PowerPoint Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 23 The Digestive System: Part A Copyright 2010 Pearson Education, Inc. Digestive System Two groups of organs 1. Alimentary canal (gastrointestinal or GI tract)
Digests and absorbs food Mouth, pharynx, esophagus, stomach, small intestine, and large intestine continuous muscular digestive tube Copyright 2010 Pearson Education, Inc. Digestive System 2. Accessory digestive organs
aid digestion physically and produce secretions that break down foodstuff in the GI tract Teeth, tongue, gallbladder Digestive glands Salivary glands Liver
pancreas Copyright 2010 Pearson Education, Inc. Mouth (oral cavity) Tongue Esophagus Liver Gallbladder Duodenum Jejunum Small intestine Ileum Anus
Copyright 2010 Pearson Education, Inc. Parotid gland Sublingual gland Salivary Submandibular glands gland Pharynx Stomach Pancreas (Spleen) Transverse colon Descending colon Ascending colon Large Cecum intestine Sigmoid colon Rectum
Vermiform appendix Anal canal Figure 23.1 Digestive Processes- 6 Essential Activities 1. Ingestion- act of putting food into the mouth 2. Propulsion-moves food through the alimentary canal and includes both swallowing and peristalsis 3. Mechanical digestion-physical process of preparing the food for chemical digestion and involves chewing, mixing, churning, and segmentation 4. Chemical digestion-series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by enzymes 5. Absorption-passage of digested end products from the lumen of the GI tract through the mucosal cells into the blood or lymph 6. Defecation-eliminates indigestible substances from the body via the anus as feces Copyright 2010 Pearson Education, Inc.
Ingestion Mechanical digestion Chewing (mouth) Churning (stomach) Segmentation (small intestine) Chemical digestion Food Pharynx Esophagus Propulsion Swallowing (oropharynx) Peristalsis Stomach (esophagus,
stomach, small intestine, large intestine) Absorption Lymph vessel Small intestine Large intestine Defecation Copyright 2010 Pearson Education, Inc. Blood vessel Mainly H2O Feces
Anus Figure 23.2 From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. Copyright 2010 Pearson Education, Inc. (b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs. Figure 23.3
GI tract regulatory mechanisms 1. Mechanoreceptors and chemoreceptors Respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion Initiate reflexes that Activate or inhibit digestive glands Stimulate smooth muscle to mix and
move lumen contents Copyright 2010 Pearson Education, Inc. GI tract regulatory mechanisms 2. Intrinsic and extrinsic controls Enteric nerve plexuses (gut brain) initiate short reflexes in response to stimuli in the GI tract Long reflexes in response to stimuli inside or outside the GI tract involve CNS centers and autonomic nerves
Hormones from cells in the stomach and small intestine stimulate target cells in the same or different organs Copyright 2010 Pearson Education, Inc. Peritoneum and Peritoneal Cavity Peritoneum: serous membrane of the abdominal cavity Visceral peritoneum on external surface of most digestive organs Parietal peritoneum lines the body wall of the abdominopelvic cavity Peritoneal cavity Between the two peritoneums Serous fluid lubricates mobile organs Copyright 2010 Pearson Education, Inc.
Abdominopelvic cavity Vertebra Dorsal mesentery Parietal peritoneum Ventral mesentery Visceral peritoneum Peritoneal cavity
Alimentary canal organ Liver (a) Schematic cross sections of abdominal cavity illustrate the peritoneums and mesenteries. Copyright 2010 Pearson Education, Inc. Figure 23.5a Peritoneum and Peritoneal Cavity Mesentery is a double layer of peritoneum Routes for blood vessels, lymphatics, and nerves Holds organs in place and stores fat Extends to the digestive organs from the body wall Retroperitoneal organs lie posterior to the peritoneum & mesentery lying against the dorsal
abdominal wall Intraperitoneal (peritoneal) organs are surrounded by the peritoneum Copyright 2010 Pearson Education, Inc. Abdominopelvic cavity Mesentery resorbed and lost Alimentary Alimentary canal organ in canal organ a retroperitoneal position (b) Some organs lose their mesentery and become retroperitoneal during development. Copyright 2010 Pearson Education, Inc.
Figure 23.5b Blood Supply: Splanchnic Circulation Splanchnic circulation serves the digestive system and includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation Arteries Hepatic, splenic, left gastric, inferior and superior mesenteric Hepatic portal circulation Drains nutrient-rich blood from digestive organs & delivers it to the liver for processing Copyright 2010 Pearson Education, Inc. Histology of the Alimentary Canal
Four basic layers (tunics) Mucosa Submucosa Muscularis externa Serosa Copyright 2010 Pearson Education, Inc. Nerve Artery Vein Mesentery Copyright 2010 Pearson Education, Inc. Intrinsic nerve plexuses Myenteric nerve plexus Submucosal nerve plexus Glands in submucosa
Mucosa Epithelium Lamina propria Muscularis mucosae Submucosa Muscularis externa Longitudinal muscle Circular muscle Serosa Epithelium Connective tissue Lumen Gland in mucosa Lymphatic Mucosa-associated
Duct of gland outside vessel lymphoid tissue alimentary canal Figure 23.6 Mucosa Innermost, moist, epithelial membrane Lines the (lumen) entire digestive tract Functions Secretes mucus, digestive enzymes and hormones Absorbs end products of digestion into the blood Protects against infectious disease Three sublayers: epithelium, lamina propria, and muscularis mucosae Copyright 2010 Pearson Education, Inc.
Mucosa Epithelium Simple columnar epithelium and mucussecreting cells Mucus Protects digestive organs from enzymes Eases food passage May secrete enzymes and hormones (e.g., in stomach and small intestine) Copyright 2010 Pearson Education, Inc. Mucosa Lamina propria Loose areolar connective tissue Capillaries for nourishment and absorption Lymphoid follicles (part of MALT) Muscularis mucosae: smooth muscle that produces local movements of mucosa
Copyright 2010 Pearson Education, Inc. Submucosa and Muscularis Externa Submucosa Dense connective tissue layer Blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus Muscularis externa Consists of smooth muscle Responsible for segmentation and peristalsis Inner circular and outer longitudinal layers Myenteric nerve plexus Sphincters in some regions Copyright 2010 Pearson Education, Inc. Serosa Visceral peritoneum is the serosa Protective outer layer of the intraperitoneal
organs Replaced by the fibrous adventitia in the esophagus Retroperitoneal organs have both an adventitia and serosa Copyright 2010 Pearson Education, Inc. Nerve Artery Vein Mesentery Copyright 2010 Pearson Education, Inc. Intrinsic nerve plexuses Myenteric nerve plexus Submucosal nerve plexus Glands in submucosa
Mucosa Epithelium Lamina propria Muscularis mucosae Submucosa Muscularis externa Longitudinal muscle Circular muscle Serosa Epithelium Connective tissue Lumen Gland in mucosa Lymphatic Mucosa-associated
Duct of gland outside vessel lymphoid tissue alimentary canal Figure 23.6 Enteric Nervous System The alimentary canal has its own nerve supply made up of enteric neurons that communicate widely with each other to regulate digestive activity Intrinsic nerve supply of the alimentary canal Submucosal nerve plexus Regulates glands and smooth muscle in the mucosa Myenteric nerve plexus Controls GI tract motility Copyright 2010 Pearson Education, Inc.
