Chapter 24, part 1 The Digestive System

Learning Objectives Identify the organs of the digestive system and their major functions Outline the mechanisms that regulate digestion Describe the anatomy of the organs and accessory organs of the digestive system Discuss the functions of the major structures and regions of the digestive system and discuss the regulation of their activities

Learning Objectives Explain the significance of the large intestine in the absorption of nutrients Describe the events involved in the digestion of organic and inorganic nutrients Summarize the effects of the aging process on the digestive system

SECTION 24-1 The Digestive System

The Digestive system includes: The muscular digestive tract Various accessory organs

Figure 24.1 The Components of the Digestive System

Functions of the digestive system Ingestion Mechanical processing Digestion Secretion Absorption Excretion

The digestive system organs and the peritoneum Mesenteries Sheets of serous membranes that support portions of the digestive tract Greater omentum lies anterior to abdominal viscera Provides padding, protection, insulation, and energy reserves Lesser omentum

Figure 24.2 Mesenteries Figure 24.2a

Figure 24.2 Mesenteries Figure 24.2b

Figure 24.2 Mesenteries Figure 24.2c

Figure 24.2 Mesenteries Figure 24.2d

Histological organization of the digestive tract Mucosa lines digestive tract (mucous epithelium) Moistened by glandular secretions Lamina propria and epithelium form mucosa Submucosa Layer of dense irregular connective tissue

Histological organization of the digestive tract Muscularis externa Smooth muscle arranged in circular and longitudinal layers adventitia Serosa Serous membrane covering most of the muscularis externa

Figure 24.3 The Structure of the Digestive Tract

Movement of digestive materials Visceral smooth muscle shows rhythmic cycles of activity Pacemaker cells Peristalsis Waves that move a bolus Segmentation Churn and fragment a bolus

Figure 24.4 Peristalsis Figure 24.4

Control of the digestive system Movement of materials along the digestive tract is controlled by: Neural mechanisms Parasympathetic and local reflexes Hormonal mechanisms Enhance or inhibit smooth muscle contraction Local mechanisms Coordinate response to changes in pH or chemical stimuli

Figure 24.5 The Regulation of Digestive Activities

SECTION 24-2 The Oral Cavity

The mouth opens into the oral or buccal cavity Its functions include: Analysis of material before swallowing Mechanical processing by the teeth, tongue, and palatal surfaces Lubrication Limited digestion

Oral cavity Lined by oral mucosa Roof of cavity = hard and soft palates Floor of cavity = tongue Uvula guards opening to pharynx

Figure 24.6 The Oral Cavity Figure 24.6a, b

The tongue primary functions include: Mechanical processing Assistance in chewing and swallowing Sensory analysis by touch, temperature, and taste receptors

Tongue movements involve Extrinsic and intrinsic tongue muscles Innervated by the hypoglossal nerve

Salivary glands (three pairs) Parotid, sublingual, and submandibular Saliva watery solution electrolytes, buffers, glycoproteins, antibodies, enzymes Functions include: Lubrication, moistening, and dissolving Initiation of digestion of complex carbohydrates PLAY Animation: Mastication Flythrough

Figure 24.7 The Salivary Glands Figure 24.7a, b

Teeth Function in mastication of bolus Contact of occlusal surfaces Contain three layers Enamel covering crown Dentin forms basic structure Root coated with cementum Periodontal ligaments hold teeth in alveoli

Eruption of teeth 20 primary teeth AKA deciduous teeth 32 teeth of secondary dentition

Figure 24.8 Teeth Figure 24.8a, b

Figure 24.9 Primary and Secondary Teeth Figure 24.9a, b

SECTION 24-3 The Pharynx

The pharynx Common passageway for food, liquids, and air Lined with stratified squamous epithelium Pharyngeal muscles assist in swallowing Pharyngeal constrictor muscles Palatal muscles

SECTION 24-4 The Esophagus

The esophagus Carries solids and liquids from the pharynx to the stomach Passes through esophageal hiatus in diaphragm The wall of the esophagus contains mucosal, submucosal, and muscularis layers

Histology of the esophagus Distinctive features of the esophageal wall include Nonkeratinized, stratified squamous epithelium Folded mucosa and submucosa Mucous secretions by esophageal glands A muscularis with both smooth and skeletal muscle portions Lacks serosa Anchored by an adventitia

Figure 24.10 The Esophagus Figure 24.10a-c

Swallowing (deglutition) Buccal phase Pharyngeal phase Esophageal phase

Figure 24.11 The Swallowing Process Figure 24.11a-h

SECTION 24-5 The Stomach

Functions of the stomach Bulk storage of undigested food Mechanical breakdown of food Disruption of chemical bonds via acids and enzymes Production of intrinsic factor

Anatomy of the stomach Cardia – superior, medial portion Fundus – portion superior to stomach-esophageal junction Body – area between the fundus and the curve of the J Pylorus – antrum and pyloric canal adjacent to the duodenum

Stomach anatomy Phloric Sphincter Guards exit from stomach Rugae Ridges and folds in relaxed stomach

Figure 24.12 The Stomach Figure 24.12b

Histology of the stomach Gastric glands Parietal cells Intrinsic factor, and HCl Chief cells Pepsinogen Pyloric glands Mucous secretion containing several hormones Enteroendocrine cells G cells secrete gastrin D cells secrete somatostatin

Figure 24.13 The Stomach Lining Figure 24.13a, b

Figure 24.13 The Stomach Lining Figure 24.13c, d

Figure 24.14 The Secretions of Hydrochloric Acid

Regulation of gastric activity Cephalic phase prepares stomach to receive ingested material Gastric phase begins with the arrival of food in the stomach Neural, hormonal, and local responses Intestinal phase controls the rate of gastric emptying

