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BIOL 301
Vivian Dayeh

CHAPTER 23 - THE DIGESTIVE SYSTEM INTRODUCTION − Digestive system is responsible for taking in food, break it down, absorb nutrients into blood and eliminate indigestible wastes − 2 main organ groups of the digestive system o Alimentary canal  GI tract  Muscular tube that extends from mouth to anus  (Food in alimentary canal considered to be outside the body because the canal is open to the external environment at both ends) • Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus o Accessory digestive organs  lie external to the GI tract and connected by ducts  Secrete saliva, bile and digestive enzymes  Teeth, tongue, salivary glands, liver, gallbladder and pancreas The digestive process − Ingestion  food enters digestive process via the mouth − Propulsion  the movement of food through peristalsis − Mechanical digestion  breakdown of food into smaller pieces o Chewing, food churning in stomach, segmentation in small intestine − Chemical digestion  breakdown of food molecules (e.g., carbs) into building blocks (e.g., simple sugars) o Occurs in mouth, stomach, small intestine − Absorption nutrients transported from alimentary canal into blood and lymphatic capillaries − Defecation  elimination of indigestible substances as feces Abdominal regions − Scheme 1: 4 lines divide abdominal wall into 9 regions (tic-tac-toe grid) o Midclavicular lines  vertical lines of grid o Subcostal plane  superior horizontal line  Connects inferior points of costal margin o Transtubercular plane  inferior horizontal line  Connects tubercles of iliac crests o Superior 3 regions  right hypochondriac, epigastric, left hypochondriac o Middlde 3 regions  right lumbar, umbilical, left lumbar o Inferior 3 regions  right iliac (inguinal), hypogastric (pubic), left iliac (inguinal) − Scheme 2: A simple method of sectioning the anterior abdominal wall into 4 regions o Right and left upper quadrants o Right and left lower quadrants Peritoneal cavity and peritoneum − Peritoneum  most extensive serous membrane o Visceral peritoneum  surrounds digestive organs/covers the external surfaces o Parietal peritoneum  lines the body wall − Peritoneal cavity  a slit-like potential space between the visceral and parietal peritoneum. o Contains lubricating serous fluid to allow the organs to glide easily along one another. Mesenteries − Double layer of peritoneum that extends to the digestive organs from the body wall. − Functions: o Holds organs in place o Are sites of fat storage o Provides a route for circulatory vessel and nerves − Most mesenteries are dorsal, extending from the alimentary canal to the posterior abdominal wall. o Greater omentum  connects the greater curvature of the stomach to the posterior abdominal wall.  Contains a great deal of fat − Ventral mesentery in the superior abdomen extends from the stomach and liver to the anterior abdominal wall (mesenteries can be called ligaments) o Falciform ligament  binds anterior aspect of liver to anterior abdominal wall and diaphragm o Lesser omentum  runs from liver to the lesser curvature of the stomach ANATOMY OF THE ALIMENTARY CANAL Histology − Same 4 layers from esophagus to anus o Mucosa/mucous membrane  innermost layer  Contains 3 sub-layers: epithelium, lamina propia, muscularis mucosae o Submucosa  external to the mucosa; layer of CT  Contains blood and lymphatic vessels, nerve fibers o Muscularis externa  external to the submucosa  2 layers of smooth muscle: • Circular muscularis  inner layer whose fibers orient the circumference of the canal (squeezes the gut) • Longitudinal muscularis  outer layer whose fibers orient along the length of the canal (shortens the gut) o Serosa  outermost layer  Is the visceral peritoneum  Parts of the alimentary canal that are not associated with the peritoneal cavity lack a serosa and have an adventitia (ordinary fibrous CT) as their outer layer e.g., esophagus. Smooth muscle − Primarily found in the walls of visceral organs e.g., urinary bladder, uterus, intestines. − 6 major locations: the iris of the eye, and in the walls of the circulatory vessels, respiratory tubes, digestive tubes, urinary organs, and reproductive organs. − Fibers elongated and one central nucleus − Grouped into sheets (right angles to each other) o Longitudinal layer  parallel to long axis of organ; externally located o Circular layer  deeper layer, fibers run around circumference of organ; constricts a hollow organ. Mouth and associated organs Mouth/oral cavity − Mucosal lined cavity − Anterior opening is the oral orifice. − Divided into 2: o Vestibule  slit between the teeth and the cheeks (or lips) o Oral cavity proper  region of mouth that lies internal to the teeth. Lips and cheeks − Composed of a core of skeletal muscles: o Lips  orbicularis oris o Cheeks  buccinators muscles − Labial frenulum  median fold that connects the internal aspect of each lip to the gum Palate − Forms roof of the mouth − Has 2 distinct parts: o Hard palate  anteriorly; forms a rigid surface against which the tongue forces food during chewing. o Soft palate  posteriorly; rises to close off the nasopharynx when swallowing Tongue − Interlacing fascicles of skeletal muscle − Houses most of the taste buds − Mix the food with saliva and form it into a compact mass called a bolus. − Contains o Intrinsic muscles  within the tongue and not attached to bone; change shape of tongue e.g., rolling, but they do not change its position. o Extrinsic muscles  external to the tongue; extend to the tongue from bones of the skull and the hyoid bone; alter the position of the tongue e.g., protrude it, retract it, and move it laterally. o Lingual frenulum  fold of mucosa on the undersurface of the tongue.  Secures tongue to floor of mouth and limits its posterior movements. Teeth − Found in sockets called alveoli in the gums of the maxilla and mandible − Humans have 2 sets of teeth/dentitions o By 21 years old, primary dentition (deciduous teeth) have been replaced by the permanent dentition. − Teeth are classified according to their shape and function: o Incisors  nipping off pieces of food o Canines  tear and pierce o Premolars (bicuspids) and molars  have broad crowns with rounded cusps for grinding. − Tooth structure o 2 main regions:  Crown  exposed region covered by a layer of enamel (hardest substance in the body) • Dentine underlies enamel cap and forms bulk of the tooth. • Pulp cavity in the center of the tooth is filled with dental pulp, a loose CT containing the tooth’s vessels and nerves.  Root  in the socket • The part of the pulp cavity in the root is the root canal. • External surface of the tooth root is covered by a calcified CT called cementum. o Attaches the tooth to the periodontal ligament or periodontum.  Anchors tooth in bony socket of the jaw. o Continuous with the gum/gingiva.  These two regions meet at the neck near the gum line. Salivary glands − Produce saliva − All salivary glands are compound tubuloalveolar glands. − Small intrinsic salivary glands  within tongue, palate, lips, cheeks o Saliva from these glands keeps the mouth moist at all times. − Large extrinsic salivary glands  lie external to the mouth but connect to it through their ducts o Secrete saliva only during eating or anticipation of a meal causing the mouth to water. o Parotid glands  largest extrinsic gland; lies anterior to the ear  Parotid duct runs parallel to zygomatic arch  Surgery on this gland can lead to paralysis because the branches of the facial nerve run through the parotid gland on their way to the muscles of facial expression.  Contain only serous cells. o Submandibular glands  Lies along medial surface of mandible  Contain both serous and mucous cells. o Sublingual glands  Lies in floor of oral cavity, inferio
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