© 2017 Pearson Education, Inc.

Digestive System Basics (16-1)
Provides fuel for body cells’ functioning Digestive system includes: Digestive tract (gastrointestinal (GI) tract or alimentary canal) Muscular tube Includes oral cavity (mouth), pharynx, esophagus, stomach, small and large intestines, rectum, anus Accessory organs contribute to digestion Teeth, tongue, salivary glands, gallbladder, liver, and pancreas

Figure 16-1 The Components of the Digestive System.
Accessory Organs of the Digestive System Major Organs of the Digestive Tract Teeth Oral Cavity (Mouth) Tongue Pharynx Salivary Glands Esophagus Liver Stomach Gallbladder Small Intestine Pancreas Large Intestine Anus

Six Functions of the Digestive System (16-1)
Ingestion Occurs when food and drink enter mouth Mechanical processing Crushing of solid food to make it easier to move along the digestive tract Increases surface area for enzymes to work Process begins in oral cavity with teeth and tongue Digestion Chemical breakdown of food to absorbable size

Six Functions of Digestive System cont. (16-1)
Secretion Release of water, acids, enzymes, and buffers into lumen of digestive tract Absorption Movement of small organic molecules, electrolytes, and water across digestive epithelium and into interstitial fluid of digestive tract Excretion Elimination of waste products from digestive tract Products are ejected as feces in process called defecation

Figure 16-2 The Structure of the Digestive Tract.
Mesenteric artery and vein Circular fold Mucosa Mucosal epithelium Circular folds Lamina propria Mesentery Villi Mucosal glands Submucosal gland Muscularis mucosae Lymphatic vessel Mucosa Artery and vein Submucosa Submucosal plexus Muscularis externa Circular muscle layer Myenteric plexus Serosa (visceral peritoneum) Longitudinal muscle layer

The Oral Cavity (16-2) Also called the buccal cavity
Part of digestive tract that receives food Lined by oral mucosa Stratified squamous epithelium Contains tongue, teeth, and gingivae, or gums Gingivae are ridges of oral mucosae surrounding base of teeth

Oral Cavity Functions (16-2)
Senses food before swallowing Mechanically processes food Lubricates food with saliva and mucus Begins enzymatic digestion of carbohydrates and lipids

Figure 16-4 The Oral Cavity.
Hard palate Soft palate Hard palate Soft palate Nasal cavity Pharyngeal tonsil Entrance to auditory tube Palatal arches Upper labium (lip) Nasopharynx Uvula Cheek Palatine tonsil Uvula Tongue Tongue Lower labium (lip) Palatine tonsil Lingual frenulum Tongue Tongue Palatal arch Gingiva Gingiva Oropharynx Vestibule Vestibule Lingual tonsil Epiglottis Hyoid bone Laryngopharynx a An anterior view of the oral cavity, as seen through the open mouth b Sagittal section of the oral cavity © 2017 Pearson Education, Inc.

Saliva Amount and Contents (16-2)
About 1.0–1.5 liters produced each day During eating, production increases to about 7 mL per minute Regulated by autonomic nervous system Composition 99.4 percent water Mucins, ions, buffers, waste products, metabolites, and enzymes Mucins absorb water and form mucus

Saliva Functions (16-2) Water lubricates mouth and dissolves chemicals
Mucus reduces friction and makes swallowing easier Buffers keep pH near 7.0 and prevent buildup of acids produced by bacteria Salivary antibodies (IgA) and lysozyme help control bacterial levels Salivary amylase (an enzyme) begins chemical digestion of starches (complex carbohydrates) Produced primarily by parotid salivary gland

The Pharynx (16-3) Commonly called the throat
Serves as common passageway for food, liquid, and air Food passes through oropharynx and laryngopharynx to esophagus Mucosa is stratified squamous epithelium Lamina propria contains mucous glands and tonsils Pharyngeal muscles cooperate with oral cavity and esophageal muscles for swallowing

The Esophagus (16-3) Muscular tube that acts as passageway from pharynx to stomach About 25 cm long and 2 cm wide Located posterior to trachea Enters abdominal cavity through esophageal hiatus in diaphragm Diaphragmatic, or hiatal, hernia involves movement of abdominal organs upward through the esophageal hiatus Lined with stratified squamous epithelium Circular muscles at either end form upper and lower esophageal sphincters

Swallowing (16-3) Also called deglutition Complex process
Can be initiated voluntarily or involuntarily Proceeds automatically once begun Tongue forms food into bolus, or small mass Compression of bolus against hard palate initiates swallowing process

The Stomach (16-4) Four primary functions
Temporary storage of ingested food Mechanical breakdown of ingested food Chemical digestion by acids and enzymes Production of intrinsic factor needed for vitamin B12 absorption Chyme is mixture of food and gastric secretions

Regions of the Stomach (16-4)
J-shaped organ with four main regions Cardia Where the esophagus connects Fundus Bulge of stomach superior to cardia Body Large area between fundus and curve of the J Pylorus Most distal portion Connects stomach to small intestine Pyloric sphincter regulates flow of chyme into small intestine

Internal Features of the Stomach (16-4)
Rugae Folds of mucosa Show prominently when stomach is empty Flatten out with stomach distention Stomach can expand to accommodate up to 1.0–1.5 liters Muscularis externa Has circular, longitudinal, and third oblique layer Extra layer strengthens stomach wall and assists in mixing and churning chyme

Figure 16-8a The Anatomy of the Stomach.
Cardia Diaphragm Esophagus Body Fundus Lesser curvature (medial surface) Lesser omentum Greater curvature (lateral surface) Greater omentum Pylorus Rugae a This anterior view of the stomach shows important superficial landmarks.

