Gastrointestinal System

• The digestive system prepares food for use by the cells through five basic activities: ingestion, mixing and movement of food, digestion, absorption, and defecation

  
• Digestive Processes


  1. Ingestion - taking food into the mouth (eating)

  2. Mixing and movement of food. Muscular contractions mix food and secretions

  3. Digestion. The breakdown of food by both chemical and mechanical processes

  4. Absorption. The passage of digested food from the gastrointestinal tract into the cardiovascular and lymphatic systems for distribution to cells

  5. Defecation. The elimination of indigestible substances from the gastrointestinal tract

  
• Organs of digestion


  • Gastrointestinal (GI) tract or alimentary canal
  • Accessory structures: teeth, tongue, salivary glands, liver, gallbladder, and pancreas

  
• Gastrointestinal (GI) tract


  • A continuous tube that begins at the mouth and ends at the anus (mouth, pharynx, esophagus, stomach, small intestine, and large intestine)
  • Contains the food from the time it is eaten until it is digested and prepared for elimination
• Accessory structures


  • Teeth protrude into the GI tract and aid in the physical breakdown of food
  • Lie totally outside the tract and produce or store secretions necessary for chemical digestion
  • Secretions are released into the tract through ducts
• The wall of GI tract consists of four layers of tissue: mucosa, submucosa, muscularis, and serosa

  
• Mouth


  • Also referred to as the oral or bucal cavity, formed by the cheeks, hard and soft palates, and tongue
  • Lips are fleshy folds around the opening of the mouth, covered on the outside by skin and on the inside by a mucous membrane
  • During chewing, the lips and cheeks help keep food between the upper and lower teeth and assist in speech
  • Hard palate consisting of the maxillae and palatine bones form most of the roof of the mouth, the rest is formed by the muscular soft palate
  • Soft palate has a projection called the uvula, and at the back opens into the oropharynx through an opening called fauces, just behind this are the palatine tonsils

  
• Tongue


  • Forms the floor of the oral cavity, composed of skeletal muscle covered with mucous membrane
  • Maneuver food for chewing, force the food to the back of the mouth for swallowing, participates in speech and taste sensation

  
• Salivary Glands


  Three pairs of glands that lie outside the mouth and pour their contents into ducts emptying into the oral cavity: parotid, submandibular, and sublingual

  
• Saliva


  99.5 percent water and 0.5 percent solutes, including the digestive enzyme salivary amylase, mucus that lubricates food, and the enzyme lysozyme that destroys bacteria
• Secretion of Saliva


  1. Parasympathetic stimulation promotes continuous secretion to keep tongue and lips moist

  2. Sympathetic stimulation dominates during stress, resulting in dryness of the mouth

  3. During dehydration, the salivary glands stop secreting to conserve water
• Teeth


  • Accessory structures of digestive system located in bony sockets of the mandible and maxillae
  • Composed primarily of dentin, a bonelike substance that gives the tooth its basic shape and rigidity
  • Enclose the pulp cavity, a space in the crown filled with pulp, a connective tissue containing blood vessels, nerves, and lymphatic vessels
  • Humans have two sets of teeth, permanent teeth appear between age 6 and adulthood, there are 32 teeth in a complete permanent set

  
• Digestion in the Mouth


  1. Mechanical: Chewing, tongue moves food, teeth grind it, saliva mixes with it, resulting in a soft, flexible mass called a bolus that is easily swallowed

  2. Chemical: Salivary amylase starts the breakdown of starch
• Pharynx


  • A tube that extends from the internal nares to the esophagus in back and the larynx in front, composed of skeletal muscle and lined by mucous membrane
  • Food that is swallowed passes from the mouth into the oropharynx and laryngopharynx before passing into the esophagus, muscular contractions help propel food into the esophagus

  
• Esophagus


  • A muscular tube that lies behind the trachea, begins at the end of the laryngopharynx, passes through the mediastinum, and ends at the top of the stomach
  • Transports food to the stomach and secretes mucus, which aids transport
• Swallowing


  1. A mechanism that moves food from the mouth to the stomach, helped by saliva and mucus, involves the mouth, oropharynx, and esophagus

  2. Divided into three stages: voluntary stage, pharyngeal stage, and esophageal stage
• Stomach


  • A J-shaped region of GI tract directly under the diaphragm, superior portion connected to the esophagus, inferior portion empties into the duodenum
  • Divided into four main areas: cardia, fundus, body, and pylorus
  • Wall composed of four basic layers: mucosa, submucosa, muscularis, and serosa
  • When empty, the mucosa lies in large folds called rugae, surface is a layer of nonciliated epithelial cells called mucous surface cells
  • Epithelial cells form narrow channels called gastric pits lined with secretory cells called gastric glands

  
• Gastric Glands


  Contain three types of exocrine gland cells: mucous neck cells (produce mucus), chief cells (produce pepsinogen), and parietal cells (produce hydrochloric acid and intrinsic factor)
• Control of the Stomach Secretion


  Cephalic phase (control by the CNS), gastric phase (control mechanism switched by receptors in the stomach), and intestinal phase (control mechanism switched by receptors located in the intestines)
• Digestion in the Stomach


  1. Mechanical: Gentle, peristaltic mixing waves macerate food, mix it with secretions, and change it to a thin liquid called chyme

  2. Chemical: Pepsin breaks down proteins into smaller fragments called peptides in the very acidic environment of the stomach (pH = 2)
• Pepsin