Enteric Nervous System Linked to the CNS via afferent visceral fibers Long ANS fibers synapse with enteric plexuses Sympathetic impulses inhibit secretion and motility Parasympathetic impulses stimulate Copyright 2010 Pearson Education, Inc. Mouth Oral (buccal) cavity Bounded by lips, cheeks, palate, and tongue Oral orifice is the anterior opening Lined with stratified squamous epithelium Copyright 2010 Pearson Education, Inc.
Soft palate Palatoglossal arch Hard palate Uvula Oral cavity Palatine tonsil Tongue Oropharynx Lingual tonsil Epiglottis Hyoid bone Laryngopharynx Esophagus Trachea (a) Sagittal section of the oral cavity and pharynx Copyright 2010 Pearson Education, Inc.
Figure 23.7a Lips and Cheeks Contain orbicularis oris and buccinator muscles helps to keep food between the teeth when we chew and plays a small role in speech Vestibule: recess internal to lips and cheeks, external to teeth and gums Oral cavity proper lies within the teeth and gums Labial frenulum: median attachment of each lip to the gum Copyright 2010 Pearson Education, Inc. Gingivae (gums) Palatine raphe Hard palate Soft palate
Uvula Palatine tonsil Sublingual fold with openings of sublingual ducts Vestibule Lower lip Upper lip Superior labial frenulum Palatoglossal arch Palatopharyngeal arch Posterior wall of oropharynx Tongue Lingual frenulum Opening of
submandibular duct Gingivae (gums) Inferior labial frenulum (b) Anterior view Copyright 2010 Pearson Education, Inc. Figure 23.7b Palate The palate forms the roof of the mouth Hard palate: palatine bones and palatine processes of the maxillae (anterior portion) Slightly corrugated to help create friction against the tongue Soft palate: fold formed mostly of skeletal muscle (posterior portion)
Closes off the nasopharynx during swallowing Uvula projects downward from its free edge Copyright 2010 Pearson Education, Inc. Tongue Functions include Repositioning and mixing food during chewing Formation of the bolus Initiation of swallowing, speech, and taste Intrinsic muscles change the shape of the tongue Extrinsic muscles alter the tongues position Lingual frenulum: attachment to the floor of the mouth Copyright 2010 Pearson Education, Inc. Tongue
Surface bears papillae 1. Filiformwhitish, give the tongue roughness and provide friction 2. Fungiformreddish, scattered over the tongue 3. Circumvallate (vallate)V-shaped row in back of tongue These three house taste buds 4. Foliateon the lateral aspects of the posterior tongue Copyright 2010 Pearson Education, Inc. Tongue Terminal sulcus marks the division between Body: anterior 2/3 residing in the oral cavity
Root: posterior third residing in the oropharynx Copyright 2010 Pearson Education, Inc. Epiglottis Palatopharyngeal arch Palatine tonsil Lingual tonsil Palatoglossal arch Terminal sulcus Foliate papillae Circumvallate papilla Midline groove of tongue Dorsum of tongue Fungiform papilla
Filiform papilla Copyright 2010 Pearson Education, Inc. Figure 23.8 Salivary Glands Extrinsic salivary glands (parotid, submandibular, and sublingual) Intrinsic (buccal) salivary glands are scattered in the oral mucosa Produce secretion (saliva) Cleanses the mouth Moistens and dissolves food chemicals Aids in bolus formation Contains enzymes that begin the breakdown of starch Copyright 2010 Pearson Education, Inc. Salivary Glands
Parotid gland Anterior to the ear external to the masseter muscle Parotid duct opens into the vestibule next to second upper molar Submandibular gland Medial to the body of the mandible Duct opens at the base of the lingual frenulum Copyright 2010 Pearson Education, Inc. Salivary Glands Sublingual gland Anterior to the submandibular gland under the tongue Opens via 1012 ducts into the floor of the mouth Copyright 2010 Pearson Education, Inc.
Tongue Teeth Parotid gland Ducts of sublingual gland Frenulum of tongue Sublingual gland Mylohyoid muscle (cut) Anterior belly of digastric muscle (a)
Submandibular gland Parotid duct Masseter muscle Body of mandible (cut) Posterior belly of digastric muscle Submandibular duct Mucous cells (b) Copyright 2010 Pearson Education, Inc.