The Gastric Wall (16-4) Lined by simple columnar epithelium with numerous mucous cells Mucous epithelium secretes alkaline mucus that protects epithelium Gastric pits Shallow depressions that open to gastric surface Mucous cells at base, or neck, undergo active mitosis, replacing mucosal cells every three to seven days Gastric glands Located in fundus, body, and pylorus Connected to gastric pits Cells produce 1.5 liters/day of gastric juice

The Gastric Gland Cells (16-4)
Parietal cells secrete: Intrinsic factor for vitamin B12 absorption Hydrochloric acid (HCl) Lowers pH of gastric juice to 1.5–2.0 Kills microorganisms and activates enzymes Chief cells secrete: Pepsinogen, activated by HCl, which is converted into proteolytic enzyme, pepsin In infants, also secrete rennin and gastric lipase that are important in digestion of milk

Figure 16-8c,d The Anatomy of the Stomach.
Layers of the Stomach Wall Lamina propria Mucosa Gastric pit Mucous cells Gastric pit (opening to gastric gland) Neck Mucous epithelium Cells of Gastric Glands Parietal cells Lamina propria Muscularis mucosae Gastric gland Submucosa G cell Artery and vein Muscularis externa Chief cells Oblique muscle Circular muscle Smooth muscle cell Lymphatic vessel Longitudinal muscle d This diagram of a gastric gland shows the sites of parietal cells and chief cells. Myenteric plexus Serosa c This diagrammatic section shows the organization of the stomach wall.

Digestion in the Stomach (16-4)
Pepsin initiates protein digestion to small peptides Salivary amylase will digest carbohydrates until pH falls below 4.5 Generally active one to two hours after a meal No nutrients are absorbed in the stomach Mucosa covered in alkaline mucus Epithelial cells lack transport mechanisms Gastric lining is impermeable to water Digestion is incomplete, nutrients are still complex

The Small Intestine (16-5)
Major site of digestion and absorption 90% of nutrient absorption takes place here About 6 m long Divided into three segments Duodenum Jejunum Ileum

Figure 16-10 The Segments of the Small Intestine.
Duodenum Circular folds Jejunum Ileum Large intestine Gross anatomy of the jejunum b A representative view of the jejunum Rectum Rectum a The positions of the duodenum, jejunum, and ileum in the abdominopelvic cavity

The Duodenum (16-5) Segment closest to the stomach About 25 cm long
“Mixing bowl” of the small intestine Receives chyme from stomach, plus liver and pancreatic secretions C-shape curves around pancreas Mostly retroperitoneal, or behind peritoneum

The Jejunum and Ileum (16-5)
About 2.5 m long Most chemical digestion and nutrient absorption here Supported by mesentery and within peritoneum Ileum Final and longest segment (averages 3.5 m in length) Ends at ileocecal valve that controls flow into the cecum, or first part of large intestine

The Intestinal Wall (16-5)
Has three features that contribute to increased surface area (from 3300 cm2 to about 2 million cm2) Circular folds, or plicae circulares Permanent transverse ridges Villi Fingerlike projections of mucosa covered with simple columnar epithelium Microvilli Modified apical surface of columnar cells Also called the brush border

Figure 16-11 The Intestinal Wall.
Circular fold Capillaries Villi Mucous cells Lacteal Brush border a A singular circular fold and multiple villi Tip of villus LM × 250 d A villus in sectional view Villi Intestinal gland Lymphoid nodule Lacteal Columnar epithelial cell Mucous cell Layers of the Small Intestine Lacteal Submucosal artery and vein Nerve Mucosa Muscularis mucosae Capillary network Lymphatic vessel Lamina propria Submucosal plexus Submucosa Circular layer of smooth muscle Lymphatic vessel Muscularis externa Myenteric plexus Smooth muscle cell Serosa Longitudinal layer of smooth muscle Arteriole Venule c Internal structure in a single villus, showing the capillary network and lacteal b The organization of the intestinal wall

Intestinal Villus (16-5) Each villus contains:
Network of blood capillaries Transport absorbed nutrients to hepatic portal system for delivery to the liver Lacteal, or lymphatic, capillary Transports absorbed fatty acids, packaged into chylomicrons Intestinal glands at the villus base Secrete intestinal juice In duodenum, duodenal glands, or submucosal glands, also secrete mucus, helping to buffer acidic chyme

