine means a higher metabolic rate © 2015 Pearson Education, Inc.

Table 14.3 Factors Determining the Basal Metabolic Rate (BMR).

Total Metabolic Rate (TMR)
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 © 2015 Pearson Education, Inc.

Body Temperature Regulation
When foods are oxidized, more than 60% of energy escapes as heat, warming the body The body has a narrow range of homeostatic temperature Must remain between 35.6°C and 37.8°C (96°F and 100°F) © 2015 Pearson Education, Inc.

The body’s thermostat is in the hypothalamus Hypothalamus initiates mechanisms to maintain body temperature Heat-loss mechanisms involve radiation of heat from skin and evaporation of sweat Heat-promoting mechanisms involve vasoconstriction of skin blood vessels and shivering © 2015 Pearson Education, Inc.

Figure 14.24 Mechanisms of body temperature regulation.
Skin blood vessels dilate: Capillaries become flushed with warm blood; heat radiates from skin surface Activates heat-loss center in hypothalamus Sweat glands activated: Secrete perspiration, which is vaporized by body heat, helping to cool the body Body temperature decreases: Blood temperature declines and hypothalamus heat-loss center “shuts off” Blood warmer than hypothalamic set point Stimulus: Increased body temperature (e.g., when exercising or the climate is hot) IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) Stimulus: Decreased body temperature (e.g., due to cold environmental temperatures) IMBALANCE Skin blood vessels constrict: Blood is diverted from skin capillaries and withdrawn to deeper tissues; minimizes overall heat loss from skin surface Blood cooler than hypothalamic set point Body temperature increases: Blood temperature rises and hypothalamus heat-promoting center “shuts off” Activates heat- promoting center in hypothalamus Skeletal muscles activated when more heat must be generated; shivering begins

Slide 1 Skin blood vessels dilate: Capillaries become flushed with warm blood; heat radiates from skin surface Activates heat-loss center in hypothalamus Sweat glands activated: Secrete perspiration, which is vaporized by body heat, helping to cool the body Body temperature decreases: Blood temperature declines and hypothalamus heat-loss center “shuts off” Blood warmer than hypothalamic set point Stimulus: Increased body temperature (e.g., when exercising or the climate is hot) IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) IMBALANCE

Slide 2 Stimulus: Increased body temperature (e.g., when exercising or the climate is hot) IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) IMBALANCE

Slide 3 Activates heat-loss center in hypothalamus Blood warmer than hypothalamic set point Stimulus: Increased body temperature (e.g., when exercising or the climate is hot) IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) IMBALANCE

Slide 4 Skin blood vessels dilate: Capillaries become flushed with warm blood; heat radiates from skin surface Activates heat-loss center in hypothalamus Sweat glands activated: Secrete perspiration, which is vaporized by body heat, helping to cool the body Blood warmer than hypothalamic set point Stimulus: Increased body temperature (e.g., when exercising or the climate is hot) IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) IMBALANCE

Slide 7 IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) Stimulus: Decreased body temperature (e.g., due to cold environmental temperatures) IMBALANCE

Slide 8 IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) Stimulus: Decreased body temperature (e.g., due to cold environmental temperatures) IMBALANCE Blood cooler than hypothalamic set point Activates heat- promoting center in hypothalamus

Slide 9 IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) Stimulus: Decreased body temperature (e.g., due to cold environmental temperatures) IMBALANCE Skin blood vessels constrict: Blood is diverted from skin capillaries and withdrawn to deeper tissues; minimizes overall heat loss from skin surface Blood cooler than hypothalamic set point Activates heat- promoting center in hypothalamus Skeletal muscles activated when more heat must be generated; shivering begins

Slide 10 IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) Stimulus: Decreased body temperature (e.g., due to cold environmental temperatures) IMBALANCE Skin blood vessels constrict: Blood is diverted from skin capillaries and withdrawn to deeper tissues; minimizes overall heat loss from skin surface Blood cooler than hypothalamic set point Body temperature increases: Blood temperature rises and hypothalamus heat-promoting center “shuts off” Activates heat- promoting center in hypothalamus Skeletal muscles activated when more heat must be generated; shivering begins

Slide 11 IMBALANCE HOMEOSTASIS = normal body temperature (35.6°C–37.8°C) Stimulus: Decreased body temperature (e.g., due to cold environmental temperatures) IMBALANCE Skin blood vessels constrict: Blood is diverted from skin capillaries and withdrawn to deeper tissues; minimizes overall heat loss from skin surface Blood cooler than hypothalamic set point Body temperature increases: Blood temperature rises and hypothalamus heat-promoting center “shuts off” Activates heat- promoting center in hypothalamus Skeletal muscles activated when more heat must be generated; shivering begins

Fever—controlled hyperthermia Results from infection, cancer, allergic reactions, CNS injuries If the body thermostat is set too high, body proteins may be denatured, and permanent brain damage may occur © 2015 Pearson Education, Inc.

Developmental Aspects of the Digestive System and Metabolism
The alimentary canal is a continuous, hollow tube present by the fifth week of 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 © 2015 Pearson Education, Inc.

Common congenital defects that interfere with normal nutrition: Cleft palate Cleft lip Tracheoesophageal fistula Common inborn errors of metabolism: Phenylketonuria (PKU) Cystic fibrosis (CF) © 2015 Pearson Education, Inc.

Problems of the digestive system: Gastroenteritis—inflammation of the gastrointestinal tract; can occur at any time Appendicitis—inflammation of the appendix; common in adolescents Metabolism decreases with old age Middle-age digestive problems Ulcers Gallbladder problems © 2015 Pearson Education, Inc.

Later middle-age problems Obesity Diabetes mellitus Activity of the digestive tract in old age Fewer digestive juices Peristalsis slows Diverticulosis and gastrointestinal cancers are more common © 2015 Pearson Education, Inc.

© 2015 Pearson Education, Inc.

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