Serous cells forming demilunes Figure 23.9 Composition of Saliva Secreted by serous and mucous cells 9799.5% water, slightly acidic solution containing ElectrolytesNa+, K+, Cl, PO4 2, HCO3 Salivary amylase and lingual lipase Mucin Metabolic wastesurea and uric acid Lysozyme, IgA, defensins, and a cyanide compound protect against microorganisms PLAY Animation: Rotatable head
Copyright 2010 Pearson Education, Inc. Control of Salivation Intrinsic glands continuously keep the mouth moist Extrinsic salivary glands produce secretions when Ingested food stimulates chemoreceptors and mechanoreceptors in the mouth Salivatory nuclei in the brain stem send impulses along parasympathetic fibers in cranial nerves VII and IX Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia) Copyright 2010 Pearson Education, Inc. Teeth The teeth tear and grind food, breaking it into smaller pieces
Primary and permanent dentitions are formed by age 21 20 deciduous teeth erupt (624 months of age) Roots are resorbed, teeth fall out (612 years of age) as permanent teeth develop 32 permanent teeth All except third molars erupt by the end of adolescence Copyright 2010 Pearson Education, Inc. (b) Deciduous teeth Copyright 2010 Pearson Education, Inc. Permanent teeth
Figure 23.10b Classes of Teeth Incisors Chisel shaped for cutting Canines Fanglike teeth that tear or pierce Premolars (bicuspids) and molars Have broad crowns with rounded cusps for grinding or crushing Copyright 2010 Pearson Education, Inc. Incisors Central (68 mo) Lateral (810 mo) Canine (eyetooth)
(1620 mo) Molars First molar (1015 mo) Second molar (about 2 yr) (a) Copyright 2010 Pearson Education, Inc. Deciduous (milk) teeth Incisors Central (7 yr) Lateral (8 yr) Canine (eyetooth) (11 yr)
Premolars (bicuspids) First premolar (11 yr) Second premolar (1213 yr) Molars First molar (67 yr) Second molar (1213 yr) Third molar (wisdom tooth) (1725 yr) Permanent teeth Figure 23.10a
Dental Formulas A shorthand way of indicating the number and relative position of teeth Ratio of upper to lower teeth for one-half of the mouth Primary: 2I,1C, 2M Permanent: 2I,1C, 2PM, 3M Copyright 2010 Pearson Education, Inc. Tooth Structure Crown: the exposed part above the gingiva (gum) Covered by enamelthe hardest substance in the body (calcium salts and hydroxyapatite crystals) Root: portion embedded in the jawbone Connected to crown by neck
Copyright 2010 Pearson Education, Inc. Tooth Structure Cementum: calcified connective tissue Covers root and attaches it to the periodontal ligament Periodontal ligament Forms fibrous joint called a gomphosis Gingival sulcus: groove where gingiva borders the tooth Copyright 2010 Pearson Education, Inc. Tooth Structure Dentin: bonelike material under enamel Maintained by odontoblasts of pulp cavity
Pulp cavity: cavity surrounded by dentin Pulp: connective tissue, blood vessels, and nerves Root canal: extends from pulp cavity to the apical foramen of the root Copyright 2010 Pearson Education, Inc. Crown Neck Enamel Dentin Dentinal tubules Pulp cavity (contains blood vessels and nerves)
Gingiva (gum) Cementum Root Root canal Periodontal ligament Apical foramen Bone Copyright 2010 Pearson Education, Inc. Figure 23.11 Tooth and Gum Disease Dental caries (cavities): gradual demineralization of enamel and dentin Dental plaque (sugar, bacteria, and debris)
adheres to teeth Acid from bacteria dissolves calcium salts Proteolytic enzymes digest organic matter Prevention: daily flossing and brushing Copyright 2010 Pearson Education, Inc. Tooth and Gum Disease Gingivitis Plaque calcifies to form calculus (tartar) Calculus disrupts the seal between the gingivae and the teeth Anaerobic bacteria infect gums Infection reversible if calculus removed Copyright 2010 Pearson Education, Inc. Tooth and Gum Disease Periodontitis
Immune cells attack intruders and body tissues Destroy periodontal ligament Activate osteoclasts Consequences Possible tooth loss, promotion of atherosclerosis and clot formation in coronary and cerebral arteries Copyright 2010 Pearson Education, Inc. Pharynx Oropharynx and laryngopharynx Allow passage of food, fluids, and air Stratified squamous epithelium lining Skeletal muscle layers: inner longitudinal, outer pharyngeal constrictors Copyright 2010 Pearson Education, Inc. Esophagus
Flat muscular tube from laryngopharynx to stomach Pierces diaphragm at esophageal hiatus Joins stomach at the cardiac orifice Provides a passageway for food and fluids Copyright 2010 Pearson Education, Inc. Esophagus Esophageal mucosa contains stratified squamous epithelium Changes to simple columnar at the stomach Esophageal glands in submucosa secrete mucus to aid in bolus movement Muscularis: skeletal superiorly; smooth inferiorly Adventitia instead of serosa Copyright 2010 Pearson Education, Inc.
(a) Copyright 2010 Pearson Education, Inc. Mucosa (contains a stratified squamous epithelium) Submucosa (areolar connective tissue) Lumen Muscularis externa Longitudinal layer Circular layer Adventitia (fibrous connective tissue) Figure 23.12a Digestive Processes: Mouth Ingestion
Mechanical digestion Mastication (chewing) is partly voluntary, partly reflexive; mixes food with saliva Chemical digestion (salivary amylase and lingual lipase) Propulsion Deglutition (swallowing) Copyright 2010 Pearson Education, Inc. Deglutition Involves the tongue, soft palate, pharynx, esophagus, and 22 muscle groups Buccal phase Voluntary contraction of the tongue and occurs in the mouth where the bolus is forced into the oropharynx
Pharyngeal-esophageal phase Involuntary and occurs when food is squeezed through the pharynx and into the esophagus Control center in the medulla and lower pons Copyright 2010 Pearson Education, Inc. Bolus of food Tongue Uvula Pharynx Bolus Epiglottis Epiglottis
Glottis Trachea Bolus Esophagus 1 Upper esophageal sphincter is contracted. During the buccal phase, the tongue presses against the hard palate, forcing the food bolus into the oropharynx where the involuntary phase begins. 2 The uvula and larynx rise to prevent food from entering respiratory passageways. The tongue blocks off the mouth. The upper esophageal sphincter relaxes, allowing food to enter the esophagus. 3 The constrictor muscles of the
pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after entry. Relaxed muscles 4 Food is moved through the esophagus to the stomach by peristalsis. Circular muscles contract Bolus of food Relaxed muscles 5 The gastroesophageal
sphincter opens, and food enters the stomach. Longitudinal muscles contract Gastroesophageal sphincter closed Gastroesophageal sphincter opens Stomach Copyright 2010 Pearson Education, Inc. Figure 23.13 Stomach: Gross Anatomy Temporary storage tank where the chemical
breakdown of proteins is initiated and food is converted to chyme The adult stomach varies from 1525 cm long; its diameter and volume vary depending on the amount of food it contains Cardiac region (cardia) Surrounds the cardiac orifice Fundus Dome-shaped region beneath the diaphragm Copyright 2010 Pearson Education, Inc. Stomach: Gross Anatomy Body Midportion Pyloric region: antrum, pyloric canal, and pylorus Pylorus is continuous with the duodenum through
the pyloric valve (sphincter) Greater curvature Convex lateral surface Lesser curvature Concave medial surface Copyright 2010 Pearson Education, Inc. Cardia Esophagus Muscularis externa Longitudinal layer Circular layer Oblique layer Lesser curvature
Fundus Serosa Body Lumen Rugae of mucosa Greater curvature Duodenum (a) Copyright 2010 Pearson Education, Inc. Pyloric Pyloric canal antrum Pyloric sphincter (valve) at pylorus
Figure 23.14a Stomach: Gross Anatomy Two omentums help to tie the stomach to other digestive organs and the body wall Lesser omentum From the liver to the lesser curvature Greater omentum Drapes from greater curvature Anterior to the small intestine Copyright 2010 Pearson Education, Inc. Falciform ligament Liver Gallbladder Spleen Stomach
Ligamentum teres Greater omentum Small intestine Cecum (a) Copyright 2010 Pearson Education, Inc. Figure 23.30a Liver Gallbladder Lesser omentum Stomach Duodenum Transverse colon Small intestine Cecum
Urinary bladder Copyright 2010 Pearson Education, Inc. (b) Figure 23.30b Stomach: Gross Anatomy ANS nerve supply Sympathetic via splanchnic nerves and celiac plexus Parasympathetic via vagus nerve Blood supply Celiac trunk Veins of the hepatic portal system Copyright 2010 Pearson Education, Inc. Stomach: Microscopic Anatomy
Four tunics Muscularis and mucosa are modified Muscularis externa Three layers of smooth muscle Inner oblique layer allows stomach to churn, mix, move, and physically break down food Copyright 2010 Pearson Education, Inc. Surface epithelium Mucosa Lamina propria Submucosa (contains submucosal plexus)
Muscularis externa (contains myenteric plexus) Serosa Muscularis mucosae Oblique layer Circular layer Longitudinal layer (a) Layers of the stomach wall (l.s.) Copyright 2010 Pearson Education, Inc. Stomach wall Figure 23.15a Stomach: Microscopic Anatomy
Mucosa Simple columnar epithelium composed of goblet cells Goblet cells produce a protective two-layer coat of alkaline mucus that traps bicarbonate-rich fluid beneath it Gastric pits lead into gastric glands Copyright 2010 Pearson Education, Inc. Gastric pits Surface epithelium (mucous cells) Gastric pit Mucous neck cells Parietal cell Chief cell
Gastric gland Enteroendocrine cell (b) Enlarged view of gastric pits and gastric glands Copyright 2010 Pearson Education, Inc. Figure 23.15b Gastric Glands Cell types Mucous neck cells (secrete thin, acidic mucus) Parietal cells Chief cells Enteroendocrine cells Copyright 2010 Pearson Education, Inc.