Figure 16-11b&c The Intestinal Wall.
Villi Intestinal gland Lymphoid nodule Lacteal Columnar epithelial cell Mucous cell Layers of the Small Intestine Lacteal Submucosal artery and vein Nerve Mucosa Muscularis mucosae Capillary network Lymphatic vessel Lamina propria Submucosal plexus Submucosa Circular layer of smooth muscle Lymphatic vessel Muscularis externa Myenteric plexus Smooth muscle cell Serosa Longitudinal layer of smooth muscle Arteriole Venule c Internal structure in a single villus, showing the capillary network and lacteal b The organization of the intestinal wall

Intestinal Secretions (16-5)
Intestinal juice produced at rate of about liters/day Moistens intestinal contents Helps buffer acids Provides liquid environment for intestinal contents Composed mostly of water from mucosa Moves into lumen by osmosis Rest is secreted by intestinal glands Stimulated by touch and stretch receptors in intestinal wall Also respond to signals in cephalic phase

Intestinal Hormones (16-5)
Gastrin released in response to incompletely digested proteins Promotes stomach motility Stimulates production of acid and enzymes Secretin released in response to acidic chyme Increases secretion of bile and buffers by liver and pancreas Cholecystokinin (CCK) released in response to high-fat chyme In pancreas, increases enzyme production In gallbladder, causes the ejection of bile

Intestinal Hormones cont. (16-5)
Gastric inhibitory peptide (GIP) released in response to high-fat and high-glucose chyme Inhibits gastric activity Causes release of insulin

Digestion in the Small Intestine (16-5)
Most important digestive processes are completed in small intestine Final products of digestion absorbed here Most enzymes and buffers come from pancreas and liver Small intestine enzymes produced by brush border cells

The Pancreas (16-6) Position and size Lies posterior to stomach
Extends from duodenum toward spleen Retroperitoneal (only anterior surface covered with peritoneum) About 15 cm long; weighing around 80 g

Histology of the Pancreas (16-6)
Two distinct groups of cells Pancreatic islets (endocrine) Secrete insulin and glucagon Only about 1% of the pancreatic cells Pancreatic acinar cells (exocrine) Produce mixture of digestive enzymes, water, and buffers as pancreatic juice Organized into pouches called pancreatic acini Duct networks converge into pancreatic duct Empties into duodenum along with the common bile duct

Diagram of the cellular organization of the pancreas.
Figure The Pancreas. Pancreatic duct Connective tissue septum Exocrine cells in pancreatic acini Endocrine cells in pancreatic islet b Diagram of the cellular organization of the pancreas. Accessory pancreatic duct Common bile duct Pancreatic duct Lobules Tail of pancreas Duct Body of pancreas Body of pancreas Pancreatic islet (endocrine) Head of pancreas Head of pancreas Pancreatic acini (exocrine) Duodenal papilla Duodenum Duodenum Pancreas LM × 75 a The gross anatomy of the pancreas The head of the pancreas is tucked into a C-shaped curve of the duodenum that begins at the pylorus of the stomach. c Light micrograph of the cellular organization of the pancreas.

Control of Pancreatic Secretions (16-6)
About 1000 mL of pancreatic juice secreted each day Regulated by hormones Secretin released from duodenum in response to presence of acidic chyme entering from stomach Triggers pancreas to release watery, alkaline (pH 7.5–8.8) fluid One of primary buffers in fluid is sodium bicarbonate Increases pH of chyme to optimal pH for digestive enzymes Cholecystokinin (CCK) stimulates production and release of pancreatic enzymes

Pancreatic Enzymes (16-6)
Classified by substances they help break down Carbohydrases (general term) digest sugars and starches Pancreatic amylase breaks down carbohydrates Pancreatic lipase breaks down lipids, or fats Nucleases break down nucleic acids Pancreatic proteases break down proteins Make up 70 percent of total pancreatic enzyme production Examples: trypsin, chymotrypsin, carboxypeptidase Secreted as inactive proenzymes and activated after reaching small intestine

The Liver (16-6) Largest visceral organ
Weighs about 1.5 kg (roughly 2.5 percent of total body weight) Found in right hypochondriac and epigastric abdominopelvic regions Wrapped in tough fibrous capsule and covered by visceral peritoneum Divided into four lobes Large left and right lobes Smaller caudate and quadrate lobes Falciform ligament marks division between left and right lobes on anterior surface Posterior margin of falciform ligament is the round ligament

Figure 16-14 The Surface Anatomy of the Liver.
Coronary ligament Right lobe Right lobe Left lobe Left lobe Falciform ligament Round ligament Liver Gallbladder a Anterior surface of the liver Left hepatic vein Coronary ligament Inferior vena cava Lobes of Liver Left lobe Common bile duct Caudate lobe Hepatic portal vein Right lobe Hepatic artery proper Quadrate lobe Gallbladder b Posterior surface of the liver

