Med Surg

• Functions of the Digestive System:

  • 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) and Accessory Digestive Organs
  • Alimentary Canal: continuous, coiled, hollow tube that runs from the stomach to the anus
  • Accessory Digestive Organs: include teeth, tongue, and several large digestive organs that assist in digestion
• Organs of the Alimentary Canal:

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anus
• Mouth:

  • Anatomy includes lips, cheeks, hard palate, soft palate, uvula, vestibule, oral cavity proper, and tongue
  • Functions include mastication (chewing) of food, mixing food with saliva, initiating swallowing, and allowing for taste
• Pharynx:

  • Serves as a passageway for foods, fluids, and air
  • Food passes from the mouth into the oropharynx and laryngopharynx
  • Food is propelled to the esophagus by two skeletal muscle layers in the pharynx
• Esophagus:

  • About 10 inches long
  • Conducts food to the stomach by peristalsis
  • Passageway for food only (respiratory system branches off after the pharynx)
• Layers of Tissue in the Alimentary Canal Organs:

  • Mucosa: innermost layer consisting of surface epithelium, connective tissue, and smooth muscle
  • Submucosa: soft connective tissue with blood vessels, nerve endings, and lymphatic vessels
  • Muscularis externa: smooth muscle with inner circular and outer longitudinal layers
  • Serosa: outermost layer containing fluid-producing cells
• Stomach:

  • C-shaped organ located on the left side of the abdominal cavity
  • Regions include cardial, fundus, body, and pylorus
  • Can stretch and hold 4 liters of food when full
  • Structure of the stomach mucosa includes simple columnar epithelium and gastric pits leading to gastric glands
• Small Intestine:

  • Body's major digestive organ
  • Longest portion of the alimentary tube
  • Site of nutrient absorption into the blood
  • Subdivisions include duodenum, jejunum, and ileum
  • Chemical digestion begins in the small intestine with enzymes and bile entering the duodenum
• Villi:

  • Fingerlike projections formed by the mucosa
  • House a capillary bed and lacteal
• Microvilli:

  • Tiny projections of the plasma membrane (brush border and its associated enzymes)
• 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
• Larger in diameter, but shorter in length at 1.5 meters, than the small intestine

  • Extends from the ileocecal valve to the anus
  • Eliminates feces
• Subdivisions:

  • Cecum
  • Appendix
  • Colon
  • Rectum
  • Anal canal
• 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 the abdominal cavity and turns at the left colic (splenic) flexure
  • Descending travels down the left side
  • Sigmoid S-shaped region; enters the 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)
• Teeth:

  • Masticate (chew) food into smaller fragments
  • Humans have two sets of teeth during a lifetime
  • Deciduous (baby or milk) teeth
  • Permanent teeth
• Teeth are classified according to shape and function:

  • Incisors cutting
  • Canines (eyeteeth) tearing or piercing
  • Premolars (bicuspids) crushing and grinding
  • Molars crushing and grinding
• Two major regions of a tooth:

  • Crown
  • Root
• 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 canal where the pulp cavity extends into the root
• Root:

  • Cement covers outer surface and attaches the tooth to the periodontal membrane (ligament)
  • Periodontal membrane holds tooth in place in the bony jaw
  • The neck is a connector between the crown and root
  • Region in contact with the gum
• Salivary Glands:

  • Three pairs of salivary glands empty secretions into the mouth
  • Parotid glands
  • Submandibular glands
  • Sublingual glands
  • Saliva mixture of mucus and serous fluids
  • Helps to moisten and bind food together into a mass called a bolus
  • Contains salivary amylase begins starch digestion
  • Lysozymes and antibodies inhibit bacteria
  • Dissolves chemicals so they can be tasted
• Pancreas:

  • Soft, pink triangular gland
  • Found posterior to the parietal peritoneum
  • 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 the stomach
  • Hormones produced by the pancreas: Insulin, Glucagon
• 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 containing 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
• Food ingestion and breakdown:

  • Food is physically broken down by chewing
  • Mixed with saliva, released in response to mechanical pressure and psychic stimuli
  • Salivary amylase begins starch digestion
• Food propulsion — swallowing and peristalsis:

  • Pharynx and esophagus serve as passageways to the stomach
  • Pharynx functions in swallowing (deglutition) with two phases: buccal phase and pharyngeal-esophageal phase
  • Buccal phase is voluntary and occurs in the mouth, forming food into a bolus forced into the pharynx by the tongue
  • Pharyngeal-esophageal phase is involuntary transport of the bolus by peristalsis, moving it towards the stomach
  • Peristalsis moves the bolus toward the stomach, opening the cardioesophageal sphincter when food presses against it
• • Propulsion in the stomach involves waves of peristalsis from the fundus to the pylorus, grinding chyme into the small intestine, and retropulsion forcing contents back into the stomach
• Food breakdown:

  • Gastric juice is regulated by neural and hormonal factors
  • Presence of food or rising pH causes release of the hormone gastrin, stimulating stomach glands to produce protein-digesting enzymes, mucus, and hydrochloric acid
  • Hydrochloric acid makes stomach contents acidic, activating pepsinogen to pepsin for protein digestion and providing a hostile environment for microorganisms
  • Pepsin and rennin are protein-digesting enzymes
  • Alcohol and aspirin are absorbed in the stomach
• Chyme breakdown and absorption:

  • Intestinal enzymes from the brush border break double sugars into simple sugars and complete some protein digestion
  • Pancreatic and brush border enzymes digest starch, proteins, fats, and nucleic acids
  • Release of pancreatic juice into the duodenum is stimulated by vagus nerves and local hormones like secretin and cholecystokinin (CCK)
  • Hormones target the liver and gallbladder to release bile, acting as a fat emulsifier for fat absorption and fat-soluble vitamins
• • Water is absorbed along the small intestine, and substances are transported to the liver by the hepatic portal vein or lymph
  • Peristalsis and segmental movements aid in moving and mixing chyme with digestive juices
• Nutrient breakdown and absorption:

  • Resident bacteria digest remaining nutrients, producing some vitamin K and B vitamins, releasing gases
  • Water, vitamins, ions, and remaining water are absorbed
  • Feces contain undigested food residues, mucus, bacteria, and water
  • Propulsion of food residue and defecation involve sluggish peristalsis, haustral contractions, and mass movements occurring three to four times per day
  • Presence of feces in the rectum causes a defecation reflex, leading to relaxation of the internal anal sphincter and voluntary (external) anal sphincter for defecation