The Digestive System 1. Lesson # 15. Chapter 24. Objectives:. 1- To explain the functions of the digestive system. 2- To describe the structure of the parietal and visceral peritoneum. 3- To describe the functional histology of the digestive tract.
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The Digestive System 1 Lesson # 15 Chapter 24 Objectives: 1- To explain the functions of the digestive system. 2- To describe the structure of the parietal and visceral peritoneum. 3- To describe the functional histology of the digestive tract. 4- To explaining how food travels through the alimentary canal. 5- To explain the mechanisms that control the digestive function.
Functions of the Digestive System • 1- Ingestion: • It occurs when materials enter the digestive tract via the mouth. • 2- Mechanical processing: • It is the crushing and shearing that make materials easier to propel along the digestive tract. • 3- Digestion: • It is the chemical breakdown of food into small organic fragments for absorption by the digestive epithelium. • 4- Secretion: • It is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by the glandular organs. • 5- Absorption: • It is the movement of organic substrates, electrolytes, vitamins, and water across the digestive epithelium into interstitial fluid of the digestive tract. • 6- Excretion: • It is the removal of waste products from the body fluids.
Digestive tract Accessory organs • The Components of the Digestive System - Oral cavity - Pharynx - Esophagus - Stomach - Small intestine - Large intestine Digestive System - Teeth - Tongue - Salivary glands - Liver - Gallbladder - Pancreas
The Digestive Organs and the Peritoneum Mesenteries: They are sheets of serous membrane that connect the visceral peritoneum of different organs and the visceral peritoneum to the parietal peritoneum. Falciform ligament Lesser omentum It attaches the liver to the diaphragm and the abdominal wall. It attaches the stomach to the liver. Transverse mesocolon Retroperitoneal organs: It supports the transverse colon. Pancreas Greater omentum Duodenum It is a pouch that hanks like an apron from the lateral and inferior borders of the stomach. It provides padding, protection, and energy reserve Mesentery proper It attaches and suspends the small intestine to the posterior wall of the abdomen. Greater omentum Sigmoid mesocolon Parietal peritoneum Visceral peritoneum Peritoneal cavity It supports the sigmoid colon. It contains the peritoneal fluid which provides lubrication. Accumulation of fluid is called ascites. Infection is called peritonitis.
Histological Organization of the Digestive Tract Stratified squamous (oral cavity, pharynx and esophagus). Epithelium Simple columnar(stomach, small intestine, and almost the entire large intestine). Areolar connective tissue with blood vessels, sensory nerve endings, lymphatic vessels, and lymphoid tissue. It supports, nourishes and protects the epithelium. Lamina propria Muscularis mucosa Smooth muscle and elastic fibers between the mucosa and submucosa. Dense irregular connective tissue, containing large blood and lymphatic vessels, exocrine glands, and a network of nerve fibers and neurons of the autonomic system called submucosal plexus or Meissner plexus. 2- Submucosa: 1- Mucosa: 3- Muscularis externa: Two or three layers of smooth muscle tissue for mechanical processing and movement of material along the digestive tract. It contains the myenteric plexus or Auerbach plexus. Parasympathetic: Increases muscle tone and activity. Sympathetic: Decreases muscle tone and activity. 4- Serosa
Changes in the Mucosa and the Number of Layers of the Muscularis Externa. The folds. Esophagus Stomach Stratified squamous epithelium (also in oral cavity, pharynx and esophagus). Diaphragm Simple columnar epithelium (it contains gastric pits and gastric glands) Three layers of smooth muscle in the muscularis externa: outer longitudinal, middle circular, and inner oblique. Two layers of smooth muscle in the muscularisexterna: outer longitudinal and inner circular. Folds of the mucosa called rugae. Longitudinal folds of the mucosa that allow for expansion Taeniae coli Large Intestine Small Intestine Transverse folds of the wall called haustra. Simple columnar epithelium with microvilli (it contains crypts of Lieberkuhn and intestinal glands) Simple columnar epithelium without villi (it is dominated by mucous cells) Two layers of smooth muscle in the muscularisexterna: outer longitudinal and inner circular. Two layers of smooth muscle in the muscularisexterna: outer longitudinal reduced to the taeniae coli, and inner circular. Transverse folds of the mucosa called plicae circulares, and fingerlike projections called villi.