Histology of the Liver (16-6)
Lobes divided into about 100,000 liver lobules Basic functional unit of liver, each about 1 mm in diameter Hepatocytes (liver cells) are covered in microvilli and arranged in plates Specialized capillaries called sinusoids empty into central vein Allow free exchange of water and solutes Contain phagocytic Kupffer cells in lining Engulf pathogens, debris, and damaged blood cells

Liver Functions (16-6) Over 200 known functions
Can be categorized into three general roles Metabolic regulation Hematological regulation Bile production

Metabolic Regulation by the Liver (16-6)
Liver has primary role in regulating composition of circulating blood Blood flows from absorptive areas of digestive tract through liver where hepatocytes: Extract nutrients and toxins from blood Store and synthesize nutrient molecules Fat-soluble vitamins (A, D, E, and K) are stored Monitor and adjust circulating levels of organic nutrients High blood glucose triggers synthesis of glycogen Low blood glucose triggers breakdown of glycogen and release of glucose © 2017 Pearson Education, Inc.

Production and Role of Bile (16-6)
Bile is synthesized by the liver Bile contains: Water and ions that dilute and buffer Bilirubin (pigment derived from hemoglobin) from destroyed RBCs Cholesterol and bile salts Bile salts break apart large fat droplets into smaller ones Process called emulsification Increases surface area of lipids for enzymatic action

The Gallbladder (16-6) Hollow, pear-shaped organ located in recess on posterior surface of liver’s right lobe Functions are bile storage and bile modification Releases bile into cystic duct, which flows into common bile duct, then enters duodenum at duodenal papilla Hepatopancreatic sphincter surrounds shared passageway of common bile duct and pancreatic duct Limits entry of secretions into duodenum until needed CCK triggers contractions of gallbladder and relaxation of sphincter Bile becomes more concentrated as it is stored Too concentrated bile can form gallstones

Figure 16-16 The Gallbladder.
Cystic duct Common hepatic duct Cut edge of lesser omentum Gallbladder Hepatic portal vein Common bile duct Common hepatic artery Liver Liver Duodenum Duodenum Common bile duct Stomach Stomach Hepatopancreatic sphincter Pancreas Pancreas Duodenal papilla Pancreatic duct a A view of the inferior surface of the liver, showing the position of the gallbladder and the ducts that transport bile from the liver to the gallbladder and duodenum. Pancreas Pancreas Intestinal lumen b This sectional view of part of the duodenum shows the duodenal papilla opening and location of the hepatopancreatic sphincter.

The Large Intestine (16-7)
Also called the large bowel Begins at ileocecal valve, ends at anus Total length of about 1.5 m divided into three parts: cecum, colon, rectum Horseshoe-shape frames the small intestine Main functions Reabsorbing water, compacting chyme into feces Absorbing vitamins freed by bacterial action Storing feces prior to defecation

The Cecum (16-7) An expanded pouch Appendix, or vermiform appendix
Chyme enters through ileocecal valve Functions to initiate compaction Appendix, or vermiform appendix Attached on posteromedial surface Contains lymphoid nodules for immune function Inflammation is called appendicitis

The Colon (16-7) Longest portion of the large intestine
Has a large diameter, lacks villi, and has abundant mucous cells External features Haustra: pouches that allow colon to expand and elongate Teniae coli: three longitudinal bands of smooth muscle Divided into four segments Ascending colon, transverse colon, descending colon, sigmoid colon

The Rectum (16-7) Forms last 15 cm of digestive tract
Expandable organ for temporarily storing feces Anal canal is last portion Contains longitudinal anal columns Anus is the exit of the anal canal Lined with stratified squamous epithelium Internal anal sphincter made of smooth muscle of muscularis externa, involuntary control External anal sphincter is voluntary, skeletal muscle

Figure 16-17 The Large Intestine.
Aorta Splenic vein Hepatic portal vein Superior mesenteric artery Superior mesenteric vein Inferior mesenteric vein Inferior vena cava Greater omentum (cut) Transverse colon Descending colon Inferior mesenteric artery Ascending colon Haustra Ileocecal valve Ileum Ileum Cecum Tenia coli Appendix Sigmoid colon a The gross anatomy and regions of the large intestine Rectum Rectum Ileocecal valve Rectum Rectum Cecum (cut open) Anal canal Anal columns Appendix Internal anal sphincter External anal sphincter b The cecum and appendix Anus c The rectum and anus

Absorption in the Large Intestine (16-7)
Major function of large intestine is reabsorption of substances Reabsorbs water Of 1500 mL of chyme that enters cecum each day, mL are reabsorbed Reabsorbs bile salts Bile salts absorbed in the cecum Transported to the liver for secretion in bile

Absorption of Organic Wastes - Toxins (16-7)
Bacterial action breaks down peptides in feces Generates ammonia, nitrogenous compounds, hydrogen sulfide Odor of feces from these compounds Ammonia and other toxins absorbed into hepatic portal circulation Liver converts them into nontoxic compounds that can be excreted by kidneys Indigestible carbohydrates in colon Provide nutrient source for bacteria Metabolic activities of those bacteria create flatus, or gas

