Chapter 14- The Digestive System

Digestive System Functions

  • Ingestion—taking in food
  • Digestion—breaking food into nutrient molecules
  • Absorption—movement of nutrients into the bloodstream
  • Defecation—excretes to rid the body of indigestible waste

Anatomy of the Digestive System

  • Two main groups of organs   * Alimentary canal (gastrointestinal, or GI, tract)—continuous, coiled, hollow tube     * These organs ingest, digest, absorb, defecate   * Accessory digestive organs     * Include teeth, tongue, and several large digestive organs     * Assist digestion in various ways

Organs of the Alimentary Canal

  • The alimentary canal is a continuous, coiled, hollow tube that runs through the ventral cavity from stomach to anus   * Mouth                                 Small intestine   * Pharynx                              Anus   * Esophagus                         Large intestine   * Stomach

Mouth

  • Anatomy of the mouth   * Mouth (oral cavity)—mucous membrane–lined cavity   * Lips (labia)—protect the anterior opening   * Cheeks—form the lateral walls   * Hard palate—forms the anterior roof   * Soft palate—forms the posterior roof   * Uvula—fleshy projection of the soft palate   * Vestibule—space between lips externally and teeth and gums internally   * Oral cavity proper—area contained by the teeth   * Tongue— attached at hyoid bone and styloid processes of the skull, and by the lingual frenulum to the floor of the mouth   * Tonsils     * Palatine—located at posterior end of oral cavity     * Lingual—located at the base of the tongue
  • Functions of the mouth   * Mastication- (chewing) of food   * Tongue mixes masticated food with saliva   * Tongue initiates swallowing   * Taste buds on the tongue allow for taste

Pharynx

  • Serves as a passageway for foods, fluids, and air
  • Food passes from the mouth posteriorly into the:   * Oropharynx—posterior to oral cavity   * Laryngopharynx—below the oropharynx and continuous with the esophagus
  • Food is propelled to the esophagus by two skeletal muscle layers in the pharynx   * Longitudinal outer layer   * Circular inner layer
  • Alternating contractions of the muscle layers (peristalsis) propel the food

Esophagus

  • Anatomy   * About 10 inches long   * Runs from pharynx to stomach through the diaphragm
  • Physiology   * Conducts food by peristalsis (slow rhythmic squeezing) to the stomach   * Passageway for food only (respiratory system branches off after the pharynx)

Stomach

  • C-shaped organ located on the left side of the abdominal cavity
  • Food enters at the cardioesophageal sphincter from the esophagus
  • Food empties into the small intestine at the pyloric sphincter (valve)
  • Regions   * Cardial (cardia)—near heart & surrounds the cardioesophageal sphincter   * Fundus—expanded portion lateral to the cardiac region   * Body—midportion     * Greater curvature--convex lat. surface     * Lesser curvature is the concave medial surface   * Pylorus—funnel-shaped terminal end
  • Stomach can stretch and hold 4 L (1 gallon) of food when full   * Rugae—internal folds of the mucosa present when the stomach is empty
  • Lesser omentum   * Double layer of the peritoneum   * From liver to the lesser curvature of stomach
  • Greater omentum   * Another extension of the peritoneum   * Covers the abdominal organs   * Fat insulates, cushions, and protects organs

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  • Structure of the stomach mucosa   * Simple columnar epithelium composed almost entirely of mucous cells   * Mucous cells produce bicarbonate-rich alkaline mucus   * Dotted by gastric pits leading to gastric glands that secrete gastric juice, including:     * Intrinsic factor, which is needed for vitamin B12 absorption in the small intestine
  • Structure of the stomach mucosa (cont.)   * Chief cells—produce protein-digesting enzymes (pepsinogens)   * Parietal cells—produce hydrochloric acid that activates enzymes   * Mucous neck cells—produce thin acidic mucus (different from the mucus produced by mucous cells of mucosa)   * Enteroendocrine cells—produce local hormones such as gastrin                                                                           

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  • Functions   * Temporary storage tank for food   * Site of food breakdown (Mostly Physical)   * Chemical breakdown of protein begins   * Delivers chyme (processed food) to the small intestine

Small Intestine The body’s major digestive organ

  • Longest portion of the alimentary tube   (2–4m, or 7–13 feet, in a living person)

  • Site of nutrient absorption into the blood

  • Muscular tube extending from the pyloric sphincter to the ileocecal valve

  • Suspended from the posterior abdominal wall by the mesentery ^^Subdivisions^^   * Duodenum   * Jejunum   * Ileum