Figure 24.15 The Phases of Gastric Secretion Figure 24.15a

Figure 24.15 The Phases of Gastric Secretion Figure 24.15b

Figure 24.15 The Phases of Gastric Secretion Figure 24.15c

Digestion and absorption in the stomach Preliminary digestion of proteins Pepsin Permits digestion of carbohydrates Very little absorption of nutrients Some drugs, however, are absorbed

SECTION 24-6 The Small Intestine and Associated Glandular Organs

Small intestine Important digestive and absorptive functions Secretions and buffers provided by pancreas, liver, gall bladder Three subdivisions: Duodenum Jejunum Ileum Ileocecal sphincter Transition between small and large intestine

Figure 24.16 Regions of the Small Intestine Figure 24.16a

Histology of the small intestine Plicae Transverse folds of the intestinal lining Villi Fingerlike projections of the mucosa Lacteals Terminal lymphatic in villus Intestinal glands Lined by enteroendocrine, goblet and stem cells

Figure 24.17 The Intestinal Wall Figure 24.17a

Figure 24.17 The Intestinal Wall Figure 24.17b, c

Figure 24.17 The Intestinal Wall Figure 24.17d, e

Intestinal juices Moisten chyme Help buffer acids Maintain digestive material in solution

Small Intestine Duodenal glands (Brunner’s glands) produce mucus, buffers, urogastrone Ileum aggregated lymphoid nodules (Peyer’s patches)

Intestinal movements Peristalsis Segmentation Gastroenteric reflexes Initiated by stretch receptors in stomach Gastroileal reflex Triggers relaxation of ileocecal valve

The pancreas Pancreatic duct penetrates duodenal wall Endocrine functions Insulin and glucagons Exocrine functions Majority of pancreatic secretions Pancreatic juice secreted into small intestine Carbohydrases Lipases Nucleases Proteolytic enzymes

Figure 24.18 The Pancreas Figure 24.18a-c

The liver Performs metabolic and hematological regulation and produces bile Histological organization Lobules containing single-cell thick plates of hepatocytes Lobules unite to form common hepatic duct Duct meets cystic duct to form common bile duct

Figure 24.19 The Anatomy of the Liver Figure 24.19a

Figure 24.19 The Anatomy of the Liver Figure 24.19b, c

Liver lobule is the basic functional unit of the liver Hepatocytes form irregular plates arranged in spoke-like fashion Bile canaliculi carry bile to bile ductules Bile ductules lead to portal areas

Figure 24.20 Liver Histology Figure 24.20a, b

The gallbladder Hollow, pear-shaped organ Stores, modifies and concentrates bile PLAY Animation: Accessory Organ

Figure 24.21 The Gallbladder Figure 24.21a, b

Coordination secretion and absorption Neural and hormonal mechanisms coordinate glands GI activity stimulated by parasympathetic innervation Inhibited by sympathetic innervation Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines

Figure 24.22 The Activities of Major Digestive Tract Hormones

SECTION 24-7 The Large Intestine

Functions of the large intestine Reabsorb water and compact material into feces Absorb vitamins produced by bacteria Store fecal matter prior to defecation

The four areas of the colon are: Ascending Transverse Descending Sigmoid

Figure 24.23 The Large Intestine Figure 24.23a

Figure 24.23 The Large Intestine Figure 24.23b, c

The rectum Last portion of the digestive tract Terminates at the anal canal Internal and external anal sphincters

Histology of the large intestine Absence of villi Presence of goblet cells Deep intestinal glands

Physiology of the large intestine Reabsorption in the large intestine includes: Water Vitamins – K, biotin, and B5 Organic wastes – urobilinogens and sterobilinogens Bile salts Toxins Mass movements of material through colon and rectum Defecation reflex triggered by distention of rectal walls

Figure 24.25 The Defecation Reflex

SECTION 24-8 Digestion and Absorption

Processing and absorption of nutrients Disassembles organic food into smaller fragments Hydrolyzes carbohydrates, proteins, lipids and nucleic acids for absorption

Carbohydrate digestion and absorption Begins in the mouth Salivary and pancreatic enzymes Disaccharides and trisaccharides Brush border enzymes Monosaccharides Absorption of monosaccharides occurs across the intestinal epithelia

Lipid digestion and absorption Lipid digestion utilizes lingual and pancreatic lipases Bile salts improve chemical digestion by emulsifying lipid drops Lipid-bile salt complexes called micelles are formed Micelles diffuse into intestinal epithelia which release lipids into the blood as chylomicrons

Protein digestion and absorption Low pH destroys tertiary and quaternary structure Enzymes used include pepsin, trypsin, chymotrypsin, and elastase Liberated amino acids are absorbed

Absorption Water Nearly all that is ingested is reabsorbed via osmosis Ions Absorbed via diffusion, cotransport, and active transport Vitamins Water soluble vitamins are absorbed by diffusion Fat soluble vitamins are absorbed as part of micelles Vitamin B12 requires intrinsic factor

Figure 24.27 Digestive Secretion and Absorption of Water

Figure 24.28 Ion and Vitamin Absorption by the Digestive Tract

You should now be familiar with: The organs of the digestive system and their major functions The mechanisms that regulate digestion The anatomy of the organs and accessory organs of the digestive system The functions of the major structures and regions of the digestive system and the regulation of their activities

You should now be familiar with: The significance of the large intestine in the absorption of nutrients The events involved in the digestion of organic and inorganic nutrients