Defecation Process (16-7)
Internal anal sphincter relaxes involuntarily Conscious relaxation of external sphincter required for defecation Other conscious actions help force fecal material into rectum Tensing abdominal muscles or elevating intra-abdominal pressure Valsalva maneuver Attempting to forcibly exhale with a closed glottis Repeated bouts of straining to defecate permanently distends veins in the anal canal, producing hemorrhoids

Water and Electrolyte Absorption (16-8)
Total of about 9000 mL water and secretions added into digestive tract each day All but 150 mL absorbed Intestinal epithelial cells constantly absorb dissolved nutrients and ions Water “follows” through osmosis Absorption of Na+ and Cl– most important factor promoting water reabsorption Other ions include K+, Mg2+, I–, HCO3–, Fe2+, and Ca2+ Ca2+ absorption under control of parathyroid hormone and calcitriol

Digestive Tract Lining (16-1)
Protects surrounding tissues from: Corrosive effects of digestive acids and enzymes Physical abrasion Bacteria that are ingested or live in digestive tract 4 layers Mucosa Submucosa Muscularis externa Serosa

Mucosa (16-1) Mucous membrane that forms the inner lining of the digestive tract Consists of: Mucosal epithelium Underlying layer of areolar tissue called the lamina propria Muscle layer called the muscularis mucosae Smooth muscle that helps move mucosa

Mucosal Epithelia (16-1) Stratified squamous in high physical stress organs Oral cavity, pharynx, esophagus, anus Rest is simple columnar with surface modifications Ducts of secretory glands open to surface of epithelium Circular folds and villi increase surface area for absorption

Submucosa (16-1) Layer of dense irregular connective tissue
Binds mucosa to muscularis externa Contains blood vessels and lymphatics Outer margin contains: Parasympathetic neurons and sensory neurons Submucosal plexus Neural network that can function without CNS Regulates secretion and motility

Muscularis Externa (16-1)
Band of smooth muscle arranged in: Inner circular and outer longitudinal layer Function to mix and propel materials Myenteric plexus between layers of muscle Contains parasympathetic ganglia, sensory neurons, interneurons, and sympathetic postganglionic fibers Parasympathetic stimulation increases activity Sympathetic stimulation decreases activity

Serosa and Adventitia (16-1)
Serous membrane covering muscularis externa along GI tract enclosed by peritoneum Also called the visceral peritoneum Continuous with the parietal peritoneum, which lines inner surfaces of body wall Adventitia Layer covering muscularis externa of regions where there is no serosa Examples: oral cavity, pharynx, esophagus, and rectum Attaches GI tract to adjacent structures

Mesenteries (16-1) Mesenteries
Double sheets of serous membrane (parietal and visceral peritoneum) Suspend portions of digestive tract Provide pathways for blood vessels, lymphatics, and nerves Help organize and stabilize attached organs

Movement of Digestive Materials (16-1)
Pacesetter cells in smooth muscle of digestive tract trigger contraction Peristalsis Waves of contraction initiated by circular layer, followed by longitudinal layer Propels bolus (food mass) down tract Segmentation A mixing action with no propulsion

Figure 16-3 Peristalsis. 1 2 3 4 Initial State Longitudinal Muscle
Circular muscle From mouth To anus 1 Contraction of circular muscles behind bolus Contraction 2 Contraction of longitudinal muscles ahead of bolus Contraction Contraction 3 A wave of contraction in circular muscle layer forces bolus forward

The Tongue (16-2) Manipulates food within oral cavity Lingual tonsils
Mechanically compresses, abrades, distorts material Assists in chewing and preparing food for swallowing Provides sensory analysis of touch, temperature, and taste Lingual tonsils Paired lymphoid nodules at base of tongue Help resist infection

The Salivary Glands (16-2)
Three pairs of glands that secrete into oral cavity Parotid salivary glands On each side of oral cavity between mandible and skin Parotid duct empties into vestibule at level of second upper molar Sublingual salivary glands Under mucous membrane on floor of mouth Numerous sublingual ducts open on either side of lingual frenulum Submandibular salivary glands In floor of the mouth along inner surfaces of mandible Ducts open into mouth behind teeth on either side of lingual frenulum

Figure 16-5 The Salivary Glands.
Parotid duct Openings of sublingual ducts Salivary Glands Parotid salivary gland Lingual frenulum Sublingual salivary gland Opening of left submandibular duct Submandibular salivary gland Submandibular duct

Figure 16-6a Teeth: Structural Components and Dental Succession.
Pulp cavity Crown Enamel Dentin Gingiva Neck Cementum Periodontal ligament Root Root canal Bone of alveolus Branches of blood vessels and nerves a Diagrammatic section through a typical adult tooth