The Movement of Digestive Materials The smooth muscle of the muscularis externa is called visceral smooth muscle. • Some cells of the visceral smooth muscle called pacesetter cells undergo spontaneous depolarization that triggers waves of contraction through the entire muscular sheet. Waves of contractions propel material from one portion of the digestive tract to another. Waves of contraction Peristalsis Waves of contractions does not propel material, but churns, fragments, and mixes the bolus with intestinal secretions. Segmentation Peristalsis: Bolus Bolus Bolus Bolus
Control of Digestive Functions • 1- Local Factors • 2- Neural Mechanisms • a- Short reflexes • b- Long reflexes • 3- Hormonal Mechanisms • 1- Local Factors • Prostaglandins, histamine, and other chemicals released into interstitial fluid, may affect adjacent cells within small segment of digestive tract, coordinating response to changing conditions (variations in local pH, chemical, or physical stimuli). • Local factors affect only a portion of tract.
2- Neural Mechanisms • They control the movement of materials along digestive tract, and secretory functions. • Motor neurons located in myenteric plexus control smooth muscle contraction and glandular secretion. • 1- Short reflexes: • Acid and semi-digested fats in the duodenum trigger a long reflex. The medulla oblongata inhibits vagal nuclei, reducing vagal stimulation of the stomach. • They are responsible for local reflexes that control small segments of digestive tract. They operate entirely outside of CNS control. • 2- Long reflexes: • They provide a higher level control that involves interneurons and motor neurons in CNS. • Stretching of the stomach activates a short reflex that increases gastric secretions. • They control large-scale peristaltic waves that move material from one region of the digestive system to another.
3- Hormonal Mechanisms • At least 18 peptide hormones affect most aspects of digestive function and some of them also affect activities of other systems. • The hormones gastrin, secretin, and other; are produced by enteroendocrine cells in the digestive tract. • They reach target organs after distribution in bloodstream. Ex: Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin (CCK) that suppress gastric secretion and motility.
Control of Digestive Functions Neural Control Mechanisms The movement of materials along the digestive tract, as well as many secretoryfunctions, is primarily controlled by local factors. These reflexes are: 1- Short reflexes are triggered by chemoreceptors or stretch receptors in the walls of the digestive tract; the controlling neurons are located in the myenteric plexus. CNS Longreflex 2- Long reflexes involving interneurons and motor neurons in the CNS provide a higher level of control over digestive and glandular activities, generally controlling large-scale peristaltic waves that move materials from one region of the digestive tract to another. Myentericplexus Shortreflex Peristalsis andsegmentationmovements Stretch receptors,chemoreceptors Buffers, acids,enzymes released Secretorycells Hormonal Control Mechanisms The digestive tract produces at least 18 hormones that affectalmost every aspect of digestivefunction, and some of themalso affect the activities ofother systems. Local Factors Local factors are the primary stimulus fordigestive activities. They coordinate theresponses to: Via thebloodstream Enteroendocrinecells 1- Changes in the pH of the contents of the lumen. These hormones are peptides produced by enteroendocrine cells, endocrine cells in the epithelium of the digestive tract. 2- Physical distortion of the wall of the digestive tract. Hormonesreleased 3- The presence of chemicals—either specific nutrients or chemical messengers released by cells of the mucosa.
Overview of Chemical Digestion 1- Carbohydrates Amylases Amylases Amylases Disaccharides Monosaccharides Starch is the most digestible carbohydrate • They are absorbed immediately 2- Proteins Oligosaccharides Peptidases Peptidases Protein Aminoacids Polypeptides • They are absorbed immediately
3- Lipids + Lipases Triglycerides Glycerol Fatty acids Lipases + Monoglycerides Fatty acids
Soft palate Uvula Upper lip Gingiva Cheek Lingual frenulum Vestibule Lower lip The Oral Cavity, the Tongue, the Salivary Glands, and the Teeth The Oral Cavity The oral cavity is lined by the oral mucosa, which has a stratified squamous epithelium. Palatine bone Palatine process of maxilla Hard palate Tongue It is the space between the cheek (or lips) and the teeth . Functions: 1- Sensory analysis of material before swallowing. 2- Mechanical processing through actions of teeth, tongue, and palatal surfaces. 3- Lubrication by mixing with mucus and salivary gland secretions. 4- Limited digestion of carbohydrates and lipids.