  • Secreted in an inactive form (pepsinogen) so it cannot digest the proteins in the chief cells that produce it, activated by hydrochloric acid
  • Stomach cells are protected by mucus secreted by mucous cells, forming a barrier between the mucosa and the gastric juices
• Gastric Emptying


  The release of chyme from the stomach into the duodenum, occurs within 2 to 6 hours after ingestion, fastest for carbohydrate-rich foods, slowest for triglyceride-rich foods
• Vomiting


  • The forcible expulsion of the contents of the upper GI tract through the mouth, stimulated by irritation and stretching of the stomach, unpleasant sights, dizziness, and certain drugs
  • Involves squeezing the stomach between the diaphragm and abdominal muscles and expelling the contents into the esophagus and through the mouth
• Absorption in the stomach includes water, electrolytes, drugs (aspirin), and alcohol
• Pancreas


  • Lies behind the stomach, secretions pass from the pancreas to the duodenum by the pancreatic duct, which unites with the common bile duct from the liver and gallbladder
  • Made up of pancreatic islets (endocrine portion producing hormones) and acini (exocrine glands secreting pancreatic juice)
• Pancreatic Juice


  • A clear, colorless liquid consisting mostly of water, some salts, sodium bicarbonate, and enzymes (pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, pancreatic lipase, ribonuclease, and deoxyribonuclease)
  • Protein-digesting enzymes are produced in inactive form to prevent them from digesting the pancreas itself, activated in the small intestine
• Regulation of Pancreatic Secretion


  Regulated by both nervous (parasympathetic fibers from the vagus nerves) and hormonal (secretin and cholecystokinin) mechanisms
• Liver


  The heaviest gland of the body, weighing about 1.4 kg in the average adult, located under the diaphragm, mostly on the right side of the body, covered by a connective tissue capsule and peritoneum
• Pancreatic juice


  Clear, colorless liquid that consists mostly of water, some salts, sodium bicarbonate, and enzymes (pancreatic amylase; trypsin, chymotrypsin, and carboxypeptidase; pancreatic lipase; and ribonuclease and deoxyribonuclease)
• Protein-digesting enzymes


  Produced in inactive form to prevent them from digesting the pancreas itself, activated in the small intestine
• Regulation of Pancreatic Secretion


  1. Regulated by both nervous and hormonal mechanisms

  2. Secretion of pancreatic enzymes is stimulated by parasympathetic fibers from the vagus (X) nerves and by the hormones secretin and cholecystokinin
• Liver


  • Heaviest gland of the body, weighing about 1.4 kg in the average adult
  • Located under the diaphragm, mostly on the right side of the body
  • Covered by a connective tissue capsule, which, in turn, is covered by peritoneum
  • Divided by the falciform ligament into two principal lobes: the right lobe and the left lobe
  • Lobes are made up of functional units called lobules
  • Lobulus consists of rows of hepatocytes arranged radially around a central vein
  • Hepatocytes produce bile
• Liver Functions


  • Maintains a normal blood glucose level
  • Stores some triglycerides and breaks down fatty acids
  • Synthesizes most plasma proteins
  • Detoxifies substances, such as alcohol, drugs
  • Excretes the bile
  • Bile salts synthesis
  • Stores vitamins (A, D, E, K, B12) and minerals
  • Phagocytosis
  • Activation of vitamin D
• Gallbladder


  • Pear-shaped sac located in a depression under the liver
  • Concentrates and stores bile until it is needed in the small intestine
  • Bile enters the small intestine through the common bile duct
  • When triglycerides enter the small intestine, cholecystokinin is released to stimulate contraction of the gallbladder's muscular layer, emptying bile into the common bile duct to flow into the small intestine
• Bile


  • Yellow, brownish, or olive-green liquid with a pH of 7.6 to 8.6
  • Consists mostly of water and bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions
  • Produced in the liver but stored in the gallbladder
  • Bile salts aid in emulsification, the conversion of triglyceride (fat) globules into a suspension of triglyceride droplets, and absorption of triglycerides following their digestion
• Regulation of Bile Secretion

  1. Parasympathetic stimulation increases the production of bile

  2. Secretin stimulates the secretion of bile

  3. Cholecystokinin (CCK) causes ejection of bile from the gallbladder
• Small Intestine


  • Tube that averages about 2.5 cm in diameter and about 6.5 m in length in cadaver and about 3 m in length in a living person (due to the difference in muscle tone)
  • Divided into three segments: duodenum, jejunum, and ileum
  • Epithelium of the mucosa consists of simple columnar epithelium that contains absorptive cells, goblet cells, hormone-producing cells, and Paneth cells
  • Membrane of absorptive cells features microvilli that greatly increase the surface area and allow larger amounts of digested nutrients to be digested and absorbed

  
• Intestinal Juice


  • Clear yellow fluid that has a pH of 7.6 (slightly alkaline) and contains water and mucus
  • Rapidly reabsorbed by the villi and provides a vehicle for the absorption of substances from chyme as they come in contact with the villi
  • Intestinal enzymes are synthesized in the epithelial cells that line the villi and most digestion of enzymes of the small intestine occurs in or on the epithelial cells that line the villi
• Mechanical digestion in the Small Intestine


  1. Movements of the small intestine are two types: segmentation and peristalsis

  2. Sympathetic impulses decrease intestinal motility
• Chemical digestion in the Small Intestine


  1. Chyme entering the small intestine contains partially digested carbohydrates and proteins

  2. Completion of digestion is a collective effort of pancreatic juice, bile, and intestinal juice