Digestive System Part 1 PDF

Digestive System Marie A. Román Martínez, PhD Department of Biology Office hours: by appointment Email: [email protected] Copyright-This presentation is intended for educational purpose only. No part of this presentation may be reproduced or transmitted in any form without written permission. Obje...

Digestive System Marie A. Román Martínez, PhD Department of Biology Office hours: by appointment Email: [email protected] Copyright-This presentation is intended for educational purpose only. No part of this presentation may be reproduced or transmitted in any form without written permission. Objectives 1. Identify the structures composing the alimentary canal (digestive tract). 10.Describe the location and functions of the liver. 11.Explain how bile release is stimulated. 2. Identify the accessory organs of the digestive 12.Describe digestion in the small intestine. system. 13.Describe the structure and functions of the 3. Compare mechanical and chemical digestion. large intestine. 4. Describe the role of digestive enzymes. 14.Describe the major disorders of the digestive 5. Describe digestion in the mouth. system. 6. Describe the basic structure of a tooth. 7. Describe the location and function of the pharynx, esophagus, stomach. 8. Explain the control of gastric secretions. 9. Describe the control and functions of pancreatic secretions. 2 Introduction Nutrients are required for healthy body function. Carbohydrates, proteins, lipids, vitamins, minerals, water. Obtained from diet. Most nutrients molecules are too big to be directly passed into blood. Goals of digestive system Digest food into smaller molecules. Absorb smaller molecules into the blood. 3 Introduction Major functions of the digestive system: Digestion: All of the mechanical and chemical processes that convert large, nonabsorbable nutrient molecules into small absorbable nutrient molecules. Absorption: Process by which nutrients pass from the digestive tract into the blood. 4 Introduction Digestive system consists of the: Digestive tract: Long tube food passes through. Major parts of the digestive tract are the esophagus, stomach, small intestine, large intestine and pharynx. Accessory organs Organs with digestive function that are not part of the digestive tract. Teeth, tongue, salivary glands, liver, gallbladder, and pancreas. 5 Digestion: An Overview Digestion involves both mechanical and chemical processes. Mechanical digestion: Physical breakdown of food into smaller pieces. Forms a greater surface area for contact with digestive secretions. 6 Digestion: An Overview Chemical digestion: Splitting of complex nonabsorbable nutrient molecules into small absorbable nutrient molecules through hydrolysis (addition of water). Each type of digestive enzyme acts upon a specific type of nutrient molecule (substrate) and speeds up its breakdown→ into smaller nutrient molecules. Many enzymes are required to successfully digest complex nutrient molecules →into absorbable nutrient molecules. Nonabsorbable nutrient molecule Digestive enzyme 7 Digestive Tract Muscular tube about 6 m (20 ft) in length. Extends from esophagus to the anus. Portions are specialized to perform different digestive functions. The hollow space within the canal through which food passes is called the lumen. The wall of the digestive tract consists of four layers: Serosa Muscular layer Submucosa Mucosa Small section of the small intestine 8 Movements Contractions of the smooth muscle layers produce two types of movements: 1. Segmentation: Involves ringlike contractions followed by relaxation at multiple places along the digestive tract. Mixes luminal contents with digestive secretions. 2. Peristalsis: Propels food along the digestive tract. Involves coordinated contraction and relaxation of circular and longitudinal muscular layers. Produces a wave of contraction that pushes luminal contents. 9 Mouth Functions of the mouth: Intake of food (ingestion). Mechanical digestion Chemical digestion Swallowing Surrounded by cheeks, palate and tongue. 10 Cheeks Form lateral walls of the mouth. Outer surfaces covered with skin. Inner surface is lined by nonkeratinized stratified squamous epithelium. Help hold food in the mouth. Contraction of muscles within them help produce facial expressions. 11 Cheeks The anterior portions of the cheeks form the lips→ surround mouth opening. Lips are sensitive and highly mobile. Pinkish color results from numerous blood vessels near their surfaces. Speech production. Detecting touch and temperature. 