Week 5 Digestive System Part 1 PDF

1 Chapter 26 Digestive System  2 Digestive System Organs  3 Digestive System Functions Six main functions Ingestion Introduction of solid and liquid nutrients into the oral cavity First...

1 Chapter 26 Digestive System  2 Digestive System Organs  3 Digestive System Functions Six main functions Ingestion Introduction of solid and liquid nutrients into the oral cavity First step in process of digesting and absorbing nutrients Motility Voluntary and involuntary muscular contractions, Mixing and moving materials through the GI tract Secretion Process of producing and releasing fluid products facilitating digestion Digestion Mechanical digestion: Chewing, churning etc. Chemical digestion: Enzymes and other chemicals, breakdown Absorption Transport of digested molecules, electrolytes, vitamins, water Move from GI tract into blood or lymph Elimination Expulsion of indigestible components that are not absorbed  4 Tunics GI tract is a hollow tube of 4 concentric layers, called tunics Innermost to outermost: Mucosa – Absorption Submucosa – contains blood vessels, lymph vessels, nerves glands and MALT Muscularis – Motility (mixing, peristalsis, sphincters) Adventitia (or serosa) – Connective tissue outer layers. If intraperitoneal = serous Digestive organs slightly differ in 26.2a Figure their Tunic layers depending on  5 Gastrointestinal Tract Wall Functions Propulsion is also known as peristalsis, which works to push food/fluid along the digestive tract Mixing does not move food through the tract, but instead pushes it back and forth to mix  6 Serous Membranes Figure 26.3a ©McGraw-Hill Education/Christine Eckel  7 Serous Membranes: Mesentery Proper and Mesocolon Figure 26.3b The mesentery is a fold of membrane that attaches the intestine to the abdominal wall and holds it in place ©McGraw-Hill Education/Christine Eckel  8 Overview of the Nervous Regulation of the Digestive System GI tract controlled by two aspects of the nervous system Enteric nervous system (ENS) Sensory and motor neurons within sub-mucosal plexus and myenteric plexus Innervates smooth muscle and glands of GI tract Coordinates mixing and propulsion reflexes Autonomic Nervous System (ANS) Parasympathetic innervation promotes GI tract activity Sympathetic innervation opposes GI tract activity  9 Overview of the Nervous Regulation of the Digestive System  10 Overview of the Nervous Regulation of the Digestive Nerve reflexes System Baroreceptors detect stretch in GI tract wall Chemoreceptors monitor chemical contents in lumen Reflexes (by ANS or ENS) are initiated in response to receptor input Short reflex – local reflex, only involves ENS; coordinate small segments of GI tract Long reflex – involves sensory input to CNS and autonomic motor output; coordinate GI tract motility, secretions, and accessory digestive organs Hormonal control Several hormones participate in regulation of digestion E.g., Gastrin, secretin, cholecystokinin, motilin  11 Overview of the Nervous Regulation of the Digestive System  12 Overview of the Hormonal Regulation of the Digestive Hormonal control System Several hormones participate in regulation of digestion Gastrin Secreted when food enters stomach. Gastric motility and gastric juice secretion Secretin Food entering small intestine. Inhibits stomach and stimulates liver and pancreas to secrete bicarbonate Cholecystokinin Secreted in response to higher fat food entering small intestine. Stimulates secretions rom pancreas and gall bladder to help digest fat Motilin Stimulates intestinal movement to help move food through  13 What did you learn? 1. How is the gastrointestinal (GI) tract distinguished from accessory digestive organs? List the structures that compose each category. 2. What is the primary difference between mechanical digestion and chemical digestion? 3. How does peristalsis differ from mixing?  14 Upper Gastrointestinal Tract Organs and Upper GI tract organs and Their accessory Functions structures Oral cavity and salivary glands Mechanical digestion begins Saliva secreted from salivary glands in response to food Contains salivary amylase, enzyme initiating digestion of starch Mixed with ingested materials to form bolus Pharynx Bolus moved to pharynx during swallowing Mucus secreted to facilitate swallowing Esophagus Bolus transported from pharynx into stomach Lubricated by mucus secretions Stomach Bolus mixed with gastric secretions by smooth muscle contractions Secretions produced by epithelial cells of stomach Chyme formed from mixing Duodenum also considered part of upper GI tract  15 Oral Cavity, Anterior View Figure 26.4a  Oral Cavity and Pharynx, 16 Sagittal Section Figure 26.4b  17 Salivary Glands Salivary glands: Produce saliva Intrinsic salivary glands (within oral cavity) Unicellular glands that continuously release secretions independent of food Contains lingual lipase, enzyme that begins digestion of fats Extrinsic salivary glands (outside of oral cavity) Produce most saliva - Parotid, submandibular, and sublingual glands Parotid salivary glands, largest salivary glands 5–30% of saliva Infection of the parotid glands causes mumps Submandibular salivary glands Produces 60–70% of saliva Sublingual salivary gland Contribute only 3–5% of saliva Figure 26.5a  Oral Cavity and Salivary 18 Glands Regulation of salivary secretions Regulated by salivary nuclei within brainstem Basal level of salivation in response to parasympathetic stimulation Receptors detect stimuli (especially acids) in oral cavity and stomach Send signals to salivary nuclei in brainstem Signals also received from higher brain centers in response to thought of food Increased parasympathetic output to salivary glands Sympathetic stimulation results in a more viscous saliva due to decreased water in the saliva  19 Saliva 1.0–1.5 L secreted daily, most produced during mealtime 99.5% water and a mixture of solutes Salivary amylase, mucin, lysozyme added to saliva by the body’s