Pepsinogen HCl Pepsin Mitochondria Parietal cell Chief cell Enteroendocrine cell (c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland Copyright 2010 Pearson Education, Inc. Figure 23.15c Gastric Gland Secretions
Glands in the fundus and body produce most of the gastric juice, which may be composed of a combination of mucus, hydrochloric acid, intrinsic factor, pepsinogen, and a variety of hormones Parietal cell secretions HCl pH 1.53.5 denatures protein in food, activates pepsin, and kills many bacteria Intrinsic factor Glycoprotein required for absorption of vitamin B12 in small intestine Copyright 2010 Pearson Education, Inc. Gastric Gland Secretions Chief cell secretions Inactive enzyme pepsinogen Activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism)
Copyright 2010 Pearson Education, Inc. Gastric Gland Secretions Enteroendocrine cells Secrete chemical messengers into the lamina propria Paracrines Serotonin and histamine Hormones Somatostatin and gastrin Copyright 2010 Pearson Education, Inc. Mucosal Barrier Layer of bicarbonate-rich mucus Tight junctions between epithelial cells Damaged epithelial cells are quickly replaced by division of stem cells
Copyright 2010 Pearson Education, Inc. Homeostatic Imbalance Gastritis: inflammation caused by anything that breaches the mucosal barrier Peptic or gastric ulcers: erosion of the stomach wall Most are caused by Helicobacter pylori bacteria Copyright 2010 Pearson Education, Inc. Bacteria Mucosa layer of stomach (a) A gastric ulcer lesion
Copyright 2010 Pearson Education, Inc. (b) H. pylori bacteria Figure 23.16 Digestive Processes in the Stomach Physical digestion Denaturation of proteins Enzymatic digestion of proteins by pepsin (and rennin in infants) Secretes intrinsic factor required for absorption of vitamin B12 Lack of intrinsic factor pernicious anemia Delivers chyme to the small intestine Copyright 2010 Pearson Education, Inc.
Regulation of Gastric Secretion Neural and hormonal mechanisms Stimulatory and inhibitory events occur in three phases: 1. Cephalic (reflex) phase: few minutes prior to food entry 2. Gastric phase: 34 hours after food enters the stomach Copyright 2010 Pearson Education, Inc. Regulation of Gastric Secretion 3. Intestinal phase: brief stimulatory effect as partially digested food enters the duodenum, followed by inhibitory effects (enterogastric reflex and enterogastrones) Copyright 2010 Pearson Education, Inc.
Stimulatory events Cephalic phase Gastric phase 1 Sight and thought of food Cerebral cortex Conditioned reflex 2 Stimulation of taste and smell receptors Hypothalamus and medulla
oblongata 1 Stomach distension activates stretch receptors Vagovagal reflexes 1 Presence of low pH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty
Stimulate Inhibit Copyright 2010 Pearson Education, Inc. Medulla Vagus nerve Vagus nerve Local reflexes 2 Food chemicals G cells (especially peptides and
caffeine) and rising pH activate chemoreceptors Intestinal phase Inhibitory events Gastrin release to blood Intestinal (enteric) gastrin release to blood Lack of
stimulatory impulses to parasympathetic center Cerebral cortex 1 Loss of appetite, depression Gastrin secretion declines G cells Overrides
parasympathetic controls Sympathetic nervous system activation 1 Excessive acidity (pH <2) in stomach 2 Emotional upset Stomach secretory activity Enterogastric
reflex Brief effect Local reflexes Vagal nuclei in medulla Pyloric sphincter 1 Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or
irritants in the duodenum 2 Distension; Release of intestinal presence of hormones (secretin, cholecystokinin, vasoactive fatty, acidic, partially intestinal peptide) digested food in the duodenum Figure 23.17 Regulation and Mechanism of HCl Secretion Three chemicals (ACh, histamine, and gastrin) stimulate parietal cells through
second-messenger systems All three are necessary for maximum HCl secretion Antihistamines block H2 receptors and decrease HCl release Copyright 2010 Pearson Education, Inc. Blood capillary Chief cell CO2 CO2 + H2O Carbonic H2CO3 anhydrase H+
K+ Stomach lumen H+-K+ ATPase H+ K+ HCO3 Alkaline tide HCI Parietal cell HCO3 Cl
Cl HCO3- Cl antiporter Cll Interstitial fluid Copyright 2010 Pearson Education, Inc. Figure 23.18 Response of the Stomach to Filling Stretches to accommodate incoming food Reflex-mediated receptive relaxation Coordinated by the swallowing center of the brain stem
Gastric accommodation Plasticity (stress-relaxation response) of smooth muscle Copyright 2010 Pearson Education, Inc. Gastric Contractile Activity Peristaltic waves move toward the pylorus at the rate of 3 per minute Basic electrical rhythm (BER) initiated by pacemaker cells (interstitial cells of Cajal) Distension and gastrin increase force of contraction Copyright 2010 Pearson Education, Inc. Gastric Contractile Activity Most vigorous near the pylorus Chyme is either
Delivered in ~ 3 ml spurts to the duodenum, or Forced backward into the stomach Copyright 2010 Pearson Education, Inc. Pyloric valve closed Propulsion: Peristaltic waves move from the fundus toward the pylorus. 1 Copyright 2010 Pearson Education, Inc. Pyloric valve
closed 2 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. Pyloric valve slightly opened Retropulsion: The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained
material backward into the stomach. 3 Figure 23.19 Regulation of Gastric Emptying As chyme enters the duodenum Receptors respond to stretch and chemical signals Enterogastric reflex and enterogastrones inhibit gastric secretion and duodenal filling Carbohydrate-rich chyme moves quickly through the duodenum Fatty chyme remains in the duodenum 6 hours or more The rate at which the stomach empties is determined by the contents of the stomach & processing in the small intestine
