The%20Digestive%20System

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The Digestive System
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Introduction

• The digestive system consists of the muscular
  digestive tract and various accessory organs.
• Digestive functions include ingestion, mechanical
  processing, digestion, secretion, absorption,
  compaction, and excretion.

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Overview of the digestive Tract

• The digestive tract includes the oral cavity,
  pharynx, esophagus, stomach, small intestine,
  large intestine, rectum and anus.

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Histological Organization

• The epithelium and underlying connective
  tissue,the lamina propria, form the mucosa of the
  digestive tract.
• Next, outward, are the submucosa, the muscularis
  externa, and the adventitia, a layer of loose
  connective tissue.
• In the peritoneal cavity, the muscularis externa
  is covered by the serosa, a serous membrane

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• Double sheets of peritoneal membrane called
  mesenteries suspend the digestive tract.

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Movement of Digestive Materials

• The neurons that innervate the smooth muscle of
  the muscularis external are not under voluntary
  control.
• The muscularis externa propels materials through
  the digestive tract by means of the contractions
  of peristalsis.
• Segmentation movements in areas of the small
  intestine churn digestive materials.

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The Oral Cavity

• The functions of the oral cavity are
• Analysis of potential foods
• Mechanical processing using the teeth, tongue and
  palatal surfaces
• Lubrication by mixing with mucus and salivary
  secretions
• Digestion by salivary enzymes

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• The oral cavity, or buccal cavity, is lined by
  oral mucosa
• The hard palate and soft palate form its roof and
  the tongue forms its floor.

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The Tongue

• The primary functions of the tongue include
• Mechanical processing
• Manipulation to assist in chewing and swallowing
• Sensory analysis.

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The Salivary Glands

• The salivary glands discharge their secretions
  into the oral cavity
• Saliva lubricates the mouth, dissolves chemicals,
  flushes the oral surfaces and helps to control
  bacteria.
• Salivation is usually controlled by the ANS.

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Teeth

• Mastication occurs through the contact of the
  opposing surfaces of the teeth
• The periodontal ligament anchors the tooth ins a
  bony socket.
• Dentin forms the basic structure of a tooth
• The crown is coated with enamel, and the root is
  covered with cementum

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The 20 primary teeth or deciduous teeth are
replaced the the 32 teeth of the secondary
dentition during development.
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The Stomach

• The stomach has four major functions
• Bulk storage of ingested food
• The mechanical breakdown of food.
• The disruption of chemical bonds by acids enzymes
• The production of intrinsic factor

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• Chyme forms in the stomach as gastric and
  salivary secretions are mixed with food.
• The four regions of the stomach are the cardia,
  fundus, body and pylorus
• The pyloric sphincter guards the exit from the
  stomach.
• In a relaxed state the stomach lining contains
  numerous rugae (ridges and folds)

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The Gastric Wall

• Within the gastric glands, parietal cells secrete
  intrinsic factor and hydrochloric acid.
• Chief cells secrete pepsinogen which acids I the
  gastric lumen convert to the enzyme pepsin
• Gastric gland endocrine cells secrete the hormone
  gastrin.

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The Regulation of Gastric Activity

• Gastric secretion includes
• The cephalic phase which prepares the stomach to
  receive ingested materials
• The gastric phase which begins with the arival of
  food in the stomach
• The intestinal phase which controls the rate of
  gastric emptying

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The Small Intestine

• The small intestine includes the duodenum, the
  jejunum, and the ileum.
• The ileocecal valve, a sphincter, marks the
  transition between the small and large intestines.

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The Intestinal Wall

• The intestinal mucosa bears transverse fold
  called plicae, or plicae circulares,
• and small projections called intestinal villi.
• They increase the surface area for absorption.
• Each villus contains a terminal lymphatic called
  a lacteal.

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Intestinal Secretions

• Intestinal glands secrete intestinal juice,
  mucus,and hormones.
• Intestinal juice moistens the chyme, helps buffer
  acids,and dissolves digestive enzymes and the
  products of digestion.
• Intestinal hormones include secretin,
  cholecystokinin(CCK) and gastric inhibitory
  peptide.

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Digestion and the Small intestine

• The most important digestive and absorptive
  functions occur in the small intestine.
• Digestive enzymes and buffers are provided by the
  pancreas, liver and gallbladder.

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The Pancreas

• The pancreatic duct penetrates the call of the
  duodenum, where it delivers the secretions of the
  pancreas.

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• The pancreas has two functions
• Endocrine (secreting insulin and glucagon into
  the blood)
• Exocrine (secreting water, ions, and digestive
  enzymes into the small intestine)
• Pancreatic enzymes include lipases,
  carbohydreases and proteases.

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The Control of Pancreataic Secretion

• When chyme arrives in the small intestine
  secretin triggers the pancreatic production of a
  fluid containing buffers, primarily sodium
  bicarbonate, what help bring the pH of the chyme
  under control.
• Also CCK is released which stimulates the
  pancreas to secrete pancreatic amylase,
  pancreatic lipase, nucleases and protease
  enzymes.

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The Liver

• Largest visceral organ in the body, with over 200
  known functions.

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Liver Functions

• The liver performs metabolic and hematological
  regulation and produces bile.
• The gallbladder stores and concentrates bile for
  release into the duodenum.

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The large Intestine

• The main functions of the large intestine are
• Reabsorb water and compact the feces
• Absorb vitamins liberated by bacteria
• Store fecal material prior to defecation.

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Three Parts of Large Intestine

• Cecum collects and stores material from the
  ileum. Vermiform appendix is attached to the
  cecum.
• Colon larger diameter and thinner wall than sm.
  Intestine/
• Rectum terminates in the anal canal leading to
  the anus.

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Physiology of the Large Intestine

• The large intestine reabsorbs water and other
  substances such as vitamins, bilirubin, bile
  salts, and toxins
• Bacteria residing in the large intestine are
  responsible for intestinal gas or flatus.

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Digestion and Absorption

• The digestive system breaks down the physical
  structure of the ingested material and then
  disassembles the component molecules into smaller
  fragments through hydrolysis.

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• Amylase breaks down complex carbohydrates. These
  are broken down into monosaccharides by other
  enzymes and are absorbed by the intestinal
  epithelium through facilitated diffusion and
  cotransport.

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• Triglycerides are emulsified into large lipid
  drops. The results interact with bile salts to
  form micelles which diffuse across the intestinal
  epitheliums.
• Protein digestion involves the gastric enzyme
  pepsin and various pancreatic proteases.

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Water and Electrolyte Absorption

• About 2-2.5 liters of water are ingested each day
  and digestive secretion provide 6-7 liters
• Nearly all is reabsorbed by osmosis.

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• Fat-soluble vitamins are enclosed within fat
  droplets are are absorbed with the products of
  lipid digestion.
• The nine water-soluble vitamins are important as
  cofactors and coenzymes in enzymatic reactions..

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Aging and the Digestive System

• Age related changes include a thinner and more
  fragile epithelium to a reduction in epithelial
  stem cell division and weaker peristaltic
  contraction as smooth muscle tone decreases.