  • Chemical digestion begins in the small intestine

  • Enzymes produced by intestinal cells and pancreas are carried to the duodenum by pancreatic ducts

  • Bile, formed by the liver, enters the duodenum via the bile duct

  • Hepatopancreatic ampulla is the location where the main pancreatic duct and bile ducts join

  • Structural modifications Increase surface area for food absorption -

  • Decrease in number (absorptive surface) toward the end of the small -

  • intestine Villi—fingerlike projections formed by the mucosa

  • House a capillary bed and lacteal

  • Microvilli—tiny projections of the plasma membrane

  • Circular folds (plicae circulares)—deep folds of mucosa and submucosa ^^Peyer’s patches^^

  • Collections of lymphatic tissue

  • Located in submucosa   * Increase in number toward the end of the small intestine   * More are needed there because remaining food residue contains much bacteria

Large Intestine

Larger in diameter, but shorter in length at **1.5m**, than the small intestine

Extends from the ileocecal valve to anus

  • Subdivisions (detailed next)

Cecum

Appendix

Colon

Rectum

Anal canal

Large Intestine

Cecum—saclike first part of the large intestine

Appendix

Hangs from the cecum

Accumulation of  lymphoid tissue that sometimes becomes inflamed

(appendicitis)

Colon

Ascending—travels up right side of abdomen and makes a turn at

the right colic (hepatic) flexure

Transverse—travels across abd. cavity and turns at the left colic

(splenic) flexure

Descending—travels down the left side

Sigmoid—S-shaped region; in pelvis

Sigmoid colon, rectum, and anal canal are located in the pelvis

Anal canal ends at the anus

Anus—opening of the large intestine

External anal sphincter—formed by skeletal muscle and is

voluntary

Internal anal sphincter—formed by smooth muscle and is

involuntary

These sphincters are normally closed except during defecation

The large intestine delivers indigestible food residues to the body’s exterior

Goblet cells produce alkaline mucus to lubricate the passage of

feces

Muscularis externa layer is reduced to three bands of muscle, called teniae coli

These bands of muscle cause the wall to pucker into haustra (pocketlike sacs)

Layers of Tissue in the Alimentary Canal Organs Summary of the four layers from innermost to outermost, from esophagus to the large intestine (detailed next)

  1. Mucosa
  2. Submucosa
  3. Muscularis externa
  4. Serosa

^^1. Mucosa^^

Innermost, moist membrane consisting of:

Surface epithelium that is mostly simple columnar epithelium(except esophagus—strat squamous epithelium)

Small amount of connective tissue (lamina propria)

 Scanty smooth muscle layer

Lines the cavity (known as the lumen)

^^2. Submucosa^^

Just beneath the mucosa

 Soft connective tissue with blood vessels, nerve endings, mucosa-

associated lymphoid tissue, and lymphatic vessels

^^3. Muscularis externa—smooth muscle^^

Inner circular layer

Outer longitudinal layer

^^4. Serosa^^—outermost layer of the wall; contains fluid-producing cells

 Visceral peritoneum—innermost layer continuous with the outermost layer

 Parietal peritoneum—outermost layer that lines the abdominopelvic cavity by

way of the mesentery

Alimentary Canal Nerve Plexuses

Alimentary canal wall contains two intrinsic nerve plexuses that are part of the autonomic nervous system

 Submucosal nerve plexus

 Myenteric nerve plexus

 Regulate mobility and secretory activity of the GI tract organs

Accessory Digestive Organs

  • Teeth
  • Salivary glands
  • Pancreas
  • Liver
  • Gallbladder

Teeth

  • Teeth masticate food into small fragments
  • Humans have two sets of teeth

     1. Deciduous (baby or milk) teeth      * A baby has 20 teeth by age 2      * First teeth to appear are the lower central incisors

  • Permanent teeth

  • Replace deciduous teeth between ages 6 and 12

  • A full set is 32 teeth (incl. wisdom)

