Lecture 9

8 Pages

Biological Sciences
Course Code
Maydianne Andrade

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1 Lecture 9 The Digestive System (based on Chapter 25) Introduction The digestive system consists of: The digestive tract (one continuous tube from mouth to anus) Accessory organs of digestion Digestive tract Mouth Pharynx Esophagus Stomach Small intestine Large intestine Accessory Organs of the Digestive Tract Teeth Tongue Salivary glands Pancreas Liver Gallbladder Functions of the Digestive System Ingestion Bringing food and liquids into the mouth Mechanical processing Chewing and swallowing food Digestion Chemical breakdown of food into nutrient form Secretion Secretion of products by the lining of the digestive tract Secretion of products by the accessory organs of digestion Absorption The movement of nutrients from the small intestine to the bloodstream Excretion The removal of waste products from the digestive tract Compaction Progressive dehydration of organic wastes The Components of the Digestive System Mouth Begins the process of mechanical digestion Esophagus Passage tube for food to enter the stomach Stomach Enzymatic breakdown of food Small intestine Enzymatic breakdown of food Absorption of nutrients into the bloodstream Salivary glands Produce an enzyme to begin digesting food Pancreas Produces numerous enzymes that enter into the small intestine to digest food © 2012 Pearson Education, Inc. 2 Liver Produces bile for the emulsification of fat in the small intestine Gallbladder Stores bile Large intestine Removes solid waste Reabsorbs water into the bloodstream to prevent dehydration Houses bacteria that produce vitamin K for blood clotting processes Histological Organization of the Digestive Tract There are four major layers of the digestive tract The Mucosa The inner lining of the digestive tract The mucosa of the small intestine makes up folds called plicae Plicae increase the surface area for increased absorption The Submucosa Surrounds the muscularis mucosae Large blood vessels and lymphatics are in this layer Submucosal plexus innervates this layer Consists of sensory neurons Consists of parasympathetic ganglia Consists of sympathetic postganglionic fibers The Muscularis Externa Surrounds the submucosa Dominated by smooth muscle fibers Innervated by myenteric plexus This is a network of parasympathetic ganglia and sympathetic postganglionic fibers The Serosa Covers the muscularis externa Outermost layer of the digestive system Movement of digestive materials through the digestive tract The muscularis externa propels material through the digestive tract This is called peristalsis Material is churned and fragmented and at the same time is propelled through the digestive tract This is called segmentation The Peritoneum The serosa (visceral peritoneum) is continuous with the parietal peritoneum The abdominal organs lie in association with the peritoneal membrane Intraperitoneal Organs Organs that lie within the peritoneal cavity Organs are surrounded completely by the visceral peritoneum Examples: Stomach Liver Retroperitoneal Organs Organs are covered by the visceral peritoneum on their anterior surface These organs lie deep to the visceral peritoneum Examples: Kidneys Ureters Abdominal aorta Mesenteries These are fused double sheets of peritoneal membrane © 2012 Pearson Education, Inc. 3 Stabilize the position of organs Stabilize the position of blood vessels Provide the attachment of blood vessels going to and from the small intestine All but the duodenum is suspended in a sheet of mesentery called the mesentery proper Mesocolon is the mesentery attached to the large intestine Transverse mesocolon is the mesentery attached to the transverse colon Sigmoid mesocolon is the mesentery attached to the sigmoid colon The ascending colon, descending colon, and rectum are attached to the posterior abdominal wall via a fused mesentery called the fusion fascia The mesentery between the stomach and the liver is the lesser omentum The mesentery that extends from the stomach and covers the rest of the abdominal organs on the anterior surface is the greater omentum Structures within the Oral Cavity Tongue Uvula Palatoglossal arches Salivary glands Teeth Salivary Glands There are three pairs of salivary glands Parotid Sublingual Submandibular All three glands produce salivary amylase, which partially digests carbohydrates The Pharynx The Swallowing Process (three phases) Buccal phase The tongue pushes the food to the oropharynx area Pharyngeal phase The epiglottis closes over the glottis and begins Esophageal phase Upper esophageal sphincter opens and the bolus begins moving down the esophagus The Esophagus The bolus moves down the esophagus toward the stomach via peristaltic action The esophagus passes through the diaphragm by passing through the esophageal hiatus The esophagus has an upper esophageal sphincter and a lower esophageal sphincter Histology of the Esophageal Wall The esophageal wall is made of: Mucosa lining Submucosa Smooth muscle layer (muscularis mucosae) Muscularis externa The esophagus does not have a serosa layer The Stomach Mesenteries of the Stomach The mesenteries associated with the stomach are called the greater and lesser omentum Greater omentum © 2012 Pearson Education, Inc. 4 Extends from the inferior border of the stomach and drapes across the surface of the small intestine Lesser omentum The stomach consists of: Lesser curvature Greater curvature Fundus Body Pylorus The stomach also consists of: Gastric rugae Circular muscles Longitudinal muscles Oblique muscles Histology of the Stomach Lamina propria – layer of loose connective tissue directly beneath the epithelium of mucous membranes o Structures within the lining of the stomach Mucous surface cells: produce copious amounts of mucus to protect the lining of the stomach Gastric pits: produce cells to continuously replace lost stomach cells Mucous neck cells: produce mucus to lubricate the food entering the stomach Parietal cells: secrete intrinsic factor and hydrochloric acid Intrinsic factor: facilitates the absorption of vitam12 B from the small intestine into the bloodstream, which is used during erythropoiesis Hydrochloric acid: kills microorganisms and activates pepsinogen Chief cells: secrete pepsinogen, which is converted to pepsin via the action of hydrochloric acid Enteroendocrine cells: these are cells of the stomach that produce hormones. The G cells produce the hormone gastrin. Gastrin causes the parietal and chief cells to release their products Regulation of the Stomach Food enters the stomach and the stomach stretches
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