Copyright 2010 Pearson Education, Inc. Presence of fatty, hypertonic, acidic chyme in duodenum Duodenal enteroendocrine cells Chemoreceptors and stretch receptors Secrete Enterogastrones (secretin, cholecystokinin, vasoactive intestinal peptide) Duodenal stimuli
decline Initial stimulus Physiological response Result Copyright 2010 Pearson Education, Inc. Target Via short reflexes Enteric neurons Contractile force and rate of stomach emptying decline Via long
reflexes CNS centers sympathetic activity; parasympathetic activity Stimulate Inhibit Figure 23.20 Small Intestine: Gross Anatomy Major organ of digestion and absorption
24 m long; from pyloric sphincter (where it joins stomach) to ileocecal valve (where it joins large intestine) Subdivisions 1. Duodenum (retroperitoneal) 2. Jejunum (attached posteriorly by mesentery) 3. Ileum (attached posteriorly by mesentery) Copyright 2010 Pearson Education, Inc. Mouth (oral cavity) Tongue Esophagus Liver
Gallbladder Duodenum Jejunum Small intestine Ileum Anus Copyright 2010 Pearson Education, Inc. Parotid gland Sublingual gland Salivary Submandibular glands gland Pharynx Stomach Pancreas (Spleen) Transverse colon
Descending colon Ascending colon Large Cecum intestine Sigmoid colon Rectum Vermiform appendix Anal canal Figure 23.1 Duodenum The bile duct and main pancreatic duct Join at the hepatopancreatic ampulla Enter the duodenum at the major duodenal papilla Are controlled by the hepatopancreatic sphincter
Copyright 2010 Pearson Education, Inc. Right and left hepatic ducts of liver Cystic duct Common hepatic duct Bile duct and sphincter Accessory pancreatic duct Mucosa with folds Gallbladder Major duodenal papilla Hepatopancreatic ampulla and sphincter Copyright 2010 Pearson Education, Inc.
Tail of pancreas Pancreas Jejunum Duodenum Main pancreatic duct and sphincter Head of pancreas Figure 23.21 Structural Modifications Increase surface area of proximal part for nutrient absorption Circular folds (plicae circulares) Villi Microvilli Copyright 2010 Pearson Education, Inc.
Structural Modifications Circular folds Permanent (~1 cm deep) Force chyme to slowly spiral through lumen Copyright 2010 Pearson Education, Inc. Vein carrying blood to hepatic portal vessel Muscle layers Circular folds Villi Lumen
(a) Copyright 2010 Pearson Education, Inc. Figure 23.22a Structural Modifications Villi Motile fingerlike extensions (~1 mm high) of the mucosa Villus epithelium Simple columnar absorptive cells (enterocytes) Goblet cells Copyright 2010 Pearson Education, Inc. Structural Modifications Microvilli Projections (brush border) of absorptive
cells Bear brush border enzymes Copyright 2010 Pearson Education, Inc. Intestinal Crypts Intestinal crypt epithelium (crypts of Lieberkhn) Secretory cells that produce intestinal juice serves as a carrier fluid for absorbing nutrients from chyme Enteroendocrine cells Intraepithelial lymphocytes (IELs) Release cytokines that kill infected cells Paneth cells Secrete antimicrobial agents (defensins and lysozyme) Stem cells Copyright 2010 Pearson Education, Inc. Microvilli
(brush border) Absorptive cells Lacteal Goblet cell Blood capillaries Mucosa associated lymphoid tissue Intestinal crypt Muscularis mucosae Duodenal gland (b) Copyright 2010 Pearson Education, Inc. Vilus
Enteroendocrine cells Venule Lymphatic vessel Submucosa Figure 23.22b Submucosa Peyers patches protect distal part against bacteria Duodenal (Brunners) glands of the duodenum secrete alkaline mucus Copyright 2010 Pearson Education, Inc. Intestinal Juice Secreted in response to distension or irritation of the mucosa Slightly alkaline and isotonic with blood
plasma Largely water, enzyme-poor, but contains mucus Facilitates transport and absorption of nutrients Copyright 2010 Pearson Education, Inc. Liver Largest gland in the body Accessory organ associated with the small intestine Digestive function is to produce bile, which is a fat emulsifier Four lobesright, left, caudate, and quadrate Copyright 2010 Pearson Education, Inc. Liver Falciform ligament
Separates the (larger) right and (smaller) left lobes Suspends liver from the diaphragm and anterior abdominal wall Round ligament (ligamentum teres) Remnant of fetal umbilical vein along free edge of falciform ligament Copyright 2010 Pearson Education, Inc. Sternum Nipple Liver Bare area Falciform ligament Left lobe of liver
Right lobe of liver Gallbladder (a) Copyright 2010 Pearson Education, Inc. Round ligament (ligamentum teres) Figure 23.24a Sternum Nipple Liver Lesser omentum
(in fissure) Left lobe of liver Porta hepatis containing hepatic artery (left) and hepatic portal vein (right) Quadrate lobe of liver Ligamentum teres Bare area Caudate lobe of liver Sulcus for inferior vena cava Hepatic vein (cut)
Bile duct (cut) Right lobe of liver Gallbladder (b) Copyright 2010 Pearson Education, Inc. Figure 23.24b Liver: Associated Structures Lesser omentum anchors liver to stomach Hepatic artery and vein at the porta hepatis Bile ducts Common hepatic duct leaves the liver Cystic duct connects to gallbladder Bile duct formed by the union of the above two ducts
Copyright 2010 Pearson Education, Inc. Right and left hepatic ducts of liver Cystic duct Common hepatic duct Bile duct and sphincter Accessory pancreatic duct Mucosa with folds Gallbladder Major duodenal papilla Hepatopancreatic ampulla and sphincter Copyright 2010 Pearson Education, Inc.