  • Teeth are classified according to shape and function

  • Incisors—cutting

  • Canines (eyeteeth)—tearing or piercing

  • Premolars (bicuspids)—grinding

  • Molars—grinding

  • Two major regions of a tooth

  • Crown

  • Root

  1. ^^Crown^^—exposed part of tooth above the gingiva (gum)    * Enamel —covers the crown    * Dentin—found deep to the enamel and forms the bulk of the tooth, surrounds the pulp cavity    * Pulp cavity—contains connective tissue, blood vessels, and nerve fibers (pulp)    * Root cavity—where the pulp cavity extends into the root
  2. ^^Root^^    * Cement—covers outer surface and attaches the tooth to periodontal membrane (ligament)    * Periodontal membrane holds tooth in place in the bony jaw

Note: The neck is a connector between the crown and root

  1. Region in contact with the gum

Salivary Glands

  • Three pairs of salivary glands empty secretions into the mouth

1. ^^Parotid glands^^

  • Found anterior to the ears
  • Mumps affect these salivary glands
  • 1. Submandibular glands
  • 2. Sublingual glands   * Both submandibular and sublingual glands  empty saliva into the floor of the mouth through small ducts
  • Saliva
  • Mixture of mucus and serous fluids
  • Helps to moisten and bind food together into a mass called a bolus
  • Contains:

     1. Salivary amylase—begins starch digestion   2. Lysozymes and antibodies—inhibit bacteria

  • Dissolves chemicals to be tasted

Pancreas

  • Soft, pink triangular gland
  • Found posterior to the parietal peritoneum   * Mostly retroperitoneal
  • Extends across the abdomen from spleen to duodenum
  • Produces a wide spectrum of digestive enzymes that break down all categories of food
  • Secretes enzymes into the duodenum
  • Alkaline fluid introduced with enzymes neutralizes acidic chyme coming from stomach
  • Hormones produced by the pancreas   * Insulin   * Glucagon

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Liver

  • Largest gland in the body
  • Located on the right side of the body under the diaphragm
  • Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament
  • Digestive role is to produce bile   * Bile leaves the liver through the common hepatic duct and enters duodenum through the bile duct   * Bile is yellow-green, watery solution of:     * Bile salts and bile pigments (mostly  bilirubin from the breakdown of hemoglobin)     * Cholesterol, phospholipids, and electrolytes   * Bile emulsifies (breaks down) fats

Gallbladder

  • Green sac found in a shallow fossa in the inferior surface of the liver   * When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder   * While in the gallbladder, bile is concentrated by the removal of water   * When fatty food enters the duodenum, the gallbladder spurts out stored bile

Functions of Digestive System

  • Overview of gastrointestinal processes and controls   * Digestion   * Absorption
  • We will cover six more specific processes next

Overview of GI Processes & Control

  • Essential processes of the GI tract
  • 1.Ingestion—place of food into the mouth
  • 2.Propulsion—movement of foods from one region of the digestive system to another

     1. Peristalsis—alternating waves of contraction and relaxation that squeeze food along the GI tract   2. Segmentation—movement of materials back and forth to foster mixing in the small intestine

  1. Food breakdown: mechanical breakdown

Examples:

 Mixing of food in the mouth by the tongue

Churning of food in the stomach

Segmentation in the small intestine

Mechanical digestion prepares food for further degradation by enzymes

  1. Food breakdown: digestion
  • Digestion occurs when enzymes chemically break down large molecules into their building blocks
  • Each major food group uses different enzymes
  • Carbohydrates are broken down to monosaccharides (simple sugars)
  • Proteins broken down to amino acids
  • Fats to fatty acids and glycerol

Absorption

  • End products of digestion are absorbed in the blood or lymph
  • Food must enter mucosal cells and then move into blood or lymph capillaries

Defecation

  • Elimination of indigestible substances from the GI tract in the form of feces

Activities Occurring in the Mouth, Pharynx and Esophagus

  • Food ingestion and breakdown

     1. Food is placed into the mouth      * Physically broken down by chewing      * Mixed with saliva, which is released in response to mechanical pressure and psychic stimuli      * Salivary amylase begins starch digestion

  • Essentially, no food absorption occurs in the mouth

  • Food propulsion—swallowing and peristalsis

  • Pharynx and esophagus have no digestive function

  • Serve as passageways to the stomach

  • Pharynx functions in swallowing (deglutition)

  • Two phases of swallowing

     1. Buccal phase   2. Pharyngeal-esophageal phase

  1. Food propulsion—swallowing and peristalsis (continued)
  • Buccal phase   * Voluntary   * Occurs in the mouth   * Food is formed into a bolus   * The bolus is forced into the pharynx by the tongue
  1. Pharyngeal-esophageal phase