12 Palate Forms the roof of the mouth and separates the oral cavity from the nasal cavity. Hard palate: Anterior portion supported by bone. Protects nasal cavity while chewing. Allows for simultaneous chewing and breathing. Soft palate: Muscular posterior portion. Uvula extends downward at back of oral cavity. Sensitive to touch stimuli. Causes soft palate to move superiorly during swallowing to close off nasal cavity. Role in triggering the gag reflex. 13 Tongue Floor of oral cavity. Skeletal muscle covered by mucous membrane. Frenulum of the tongue: Attaches bottom of tongue to floor of mouth. Limits backward movement of the tongue. Lingual papillae: Projections on top surface of tongue. Provide a rough texture, which aids in food manipulation. Location for most taste buds. Lingual glands: Produce lingual lipase. 14 Teeth Types of permanent teeth: Incisor teeth Biting off food. Canine teeth Grasp and tear food. Premolar teeth Molar teeth Both teeth crush and grind food 15 Teeth Each tooth consists of two major parts: a root and a crown 1. Crown Portion not covered by gingiva (or gum) 2. Root Embedded in dental alveolus Attachment of root to jaw: Cement Periodontal ligaments Composition of a tooth: Dentin a hard calcified tissue similar to bone. Enamel is the hardest substance in the body and it is appropriately located to resist abrasion caused by chewing hard foods. Pulp cavity: Central portion Blood vessels and nerves enter through root canal and pass into pulp cavity of the crown. 16 Filled with dental pulp Salivary Glands Secrete saliva into the mouth. Secretion regulated by a positivefeedback mechanism. Mechanism is activated by the presence of food in the mouth or thought or smell of food. Saliva components: Water: 99.5% Aids in the sense of taste. Dissolves food Cleanses mouth pH: 6.2 -7.6 Mucus: Binds food particles. Provides lubrication. Salivary amylase: Starts carbohydrate digestion. Lysozyme: Kills certain types of bacteria. 17 Salivary Glands There are three major salivary glands: Parotid gland Submandibular gland Sublingual gland 18 Salivary Glands Parotid gland Largest salivary gland. In front of each ear, on top of the masseter. Secrete saliva rich in salivary amylase. Deliver saliva into oral vestibule near the upper second molar teeth. 19 Salivary Glands Submandibular gland Located in floor of mouth. Secrete watery saliva with little mucus. Deliver saliva into the front of the mouth at the base of the frenulum of the tongue. 20 Salivary Glands Sublingual gland Located in floor of mouth inferior to the tongue. Smallest salivary gland. Secrete mostly mucus. Deliver saliva into the floor of the mouth below the tongue. 21 Digestion and Absorption in Mouth Mechanical digestion: breaking food into smaller pieces and mixing it with saliva during mastication. Mastication increases surface area of food particles and mixes food with saliva. Chemical digestion: starts in the mouth with the breakdown of certain polysaccharides. Digestion of starch and glycogen into maltose by salivary amylase. Absorption of some vitamins, monosaccharides, alcohol, certain types of drugs does occur in the mouth. Enzyme inactivated by strong acidity of gastric juice in stomach. Lingual lipase→ acts on triglycerides and speeds up their breakdown into monoglycerides and fatty acids. Secreted by the tongue and requires the strong acidity of gastric juice to be activated. 22 Pharynx Passageway connecting nasal and oral cavities with esophagus and larynx. Transports food from mouth to esophagus during swallowing. Swallowing reflex Food is pushed into pharynx by tongue. Soft palate moves upward: Blocks off nasal cavity Directs food downward into pharynx. Larynx is elevated: Causes the epiglottis to fold over the larynx opening. Directs food into esophagus. 23 Esophagus Muscular tube extending downward from the pharynx to join the stomach. Uses peristalsis to propel food toward the stomach. Esophageal mucosa produced mucus for lubrication and ease in food passage. Lower esophageal sphincter (LES) Guards junction of stomach and esophagus. Constricted to prevent regurgitation of stomach contents. Relaxes to allow food into the stomach. Physiological sphincter. Failure to close properly results in gastroesophageal reflux (GER) or “heartburn”. 