Tail of pancreas Pancreas Jejunum Duodenum Main pancreatic duct and sphincter Head of pancreas Figure 23.21 Liver: Microscopic Anatomy Liver lobules Hexagonal structural and functional units Made of plates of liver cells (hepatocytes) Filter and process nutrient-rich blood Composed of plates of hepatocytes (liver cells)
Longitudinal central vein Copyright 2010 Pearson Education, Inc. (a) Lobule Copyright 2010 Pearson Education, Inc. (b) Central vein Connective tissue septum Figure 23.25a, b Liver: Microscopic Anatomy
Portal triad at each corner of lobule Bile duct receives bile from bile canaliculi Portal arteriole is a branch of the hepatic artery Hepatic venule is a branch of the hepatic portal vein Liver sinusoids are leaky capillaries between hepatic plates Kupffer cells (hepatic macrophages) in liver sinusoids Copyright 2010 Pearson Education, Inc. Interlobular veins (to hepatic vein) Central vein Sinusoids Bile canaliculi
Plates of hepatocytes Bile duct (receives bile from bile canaliculi) Fenestrated lining (endothelial cells) of sinusoids Portal vein Hepatic macrophages in sinusoid walls Bile duct Portal venule
Portal arteriole Portal triad (c) Copyright 2010 Pearson Education, Inc. Figure 23.25c Liver: Microscopic Anatomy Hepatocyte functions Process bloodborne nutrients Store fat-soluble vitamins Perform detoxification Produce ~900 ml bile per day Copyright 2010 Pearson Education, Inc. Bile
Yellow-green, alkaline solution containing Bile salts: cholesterol derivatives that function in fat emulsification and absorption Bilirubin: bile pigment formed from heme Cholesterol, neutral fats, phospholipids, and electrolytes Copyright 2010 Pearson Education, Inc. Bile Enterohepatic circulation Recycles bile salts Bile salts duodenum reabsorbed from ileum hepatic portal blood liver secreted into bile Copyright 2010 Pearson Education, Inc. The Gallbladder
Accessory organ associated with the small intestine Thin-walled muscular sac on the ventral surface of the liver Stores and concentrates bile (produced by liver) by absorbing its water and ions Releases bile as needed via the cystic duct, which flows into the bile duct Bile enters the small intestine when the gallbladder contracts after stimulated by cholecystokinin (CCK) Copyright 2010 Pearson Education, Inc. Pancreas Accessory gland that is retroperitoneal Location Mostly retroperitoneal, deep to the greater curvature of the stomach Head is encircled by the duodenum; tail abuts the spleen
Copyright 2010 Pearson Education, Inc. Pancreas Endocrine function Pancreatic islets secrete insulin and glucagon Exocrine function Acini (clusters of secretory cells) secrete pancreatic juice Zymogen granules of secretory cells contain digestive enzymes Copyright 2010 Pearson Education, Inc. Small duct Acinar cells Basement
membrane Zymogen granules Rough endoplasmic reticulum (a) Copyright 2010 Pearson Education, Inc. Figure 23.26a Pancreatic Juice Break downs all categories of foodstuffs and electrolytes Watery alkaline solution (pH 8) neutralizes chyme Electrolytes (primarily HCO3) Enzymes Amylase, lipases, nucleases are secreted in
active form but require ions or bile for optimal activity Proteases secreted in inactive form Copyright 2010 Pearson Education, Inc. Pancreatic Juice Secretion of pancreatic juice is regulated by local hormones and the parasympathetic nervous system Protease activation in duodenum Trypsinogen is activated to trypsin by brush border enzyme enteropeptidase Procarboxypeptidase and chymotrypsinogen are activated by trypsin Copyright 2010 Pearson Education, Inc. Stomach
Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen Trypsin (inactive) Chymotrypsin Chymotrypsinogen (inactive) Carboxypeptidase Procarboxypeptidase (inactive) Copyright 2010 Pearson Education, Inc. Figure 23.27
Regulation of Bile Secretion Bile secretion is stimulated by Bile salts in enterohepatic circulation Secretin from intestinal cells exposed to HCl and fatty chyme Copyright 2010 Pearson Education, Inc. Regulation of Bile Secretion Gallbladder contraction is stimulated by Cholecystokinin (CCK) from intestinal cells exposed to proteins and fat in chyme Vagal stimulation (minor stimulus) CKK also causes the hepatopancreatic sphincter to relax Copyright 2010 Pearson Education, Inc.
Regulation of Pancreatic Secretion CCK induces the secretion of enzyme-rich pancreatic juice by acini Secretin causes secretion of bicarbonate-rich pancreatic juice by duct cells Vagal stimulation also causes release of pancreatic juice (minor stimulus) Copyright 2010 Pearson Education, Inc. Slide 1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine
cells. 1 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3-rich pancreatic juice.
3 Copyright 2010 Pearson Education, Inc. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 4 CCK (via bloodstream) causes gallbladder to contract and
hepatopancreatic sphincter to relax; bile enters duodenum. 6 During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder. 5 Figure 23.28 Digestion in the Small Intestine Food takes 3 to 6 hours to complete its digestive path through the small intestine, the
site of virtually all nutrient absorption Chyme from stomach contains Partially digested carbohydrates and proteins Undigested fats Copyright 2010 Pearson Education, Inc. Requirements for Digestion and Absorption in the Small Intestine Optimal digestive activity in the small intestine depends on a slow, measured delivery of hypertonic chyme from the stomach Most substances required for chemical digestion (bile, enzymes, and bicarbonate) within the small intestine are imported from the pancreas and the liver Mixing Copyright 2010 Pearson Education, Inc.
Motility of the Small Intestine Segmentation Most common motion of the small intestine Initiated by intrinsic pacemaker cells Mixes and moves contents slowly and steadily toward the ileocecal valve Intensity is altered by long and short reflexes Wanes in the late intestinal (fasting) phase Copyright 2010 Pearson Education, Inc. Microvilli (b) Copyright 2010 Pearson Education, Inc.
Absorptive cell Figure 23.3b Motility of the Small Intestine Peristalsis Initiated by motilin in the late intestinal phase Each wave starts distal to the previous (the migrating motility complex) Meal remnants, bacteria, and debris are moved to the large intestine Copyright 2010 Pearson Education, Inc. From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.