  • Involuntary transport of the bolus by peristalsis

  • Nasal and respiratory passageways are blocked

  • Food propulsion—swallowing and peristalsis (continued)

  • Peristalsis moves the bolus toward the stomach

  • The cardioesophageal sphincter is opened when food presses against it

Activities in the Stomach

  • Food breakdown   * Gastric juice is regulated by neural and hormonal factors   * Presence of food or rising pH causes the release of the hormone gastrin   * Gastrin causes stomach glands to produce:     * Protein-digesting enzymes     * Mucus     * Hydrochloric acid   * Hydrochloric acid makes the stomach contents very acidic   * Acidic pH     * Activates pepsinogen to pepsin for protein digestion     * Provides a hostile environment for microorganisms   * Protein-digestion enzymes     * Pepsin—an active protein-digesting enzyme     * Rennin—works on digesting milk protein in infants; not produced in adults   * Alcohol and aspirin are virtually the only items absorbed in the stomach
  • Food propulsion   * Peristalsis: waves of peristalsis occur from the fundus to the pylorus, forcing food past the pyloric sphincter   * Grinding: the pylorus meters out chyme into the small intestine (3 ml at a time)   * Retropulsion: peristaltic waves close the pyloric sphincter, forcing contents back into the stomach; the stomach empties in 4–6 hours

Activities in the Small Intestine

  • Chyme breakdown and absorption   * Intestinal enzymes from the brush border function to:     * Break double sugars into simple sugars     * Complete some protein digestion   * Intestinal enzymes and pancreatic enzymes help to complete digestion of all food groups   * Pancreatic enzymes play the major role in the digestion of fats, proteins, and carbohydrates   * Alkaline content neutralizes acidic chyme and provides the proper environment for the pancreatic enzymes to operate
  • Chyme breakdown and absorption   * Release of pancreatic juice from the pancreas into the duodenum is stimulated by:     * Vagus nerves     * Local hormones that travel via the blood to influence the release of pancreatic juice (and bile)       * Secretin       * Cholecystokinin (CCK)   * Hormones (secretin and CCK) also target the liver and gallbladder to release bile     * Bile       * Acts as a fat emulsifier       * Needed for fat absorption and absorption of fat-soluble vitamins (K, D, E, and A)   * A summary table of hormones is presented next   * Water is absorbed along the length of the small intestine   * End products of digestion     * Most substances are absorbed by active transport through cell membranes     * Lipids are absorbed by diffusion   * Substances are transported to the liver by the hepatic portal vein or lymph
  • Chyme propulsion   * Peristalsis is the major means of moving food   * Segmental movements     * Mix chyme with digestive juices     * Aid in propelling food

Activities in the Large Intestine

  • Nutrient breakdown and absorption   * No digestive enzymes are produced   * Resident bacteria digest remaining nutrients     * Produce some vitamin K and some B vitamins     * Release gases   * Water, vitamins, ions, and remaining water are absorbed   * Remaining materials make feces   * Feces contains:     * Undigested food residues     * Mucus     * Bacteria     * Water
  • Propulsion of food residue and defecation   * Sluggish peristalsis begins when food residue arrives   * Haustral contractions are the movements occurring most frequently in the large intestine   * Mass movements are slow, powerful movements that occur three to four times per day   * Presence of feces in the rectum causes a defecation reflex     * Internal anal sphincter is relaxed     * Defecation occurs with relaxation of the voluntary (external) anal sphincter

Part III; Developmental Aspects of Digestive System and Metabolism

  • The alimentary canal is a continuous, hollow tube present by the fifth week of development
  • Digestive glands bud from the mucosa of the alimentary tube
  • The developing fetus receives all nutrients through the placenta
  • In newborns, feeding must be frequent, peristalsis is inefficient, & vomiting is common
  • Newborn reflexes   * Rooting reflex helps the infant find the nipple   * Sucking reflex helps the infant hold on to the nipple and swallow
  • Teething begins around age 6 months
  • Problems of the digestive system   * Gastroenteritis—inflammation of the GI tract; can occur at any time   * Appendicitis—inflammation of the appendix; common in adolescents
  • Metabolism decreases with old age
  • Middle-age digestive problems   * Ulcers   * Gallbladder problems
  • Later middle-age problems   * Obesity   * Diabetes mellitus
  • Activity of the digestive tract in old age   * Fewer digestive juices   * Peristalsis slows   * Diverticulosis and gastrointestinal cancers are more common

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