Figure 16-6b,c Teeth: Structural Components and Dental Succession.
Centralincisors (7–8 yr) Lateral incisor (8–9 yr) Cuspid (11–12 yr) 1st Premolar (10 –11 yr) Upper dental arch 2nd Premolar (10 –12 yr) Central incisors (7.5 mo) UPPER JAW 1st Molar (6–7 yr) 2nd Molar (12–13 yr) Lateral incisor (9 mo) Hard palate Cuspid (18 mo) 3rd Molar (17–21 yr) Hard palate Primary 1st molar (14 mo) 3rd Molar (17–21 yr) Primary 2nd molar (24 mo) 2nd Molar (11–13 yr) 1st Molar (6–7 yr) Primary 2nd molar (20 mo) 2nd Premolar (11–12 yr) Lower dental arch Primay 1st molar (12 mo) 1st Premolar (10 –12 yr) Cuspid (16 mo) Cuspid (9–10 yr) Lateral incisor (7 mo) Lateral incisor (7–8 yr) Central incisors (6 mo) LOWER JAW Central incisors (6–7 yr) b Primary teeth, with the age at eruption given in months c Adult teeth, with the age at eruption given in years

Phases of Swallowing (16-3)
Buccal phase Voluntary phase Movement of bolus into back of oral cavity and into oropharynx Soft palate closes over nasopharynx Pharyngeal phase Epiglottis folds over larynx Food and liquid are directed past closed glottis Uvula and soft palate block nasopharynx Esophageal phase Bolus is pushed into esophagus and toward stomach Pharyngeal and esophageal phases are involuntary due to swallowing reflex

Figure 16-7 The Swallowing Process.
Slide 4 1 Buccal Phase The buccal phase begins with the compression of the bolus against the hard palate. Retraction of the tongue then forces the bolus into the oropharynx and assists in elevating the soft palate, thereby sealing off the nasopharynx. Once the bolus enters the oropharynx, reflex responses begin and the bolus is moved toward the stomach. Hard palate Soft palate Bolus Tongue Oropharynx Epiglottis Trachea 2 Pharyngeal Phase The pharyngeal phase begins as the bolus comes into contact with the palatal arches and the posterior pharyngeal wall. Elevation of the larynx and folding of the epiglottis direct the bolus past the closed glottis. At the same time, the uvula and soft palate block passage back to the nasopharynx. Uvula Tongue Bolus Epiglottis Larynx 3 Esophageal Phase The esophageal phase begins as the contraction of pharyngeal muscles forces the bolus through the entrance to the esophagus. Once in the esophagus, the bolus is pushed toward the stomach by peristalsis. Peristalsis in esophagus Trachea 4 Bolus Enters Stomach The approach of the bolus triggers the opening of the lower esophageal sphincter. The bolus then continues into the stomach. Thoracic cavity Lower esophageal sphincter Stomach © 2017 Pearson Education, Inc.

Mesenteries Associated with the Stomach (16-4)
Greater omentum Very large peritoneal pouch Extends from greater curvature of stomach down over abdominal viscera Lesser omentum Smaller peritoneal pouch Extends from lesser curvature of stomach to liver

Figure 16-8b The Anatomy of the Stomach.
Diaphragm Liver Lesser omentum Pancreas Stomach Mesentery Duodenum Transverse colon Mesentery Rectum Greater omentum Parietal peritoneum Small intestine Uterus Bladder b The stomach is surrounded by the peritoneal cavity. Its position is maintained by the greater and lesser omenta.

Regulation of Gastric Activity (16-4)
Production of acid and enzymes Controlled by central nervous system Regulated by reflexes involving stomach wall Regulated by hormones of digestive tract Involves three overlapping phases Cephalic phase Gastric phase Intestinal phase

Cephalic Phase (16-4) Triggered by sight, smell, taste, thought of food Prepares stomach to receive food Parasympathetic stimulation of gastric cells increases production of gastric juice Rates up to 500 mL/hour Generally only lasts short period

Figure 16-9-1 Regulation of Gastric Activity
Sight, smell, taste, or thoughts of food Food 1 CEPHALIC PHASE Central nervous system Vagus nerve (N X) The cephalic phase of gastric secretion begins when you see, smell, taste, or think of food. This phase is directed by the parasympathetic division of the autonomic nervous system. It prepares the stomach to receive food. In response to stimulation, the production of gastric juice speeds up, reaching rates of about 500 mL/h, or about 2 cups per hour. This phase generally lasts only minutes. Submucosal plexus Mucous cells Mucus Chief cells Pepsinogen Parietal cells HCl Gastrin KEY Neural stimulation G cells Secretion

Gastric Phase (16-4) Begins when food enters stomach
Stretch reflexes increase myenteric stimulation of mixing waves Submucosal plexus Stimulates parietal and chief cells Stimulates G cells to produce gastrin Results in rapid increase in gastric juice production Phase may continue for several hours

GASTRIC PHASE The gastric phase begins when food arrives in the stomach. The stimulation of stretch receptors in the stomach wall and of chemoreceptors in the mucosa triggers local reflexes in the submucosal and myenteric plexuses. This results in mixing waves from the muscularis externa, and the secretion of mucus, pepsinogen, and HCl from the cells of the gastric glands. Submucosal and myenteric plexuses Distension Stretch receptors Elevated pH Chemoreceptors carried by bloodstream Mucous cells Mucus Chief cells Pepsinogen Mixing Parietal cells waves Gastrin HCI KEY Partly digested peptides G cells Neural stimulation Hormonal stimulation