Copyright 2010 Pearson Education, Inc. Figure 23.3a Motility of the Small Intestine Local enteric neurons coordinate intestinal motility Cholinergic sensory neurons may activate the myenteric plexus Causes contraction of the circular muscle proximally and of longitudinal muscle distally Forces chyme along the tract Copyright 2010 Pearson Education, Inc. Motility of the Small Intestine Ileocecal sphincter relaxes and admits chyme into the large intestine when Gastroileal reflex enhances the force of
segmentation in the ileum Gastrin increases the motility of the ileum Ileocecal valve flaps close when chyme exerts backward pressure Copyright 2010 Pearson Education, Inc. Large Intestine Absorbs water from indigestible food residues and eliminates the latter as feces Unique features Teniae coli Three bands of longitudinal smooth muscle in the muscularis Haustra Pocketlike sacs caused by the tone of the teniae coli Epiploic appendages Fat-filled pouches of visceral peritoneum
Copyright 2010 Pearson Education, Inc. Large Intestine Regions/subdivisions Cecum (pouch with attached vermiform appendix) Colon Rectum Anal canal Copyright 2010 Pearson Education, Inc. Left colic (splenic) flexure Transverse mesocolon Epiploic appendages
Right colic (hepatic) flexure Transverse colon Superior mesenteric artery Haustrum Descending colon Ascending colon IIeum Cut edge of mesentery
Teniae coli IIeocecal valve Cecum Sigmoid colon Vermiform appendix Rectum Anal canal (a) Copyright 2010 Pearson Education, Inc. External anal sphincter Figure 23.29a
Colon Ascending colon and descending colon are retroperitoneal Transverse colon and sigmoid colon are anchored via mesocolons (mesenteries) Copyright 2010 Pearson Education, Inc. Greater omentum Transverse colon Transverse mesocolon Descending colon Jejunum Mesentery Sigmoid mesocolon Sigmoid colon
Ileum (c) Copyright 2010 Pearson Education, Inc. Figure 23.30c Liver Lesser omentum Pancreas Stomach Transverse mesocolon Duodenum Transverse colon Mesentery Greater omentum Jejunum Ileum Visceral peritoneum
Parietal peritoneum (d) Copyright 2010 Pearson Education, Inc. Urinary bladder Rectum Figure 23.30d Rectum and Anus Rectum Three rectal valves stop feces from being passed with gas Anal canal The last segment of the large intestine Sphincters Internal anal sphinctersmooth muscle External anal sphincterskeletal muscle
Copyright 2010 Pearson Education, Inc. Rectal valve Rectum Hemorrhoidal veins Levator ani muscle Anal canal External anal sphincter Internal anal sphincter Anal columns Pectinate line Anal sinuses Anus (b) Copyright 2010 Pearson Education, Inc.
Figure 23.29b Large Intestine: Microscopic Anatomy Mucosa of simple columnar epithelium except in the anal canal (stratified squamous) Mucosa is thick and has abundant deep crypts with a large number of mucusproducing goblet cells Superficial venous plexuses of the anal canal form hemorrhoids if inflamed Copyright 2010 Pearson Education, Inc. Bacterial Flora Enter from the small intestine or anus Colonize the colon Ferment indigestible carbohydrates Release irritating acids and gases Synthesize B complex vitamins and vitamin K
Copyright 2010 Pearson Education, Inc. Functions of the Large Intestine Vitamins, water, and electrolytes are reclaimed Major function is propulsion of feces toward the anus Colon is not essential for life Copyright 2010 Pearson Education, Inc. Motility of the Large Intestine Haustral contractions Slow segmenting movements Haustra sequentially contract in response to distension Copyright 2010 Pearson Education, Inc.
Motility of the Large Intestine Gastrocolic reflex Initiated by presence of food in the stomach Activates three to four slow powerful peristaltic waves per day in the colon (mass movements) Copyright 2010 Pearson Education, Inc. Defecation Mass movements force feces into rectum Distension (stretching) initiates spinal defecation reflex Parasympathetic signals Stimulate contraction of the sigmoid colon and rectum Relax the internal anal sphincter Conscious control allows relaxation of
external anal sphincter Copyright 2010 Pearson Education, Inc. Impulses from cerebral cortex (conscious control) Distension, or stretch, of the rectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 1 Sensory nerve fibers
A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter. 2 Voluntary motor nerve to external anal sphincter Sigmoid colon Stretch receptors in wall Rectum External anal
sphincter (skeletal muscle) Involuntary motor nerve (parasympathetic division) Internal anal sphincter (smooth muscle) If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so that feces may pass. 3 Copyright 2010 Pearson Education, Inc. Figure 23.31 Chemical Digestion Catabolic
Accomplished by enzymes, secreted by intrinsic and accessory glands of the alimentary canal, used in hydrolysis reactions Large food molecules are broken down to chemical building blocks (monomers), which are small enough to be absorbed by the GI tract lining Copyright 2010 Pearson Education, Inc. Chemical Digestion and Absorption of Carbohydrates Begins in the mouth, where salivary amylase breaks large polysaccharides into smaller fragments Digestive enzymes Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)
Copyright 2010 Pearson Education, Inc. Chemical Digestion and Absorption of Carbohydrates Absorption Glucose and galactose are transported into the epithelial cells by common protein carriers and are then moved by facilitated diffusion into the capillary blood Secondary active transport (cotransport) with Na+ Facilitated diffusion of some monosaccharides Enter the capillary beds in the villi Transported to the liver via the hepatic portal vein Copyright 2010 Pearson Education, Inc. Chemical Digestion and Absorption of Carbohydrates
Monosaccharides-simple sugars; absorbed immediately (glucose, galactose, and fructose) Disaccharides-two monosaccharides bonded together (maltose, lactose, and sucrose) Starch-digestible polysaccharide found in the diet Other polysaccharides-not able to be broken down by humans (cellulose) Copyright 2010 Pearson Education, Inc. Carbohydrate digestion Foodstuff Enzyme(s) and source Site of action
Starch and disaccharides Oligosaccharides and disaccharides Lactose Maltose Sucrose Galactose Glucose Fructose Copyright 2010 Pearson Education, Inc. Salivary amylase Pancreatic amylase Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, and sucrase)
Mouth Small intestine Small intestine Path of absorption Glucose and galactose are absorbed via cotransport with sodium ions. Fructose passes via facilitated diffusion. All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the
villi, and are transported to the liver via the hepatic portal vein. Figure 23.32 (1 of 4) Chemical Digestion and Absorption of Proteins Proteins digested into amino acids in the GI tract include not only dietary proteins but also enzyme proteins secreted into the GI tract lumen. Pepsin, secreted by the chief cells, begins the chemical digestion of proteins in the stomach Rennin is produced in infants and breaks down milk proteins Copyright 2010 Pearson Education, Inc.
Chemical Digestion and Absorption of Proteins Pancreatic proteases Trypsin, chymotrypsin, and carboxypeptidase Further break down proteins in the small intestine Brush border enzymes Aminopeptidases, carboxypeptidases, and dipeptidases Work on freeing single amino acids in the small intestine Absorption of amino acids is coupled to active transport of Na+ Copyright 2010 Pearson Education, Inc. Amino acids of protein fragments Brush border enzymes Apical membrane (microvilli)
Lumen of intestine Pancreatic proteases 1 Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. Na+ Na+
Absorptive epithelial cell 2 The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis). Amino acid carrier 3 The amino acids leave the Active transport Passive transport Copyright 2010 Pearson Education, Inc. Capillary
villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. Figure 23.33 Protein digestion Foodstuff Protein Large polypeptides Small polypeptides, small peptides Amino acids (some dipeptides and tripeptides) Copyright 2010 Pearson Education, Inc.