24 Stomach Stomach is a J-shaped pouchlike portion of digestive tract. Located below the diaphragm in the left upper quadrant of the abdominopelvic cavity. Basic functions: Temporary food storage Mixing food with gastric juice Starting protein digestion 25 Stomach Structure Subdivisions: Cardia Fundus Body Pyloric part Pyloric sphincter Thickened ring of circular smooth muscle cells. At junction of stomach and duodenum. Constricts to close stomach outlet. Relaxes to allow food to pass into duodenum. 26 Stomach Specializations: Its muscular layer allows the stomach to better mix food with gastric secretions. Mucosa is thicker than in other areas of digestive tract. Mucosa and submucosa are organized into gastric rugae. Allow stomach lining to stretch as it fill with food. Mucosa contains numerous gastric pits that receive secretions from gastric glands. 27 Gastric Juice Secretion of the gastric glands: Mucous neck cells near gastric pit opening. Secrete mucus to protect mucosa from actions of digestive secretions. Parietal cells In mid-portion of glands. Secrete hydrochloric acid (HCl) and intrinsic factor. Chief cells In deepest portion of glands. Secrete digestive enzymes Pepsinogen (inactive form of pepsin), gastric lipase, rennin. Gastric juice converts food into chyme (a semiliquid substance). Released intermittently into the duodenum through the pyloric sphincter. Stomach https://anatomy.mheducation.com/html/apr.html?animal=huma 28 n&id=3966 Control of Gastric Secretion Secretion is under neural and hormonal control. Regulated by a positive-feedback mechanism. Gastric juice is produced continuously. Secretion increases whenever food is on the way to or already in the stomach. Parasympathetic action potentials increase with food stimuli. Triggered by sight, smell, or thought of food; food in mouth, food in stomach. Action potentials directly stimulate gastric glands to increase secretion of gastric juice. Actions potentials, food in stomach, and stomach stretching cause stomach cells to produce gastrin (a hormone). Gastrin is absorbed into the blood and stimulates gastric gland secretion. 29 Control of Gastric Secretion Parasympathetic action potentials decrease, and sympathetic action potentials increase as stomach empties. Decreases gastric juice secretion. Intestinal mucosa secretes two hormones in response to chyme. 1. Cholecystokinin (CCK) 2. Secretin Both help decrease gastric juice secretion and stomach motility. 30 31 Digestion and Absorption in Stomach Hydrochloric acid (pH= 2) Activates enzyme pepsin Pepsin →digestive enzyme secreted in an inactive form that prevents digestion of the cells secreting it. Breaks proteins into smaller peptides (shorter amino acids). Intrinsic factor Essential for absorption of vitamin B12 by the small intestine. Lingual lipase Produced by the tongue but typically activates in the stomach. Breaks triglycerides into fatty acids and monoglycerides. Endoscopy https://ww w.youtube. com/watch ?v=DUVDK oKSEkU 32 Digestion and Absorption in Stomach Rennin, and infant enzyme. Curdles milk proteins Keeps it in stomach longer. Makes proteins more easily digested by pepsin. Gastric lipase, an infant enzyme. Breaks triglycerides into fatty acids and monoglycerides. Stomach absorbs water, minerals, some drugs, alcohol. 33 Pancreas Pancreas is located behind the pyloric part of stomach. Has both endocrine and exocrine functions. Digestive function (exocrine function) is to secrete pancreatic juice. Transport of pancreatic juice: Pancreatic juice is collected by the pancreatic duct. Pancreatic duct joins with the bile duct where they empty into the duodenum. Accessory pancreatic duct allows pancreatic juice to enter duodenum independent of bile. 34 Control of Pancreatic Secretion Pancreatic secretion is controlled by both neural and hormonal mechanisms. Hormonal control: Neural control: Parasympathetic action potentials stimulate pancreas to secrete pancreatic juice. 1. Secretin: Released by intestinal mucosa in response to acidic chyme. Causes pancreas to produce juice rich in bicarbonate ions. Neutralize acidity of chyme. 2. Cholecystokinin (CCK): Secreted by intestinal mucosa in response to lipid-rich chyme. Stimulates secretion of pancreatic juice rich in digestive enzymes. 35 Digestion by Pancreatic Enzymes Pancreatic amylase: Breaks starch and glycogen (both polysaccharides) into maltose (a disaccharide). Pancreatic lipase: Breaks triglycerides into monoglycerides and fatty acids. These molecules are absorbable. Trypsin: Breaks proteins into peptides. Secreted in an inactive form. Prevents the pancreatic cells from being digested by their own enzymatic secretions. Activated when mixed with intestinal secretions. 36