They contain taste buds They contain taste buds The Tongue Root or Pharyngeal portion They provide friction Body or Oral portion Functions: 1- Mechanical processing by compression, abrasion, and distortion. 2- Manipulation of food to assist in chewing and swallowing. 3- Sensory analysis of food by touch, temperature, and taste receptors. 4- Secretion of mucus and lingual lipase. 5- Helping in speech.
Mucous cells Serous cells Parotid duct Parotid gland Sublingual gland Submandibular gland Duct Salivary Glands They produce mucins They produce enzymes Functions of Saliva: 1- Cleanses the mouth and protects the mouth (it contains antibodies and lysozyme). 2- Lubricates the mouth. 3- Moistens and lubricates materials (food) in the mouth. 4- Dissolves chemicals that stimulate taste buds. 5- Initiates the digestion of carbohydrates (salivary amylase)
Dentin Enamel Crown Gingiva or gum Neck Periodontal ligament Root Cementum Root canal Blood vessels and nerve The Teeth Pulp cavity Functions: Mechanical break down of food (chewing or mastication)
Central incisor Cuspid or canine Lateral incisor Bicuspids or premolars Molars Total: 20 primary, temporary or deciduous teeth Types of Teeth (Blade-shape teeth: Clipping and cutting) (Conical with a sharp ridgeline and a pointed tip: Tearing or slashing) Upper dental arch (Flattened crown with prominent ridges: Crushing, smashing and grinding) (Very large flattened crowns with prominent ridges: Crushing and grinding) Lower dental arch Total: 32 permanent or secondary teeth
The Pharynx Functions: Air passageway between nasal cavity and the larynx. Posterior nasal aperture Food passageway between the oral cavity and the esophagus. Pharynx: Nasopharynx It is a passageway for air. Oropharynx It is lined by a pseudostratified columnar epithelium. Laryngopharynx It is a passageway for air, food and drink. It is lined by a stratified squamous epithelium. It is a passageway for air, food and drink. Esophagus It is lined by a stratified squamous epithelium. Larynx Trachea
The Esophagus Function: It transports solids and liquids from the pharynx to the stomach. Stratified squamous epithelium (also in oral cavity, pharynx and esophagus). Diaphragm Esophageal hiatus Two layers of smooth muscle in the muscularisexterna: outer longitudinal and inner circular. Longitudinal folds of the mucosa that allow for expansion. Mucosa Serosa: The peritoneum is a serosa. The visceral layer lines several organs of the digestive system (stomach, small intestine, liver). Adventitia: There is no serosa in oral cavity, pharynx, esophagus, and rectum. They have adventitia, a fibrous sheath of collagen fiber that attaches them to the adjacent structures.
Swallowing or Deglutition Swallowing, also called deglutition, is a complex process that can be initiated voluntarily but proceeds automatically once it begins. • It is divided into three phases: • 1- Buccal phase • 2- Pharyngeal phase • 3- Esophageal phase The buccal phase is strictly voluntary. It begins with the compression of the bolus against the hard palate and the elevation of the soft palate. The bolus enters the oropharynx. The esophageal phase begins as the contraction of the esophageal muscles forces the bolus through the entrance of the esophagus. When the bolus enters the oropharynx, the swallowing reflex is triggered and the process becomes involuntary. Once in the esophagus, the bolus is pushed toward the stomach by a peristaltic wave. The epiglottis closes the larynx, and the soft palate and the uvula close the nasopharynx. The approach of the bolus triggers the opening of the lower esophageal sphincter, and the bolus enters the stomach. The pharyngeal muscles propel the bolus into the esophagus in less than one second.