Intestinal Phase (16-4) Begins when chyme enters small intestine
Mostly inhibitory controls, slowing gastric emptying Enterogastric reflex inhibits gastrin production Intestinal hormones secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP) reduce gastric activity Ensures efficient intestinal functions Secretion, digestion, and absorption

INTESTINAL PHASE The intestinal phase of gastric secretion begins when chyme first enters the duodenum of the small intestine. The function of the intestinal phase is to control the rate of gastric emptying to ensure that the secretory, digestive, and absorptive functions of the small intestine can proceed efficiently. Enterogastric reflex Myenteric plexus carried by bloodstream Chief cells Parietal cells Duodenal stretch and chemoreceptors Peristalsis CCK Presence of lipids and carbohydrates GIP KEY Secretin Decreased pH Inhibition

Intestinal Movements (16-5)
Weak peristaltic contractions move chyme toward jejunum Local reflexes not under CNS control Gastroenteric reflex Initiated by distention of stomach Increases glandular secretion and peristaltic contractions along length of small intestine Empties duodenum Gastroileal reflex Triggered by gastrin Relaxes ileocecal valve Material pushed from ileum into large intestine

Figure 16-12 The Activities of Major Digestive Tract Hormones.
Ingested food Hormone Action Food in stomach Acid production by parietal cells KEY stimulates inhibits Gastrin Stimulation of gastric motility; mixing waves increase in intensity Release of insulin from pancreas GIP Pancreas Release of pancreatic enzymes and buffers Chyme in duodenum Secretin and CCK Bile secretion and ejection of bile from gallbladder facilitates facilitates facilitates NUTRIENT UTILIZATION BY ALL TISSUES Material arrives in jejunum Nutrient absorption

Portal Areas of the Liver (16-6)
Also called portal triad Found at each of six corners of lobule Includes three parts Branch of hepatic portal vein Branch of hepatic artery proper Small branch of bile duct

Blood Flow through the Liver (16-6)
Blood comes from hepatic artery and hepatic portal vein As it flows through sinusoids, hepatocytes: Absorb nutrient molecules Secrete plasma proteins Blood flows from sinusoids into central vein Merge to form hepatic veins Drain into inferior vena cava

Bile Production in the Liver (16-6)
Hepatocytes secrete bile into narrow channels called bile canaliculi Located between adjacent liver cells Carry bile into bile duct in portal triad Bile flows into common hepatic duct, then to: Common bile duct into the duodenum or Cystic duct into the gallbladder

Figure 16-15 Liver Histology.
1 mm a A diagrammatic view of liver structure, showing relationships among lobules Interlobular septum Bile duct Branch of hepatic portal vein Bile ductules Portal area Portal Area Branch of hepatic portal vein (containing blood) Branch of hepatic artery proper Bile duct Central vein Hepatocytes Sinusoids Kupffer cells Bile canaliculi Portal Area Bile duct Branch of hepatic portal vein Portal area LM × 320 Branch of hepatic artery proper c A sectional view showing the vessels and ducts within a portal area b A single liver lobule and its cellular components

Hematological Regulation by the Liver (16-6)
Largest blood reservoir in the body Receives about 25 percent of cardiac output Phagocytic Kupffer cells remove old or damaged RBCs, debris, and pathogens from blood Hepatocytes synthesize plasma proteins Determine osmotic pressure of plasma Function as nutrient transporters Key elements of clotting and complement cascades

Table 16.2

Movements of the Large Intestine (16-7)
Gastroileal and gastroenteric reflexes move material into cecum Transit time through large intestine very slow Allows for water reabsorption Mass movements Powerful peristaltic contractions Occur a few times per day Triggered by distention of stomach and duodenum Forces feces into rectum, producing urge to defecate

Processing and Absorption of Nutrients (16-8)
Balanced diets include all ingredients needed to maintain homeostasis Carbohydrates, proteins, lipids Broken down into absorbable forms by enzymes Process called hydrolysis Used by cells to generate ATP or build complex carbohydrates, proteins, lipids Water, electrolytes (minerals), vitamins No processing, but require special transport mechanisms

Absorption of Vitamins (16-7)
Vitamins are organic molecules essential to metabolic reactions Bacteria in colon make three key vitamins Vitamin K Needed for production of clotting factors Biotin Essential for glucose metabolism Vitamin B5 (pantothenic acid) Required for synthesis of neurotransmitters and steroid hormones

Absorption of Organic Wastes - Bilirubin (16-7)
Bacteria convert bilirubin into other products Some are absorbed into bloodstream Excreted in urine, giving yellow color Some remain in the colon Give feces brown color

Defecation Reflex (16-7) Involves two positive feedback loops
Shorter loop Stretch receptors in rectal walls stimulate local peristalsis Moves feces toward anus, distends rectum Longer loop Stretch receptors stimulate parasympathetic reflex in sacral spinal cord Stimulate increasing peristalsis and increased distention in rectum