cells Functions of saliva: Moistens ingested food to help become bolus Salivary amylase initiates chemical breakdown of starch Food molecules dissolved here so taste receptors stimulated Cleanses oral cavity structures Antibacterial substances inhibit bacterial growth (lysozyme, IgA antibodies) Figure 26.5a  20 Oral Mechanical Digestion Mechanical digestion: Mastication (chewing) Mechanically reduces bulk to facilitate swallowing Increases surface area to facilitate exposure to digestive enzymes Promotes salivation Requires coordinated activities of teeth, lips, tongue, cheeks, jaws Controlled by nuclei in medulla and pons, mastication center Swallowing (Deglutition) has several phases: Voluntary phase: Mouth, mastication; Pharyngeal phase: In throat, reflexive, involuntary; Esophageal phase: In esophagus, reflexive, involuntary Figure 26.5a Deciduous Teeth (AKA Baby 21 Teeth) Figure 26.6b  22 Permanent Teeth Incisor – designed for slicing/cutting into food Canines – designed for puncturing/tearing food Molars – designed for crushing and grinding food Figure 26.6c  23 Teeth - Anatomy of a Molar Figure 26.6a The Pharynx and Esophagus 24 Figure 26.7a Pharynx: passageway for both food and water. 3 distinct parts. Ends at larynx/esophagus (superior esophageal sphincter) Esophagus: Normally a collapsed tubular passageway Superior esophageal sphincter is closed when we breath to allow air to enter the trachea and not the esophagus Inferior esophageal sphincter – contracts, along with the muscles in diaphragm to help prevent food from refluxing back into the esophagus 26.8 Figure Phases of Swallowing 25  Clinical View: 26 Reflux Esophagitis and Gastroesophageal Reflux Disease Inflammation of esophagus due to acidic chyme refluxing into esophagus Pain posterior to sternum, heartburn Seen most frequently Overweight individuals, smokers, after large meals Hiatal hernias (portion of stomach protruding through diaphragm) Treatment Lifestyle changes, head elevation, limiting meal size - If untreated/poorly managed, can lead to chronic reflux esophagitis Erodes esophageal tissue Scar tissue buildup, leading to narrowed lumen Increases risk of cancerous growth  27 Gross Anatomy of the Stomach Cardia: small, narrow superior entrance to the stomach Pylorus: narrow, terminal region of the stomach Gastric Folds (Rugae): internal lining of the stomach, only present when stomach is empty Fundus: dome shaped upper region Body: largest part of the stomach Muscularis layer – next slide Figure 26.9a  28 Stomach Wall, Sectional View Gastric Pits : Indentations in stomach mucosa, contains multiple types of gastric glands Figure 26.10a Muscular layer differs from general GI make-up, in that it has 3 layers instead of 3 These three layers work in the churning/mixing of the food in the stomach Muscular layers thicken from body of stomach to pylorus Outer serous layer produces serous fluid to lubricate the external surface of stomach to decrease friction with other organs  29 Gastric Pit and Gland Helps protect stomach epithelial cells from acid in stomach (helps to prevent ulceration and mechanical/abrasive injuries to stomach lining Helps protect stomach epithelial cells from acid in stomach (helps to prevent ulceration and mechanical/abrasive injuries to stomach lining Intrinsic Factor: required for B12 absorption Hydrochloric Acid: acidic nature of the stomach Pepsinogen: inactive precursor to pepsin. Required for protein digestion Gastric Lipase: Limited role. Digests about 10-15% of fat Gastrin secretes stomach motility and secretions Figure 26.10c  30 Stomach: Gastric Mixing and Emptying Gastric mixing Gastric emptying Form of mechanical digestion Movement of acidic chyme from stomach into duodenum Changes semi-digested bolus into chyme Pressure gradient moving contents toward pylorus Churned and mixed, leading to reduction in size of Gradient increasing force against pyloric sphincter swallowed particles Sphincter opens, with entrance of small volume of chyme Sphincter closes, with retropulsion Reverse flow of some contents back toward stomach Figure 26.13  31 Regulation Digestive Processes: Stomach Stomach is essentially a holding bag for partially digested food until the food is moved into the small intestine where digestion is completed Pacemaker cells in stomach Specialized cells that spontaneously depolarize less than 4 times per minute Establish basic rhythm of muscular contraction Signals spreading through smooth muscle cells in muscularis layer These cells are ALWAYS depolarizing The FORCE of contraction and gastric gland secretions are Regulated by nervous reflexes and hormones Organized in three phases: cephalic, gastric, and intestinal  32 Regulation Digestive Processes: Stomach  33 Clinical View: Peptic Ulcers Peptic ulcer Solitary erosion of portion of stomach or duodenum Called Gastric ulcers in stomach, duodenal ulcers in duodenum Symptoms of burning pain in epigastric region Nausea, vomiting, bleeding May erode and cause perforation, medical emergency  34 Clinical View: Vomiting Vomiting Rapid expulsion of gastric contents through oral cavity Controlled by vomiting center in the medulla oblongata Responds to head injury, motion sickness, infection, toxicity, food irritation Closure of nasal passages and the glottis Skeletal muscle contraction increasing intragastric pressure Gastric contents forced into and through the esophagus  35 What did you learn? 1. What structures are considered part of the upper GI tract? How is the ingested material referred to as it moves through each of the structures of the upper GI tract? 2. What are the roles of the tongue, teeth, and salivary glands in forming a bolus? 3. How is the bolus moved from the oral cavity into the stomach, as described in the three phases of swallowing? 4. List the secretory cell types in the stomach, their products, and the function of the products. 5. Which neural reflex is initiated by food in the stomach, and what does it control?