Figure 24.1 The Components of the Digestive System

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

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

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

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

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

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

Figure 24.11 The Swallowing Process Figure 24.11a-h

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

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 Mucous secretion containing several hormones Enteroendocrine cells G cells secrete gastrin D cells secrete somatostatin

Figure 24.12 The Stomach Figure 24.12b

Figure 24.13 The Stomach Lining Figure 24.13a, b

Figure 24.13 The Stomach Lining Figure 24.13c, d

Histology of the stomach Gastric glands Parietal cells Intrinsic factor, and HCl Chief cells Pepsinogen Pyloric glands

Figure 24.14 The Secretions of Hydrochloric Acid

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

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

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

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

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

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