Absorption of Vitamins (16-8)
Fat-soluble vitamins: A, D, E, and K Absorbed in micelles along with lipids Vitamin K also produced in colon by bacteria Water-soluble vitamins: B vitamins and C All but B12 are easily absorbed by digestive epithelium B12 requires intrinsic factor (protein secreted by parietal cells of stomach) for absorption Bacteria in gut are also source of water-soluble vitamins

Carbohydrate Digestion and Absorption (16-8)
Begins in mouth during mastication Salivary amylase breaks down complex carbohydrates into di- or trisaccharides Pancreatic amylase continues process Brush border enzymes on intestinal microvilli complete breakdown into monosaccharides Monosaccharides absorbed through facilitated diffusion or cotransport Transported into capillaries by diffusion

Figure 16-18a Chemical Events in Digestion
REGION CARBOHYDRATES ORAL CAVITY Salivary amylase ESOPHAGUS STOMACH Disaccharides Trisaccharides SMALL INTESTINE Pancreatic alpha-amylase Disaccharides Trisaccharides INTESTINAL MUCOSA Lactase Maltase, Sucrase Brush border FACILITATED DIFFUSION AND COTRANSPORT Monosaccharides Cell body FACILITATED DIFFUSION ROUTE TO BLOODSTREAM Capillary Carbohydrates and amino acids are absorbed and transported by intestinal capillaries. Lipids form chylomicrons that diffuse into lacteals and are delivered to the left subclavian vein by the thoracic duct. Monosaccharides

Lipid Digestion and Absorption (16-8)
Involves lingual lipase from glands under tongue and pancreatic lipase from pancreas Triglycerides enter duodenum in large fat droplets Bile salts emulsify droplets; pancreatic lipase breaks apart triglycerides Triglycerides broken into fatty acids and monoglycerides These combine with bile salts to form micelles Micelles diffuse into epithelial cells and are converted into triglycerides Coated with proteins to form chylomicrons Secreted by exocytosis into interstitial fluids Absorbed into lacteals and transported through lymphatic system to left subclavian vein

Figure 16-18b Chemical Events in Digestion
REGION LIPIDS ORAL CAVITY Lingual lipase ESOPHAGUS STOMACH SMALL INTESTINE Bile salts and pancreatic lipase Monoglycerides, Fatty acids in micelles INTESTINAL MUCOSA DIFFUSION Brush border Monoglycerides, Fatty acids Triglycerides Cell body Chylomicrons EXOCYTOSIS ROUTE TO BLOODSTREAM Lacteal Carbohydrates and amino acids are absorbed and transported by intestinal capillaries. Lipids form chylomicrons that diffuse into lacteals and are delivered to the left subclavian vein by the thoracic duct. Chylomicrons

Protein Digestion and Absorption (16-8)
Complex and time-consuming process Initiated by mastication in mouth and exposure to HCl in stomach Pepsin (an enzyme in the stomach) Breaks proteins into large peptides Pancreatic proteolytic enzymes Break down polypeptide chains into short peptide chains and amino acids Peptidases on brush border (microvilli in small intestine) Complete process by cleaving off individual amino acids Amino acids absorbed through facilitated diffusion and cotransport Transported into capillaries by diffusion

Figure 16-18c Chemical Events in Digestion
REGION PROTEINS ORAL CAVITY ESOPHAGUS STOMACH Pepsin Polypeptides SMALL INTESTINE Trypsin Chymotrypsin Elastase Carboxypeptidase Short peptides, Amino acids INTESTINAL Dipeptidases MUCOSA Brush border FACILITATED DIFFUSION AND COTRANSPORT Amino acids Cell body FACILITATED DIFFUSION AND COTRANSPORT ROUTE TO BLOODSTREAM Capillary Carbohydrates and amino acids are absorbed and transported by intestinal capillaries. Lipids form chylomicrons that diffuse into lacteals and are delivered to the left subclavian vein by the thoracic duct. Amino acids

Table 16.3

Digestive System Age-Related Changes (16-9)
Division rate of epithelial stem cells declines More susceptible to damage by abrasion, acids, enzymes Peptic ulcers more likely Smooth muscle tone decreases Slows rate of peristalsis, leads to constipation Straining leads to hemorrhoids Weakening muscular sphincters can lead to more frequent “heartburn” Cumulative damage becomes apparent Tooth loss due to dental caries (“cavities”) or gingivitis Cirrhosis or other liver disease

Digestive System Age-Related Changes cont. (16-9)
Increase in cancer rate Rates of colon and stomach cancers rise with age Oral, esophageal, pharyngeal cancers more common in smokers Dehydration Osmoreceptor sensitivity declines Aging of other systems affects digestive tract Loss in bone mass and calcium can lead to tooth loss Loss of taste and olfactory sensations changes diets

Digestive System Integration with Other Systems (16-10)
Functionally linked to all other systems Anatomically connected to nervous, cardiovascular, endocrine, and lymphatic systems Endocrine function producing hormones Extensive structural and functional connections to cardiovascular system

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