Enzyme(s) and source Pepsin (stomach glands) in presence of HCl Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Site of action
Path of absorption Amino acids are absorbed by cotransport with Stomach sodium ions. Some dipeptides and tripeptides are absorbed via cotransport with H++ Small and hydrolyzed to amino intestine acids within the cells. Amino acids leave the epithelial cells by Small facilitated diffusion, enter intestine
the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Figure 23.32 (2 of 4) Chemical Digestion and Absorption of Lipids Small intestine= sole site for lipid digestion Pre-treatmentemulsification by bile salts Enzymespancreatic lipases are the enzymes that digest fats after they have been pretreated with bile Absorption of glycerol and short chain fatty acids Absorbed into the capillary blood in villi Transported via the hepatic portal vein Copyright 2010 Pearson Education, Inc.
Chemical Digestion and Absorption of Lipids Absorption of monoglycerides and fatty acids Cluster with bile salts and lecithin to form micelles Released by micelles to diffuse into epithelial cells Combine with proteins to form chylomicrons Enter lacteals and are transported to systemic circulation Copyright 2010 Pearson Education, Inc. Fat globule 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.
Bile salts Fat droplets coated with bile salts Epithelial cells of small intestine Copyright 2010 Pearson Education, Inc. 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which ferry them to the intestinal mucosa. Micelles made up of fatty
acids, monoglycerides, and bile salts 3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons. 4 Chylomicrons are extruded from the Lacteal epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph. Figure 23.34 Fat digestion Foodstuff
Enzyme(s) and source Unemulsified fats Emulsification by the detergent action of bile salts ducted in from the liver Pancreatic lipases Monoglycerides Glycerol and fatty acids and fatty acids Copyright 2010 Pearson Education, Inc.
Site of action Path of absorption Fatty acids and monoglycerides enter the intestinal cells via diffusion. Small intestine Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are Small extruded by exocytosis. intestine
The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein. Figure 23.32 (3 of 4) Chemical Digestion and Absorption of Nucleic Acids Enzymes Pancreatic ribonuclease & deoxyribonuclease present in pancreatic juice
Hydrolyze DNA & RNA to their nucleotide monomers Absorption Active transport Transported to liver via hepatic portal vein Copyright 2010 Pearson Education, Inc. Nucleic acid digestion Foodstuff Enzyme(s) and source Nucleic acids Pentose sugars, N-containing bases,
phosphate ions Copyright 2010 Pearson Education, Inc. Pancreatic ribonuclease and deoxyribonuclease Brush border enzymes (nucleosidases and phosphatases) Site of action Path of absorption Units enter intestinal cells by active transport via Small
intestine membrane carriers. Units are absorbed into capillary blood in the villi Small and transported to the intestine liver via the hepatic portal vein. Figure 23.32 (4 of 4) Vitamin Absorption In small intestine dietary vitamins are absorbed Fat-soluble vitamins (A, D, E, and K) are carried by micelles and then diffuse into absorptive cells Water-soluble vitamins (vitamin C and B vitamins) are absorbed by diffusion or by passive or active transporters. Vitamin B12 binds with intrinsic factor, and is
absorbed by endocytosis In large intestine, Vitamin K and B vitamins from bacterial metabolism are absorbed Copyright 2010 Pearson Education, Inc. Nutrient Absorption Absorption occurs along the entire length of the small intestine, and most of it is completed before the chyme reaches the ileum Several types of carriers transport the different amino acids before entering the capillary blood by diffusion Monoglycerides and free fatty acids of lipid digestion become associated with bile salts and lecithin to form micelles, which are necessary for lipid absorption Pentose sugars, nitrogenous bases, and phosphate ions are transported actively across the epithelium by special transport carriers in the villus epithelium
Copyright 2010 Pearson Education, Inc. Electrolyte Absorption Mostly along the length of small intestine Iron and calcium are absorbed in duodenum Na+ is coupled with absorption of glucose and amino acids Ionic iron is stored in mucosal cells with ferritin K+ diffuses in response to osmotic gradients Ca2+ absorption is regulated by vitamin D and parathyroid hormone (PTH) Copyright 2010 Pearson Education, Inc. Water Absorption Most abundant substance in chyme 95% is absorbed in the small intestine by osmosis Net osmosis occurs whenever a concentration gradient is established by active transport of
solutes Water uptake is coupled with solute uptake Copyright 2010 Pearson Education, Inc. Malabsorption of Nutrients Causes Anything that interferes with delivery of bile or pancreatic juice Damaged intestinal mucosa (e.g., bacterial infection) Gluten-sensitive enteropathy (celiac disease) Gluten damages the intestinal villi and brush border Treated by eliminating gluten from the diet (all grains but rice and corn) Copyright 2010 Pearson Education, Inc.
Developmental Aspects In the third week Endoderm has folded and foregut and hindgut have formed Midgut is open and continuous with the yolk sac Mouth and anal openings are nearly formed In the eighth week Accessory organs are budding from endoderm Alimentary canal is a continuous tube stretching from the mouth to the anus Copyright 2010 Pearson Education, Inc. Developmental Aspects The epithelial lining of the developing alimentary canal forms from the endoderm with the rest of the wall arising from the mesoderm The anteriormost endoderm touches the depressed area of the surface ectoderm where
the membranes fuse to form the oral membrane and ultimately the mouth The end of the hindgut fuses with an ectodermal depression, called the proctodeum, to form the cloacal membrane and ultimately the anus Copyright 2010 Pearson Education, Inc. Brain Oral membrane Heart Yolk sac Cloacal membrane Body stalk (a)
Copyright 2010 Pearson Education, Inc. Lung bud Stomodeum Foregut Stomach Liver Site of liver development Midgut Spinal cord Bile duct
Gallbladder Hindgut Cystic duct Ventral pancreatic bud Proctodeum Dorsal pancreatic bud Duodenum Endoderm (b) Figure 23.35
Developmental Aspects During old age GI tract activity (motility) declines, digestive juice production decreases, absorption is less efficient, and peristalsis is slowed, resulting in less frequent bowel movements and often constipation Diverticulosis, fecal incontinence, and cancer of the GI tract are fairly common Copyright 2010 Pearson Education, Inc. Cancer Stomach and colon cancers rarely have early signs or symptoms Metastasized colon cancers frequently cause secondary liver cancer Prevention
Regular dental and medical examination Copyright 2010 Pearson Education, Inc.