Chapter 14: The Digestive System
Functions • Ingestion—taking in food • Digestion—breaking food down both physically and chemically • Absorption—movement of nutrients into the bloodstream • Defecation—rids the body of indigestible waste
Organs Two main groups: • Alimentary canal (gastrointestinal or GI tract) • continuous coiled hollow tube • Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus • Accessory digestive organs • Teeth, salivary glands, pancreas, liver, gallbladder
Mouth Anatomy: • Lips (labia) • Cheeks • Hard palate • Soft palate • Uvula • Helps with speech and preventing food/liquid from entering nasal cavity when swallowing
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 & Lingual
Function: • Mastication (= chewing) of food • Digestion by saliva • Swallowing by the tongue • Sense of taste • Gustatory receptors
Pharynx • Only oropharynx and laryngopharynx are part of digestive system • Passageway for air and food • 2 muscle layers alternate contractions to push food down to esophagus (=peristalsis) • Longitudinal inner layer • Circular outer layer
Figure 14.2a
Esophagus • ~10 inches long • Runs from pharynx to stomach through the diaphragm • Passageway for food only • moves food through slow rhythmic squeezing (= peristalsis)
Layers of Alimentary Canal Past the Esophagus 4 layers from inside to outside: • Mucosa • Mostly moist epithelial tissue • Submucosa • Connective tissue with blood vessels, nerve endings, and lymph vessels • Muscularisexterna • Muscle tissue made of inner circular layer and outer longitudinal layer • Serosa • Fluid producing cells make up visceral peritoneum layer that runs into parietal peritoneum (= lining of abdominopelvic cavity)
Figure 14.3
Nervous System’s Role • Alimentary canal controlled by autonomic nervous system • 2 networks: • Myenteric nerve plexus: between circular and longitudinal muscles • Submucosalnerve plexus: in submucosa layer • Function is to regulate mobility and secretory activity
Stomach • Located on the left side of the abdominal cavity • Food enters at the cardioesophagealsphincter (sphincter = valve) • Food empties into the small intestine at the pyloric sphincter
Regions: • Cardiac region -near the heart • Fundus – rounded part next to the cardiac region • Body - middle • Pylorus - funnel-shaped end
Rugae= internal folds of the mucosa • External regions • Lesser curvature—concave medial surface • Greater curvature—convex lateral surface
Layers of peritoneum attached to the stomach • Lesser omentum- attaches the liver to the lesser curvature • Greater omentum- attaches the greater curvature to the posterior body wall • Sticky wall that collects fat to insulate, cushion, and protect abdominal organs
Physiology • Temporary storage tank for food • Site of food breakdown • The enzyme pepsinogen and hydrochloric acid break down proteins • Gastrin is hormone that stimulates HClsecrection • Alkaline mucus lines the inside of the stomach to protect it from HCl • Delivers chyme (processed food) to the small intestine
Structure of the Stomach Mucosa Figure 14.4c
Small Intestine • Muscular tube extending from the pyloric sphincter to the ileocecal valve • Suspended from the posterior abdominal wall by the mesentery • The body’s major digestive organ • Function: nutrient absorption into the blood
Divisions: • Duodenum • Attached to the stomach • Curves around the head of the pancreas • Jejunum • Middle • Ileum • From jejunum to large intestine
Chemical digestion begins in the small intestine • Enzymes are produced by • Intestinal cells • Pancreas • Pancreatic ducts carry enzymes to the small intestine • Bile, made by the liver, enters through bile duct
3 structural modifications that increase surface area (fom largest to smallest): • Circular folds (plicaecirculares) • Villi – fingerlike projections on circular folds • Microvilli – smaller fingerlike projections on villi that absorb; make the brush border
Figure 14.7a
Large Intestine • Larger in diameter, but shorter in length, than the small intestine • Frames the internal abdomen • Function: absorbs water & processes waste material
Parts: • Cecum- saclike first part of the large intestine • Appendix • Vestigial structure • Accumulation of lymphatic tissue that hangs from the cecum • Colon • Ascending—travels up right side of abdomen • Transverse—travels horizontally • Descending—travels down the left side • Sigmoid—enters the pelvis • Rectum and anal canal • External anal sphincter (skeletal muscle) and internal involuntary sphincter (smooth muscle)
Large Intestine Figure 14.8
No villi present • Goblet cells produce alkaline mucus which lubricates the passage of feces • Muscularisexterna layer is reduced to three bands of muscle called teniae coli • Causes the wall to pucker into pocket like sacs called haustra
Teeth • Function is to masticate (chew) food • Humans have two sets of teeth: • 20 deciduous (baby) teeth by age 2 • 32 permanent teethby age 12
Classification of teeth: • Incisors—cutting • Canines—tearing or piercing • Premolars—grinding • Molars—grinding
Salivary Glands • Three pairs of salivary glands empty secretions into the mouth • Parotid glands • Submandibular glands • Sublingual glands
Saliva • 98% water, 2% electrolytes, mucus, and enzymes • Dissolves chemicals so they can be tasted • Helps to form a food bolus • Your body produces 1-2 liters per day
Pancreas • Located deep to stomach • Produces enzymes that break down carbohydrates • Enzymes secreted into the duodenum • Alkaline fluid mixed with enzymes neutralizes acidic chyme coming from stomach • Produces hormones that regulate blood sugar: • Insulin (takes sugar from blood to tissues) • Glucagon (takes sugar from tissues to blood)
Figure 14.6
Pancreas Figure 14.1
Liver • Largest gland in the body • Located on the right side of the body under the diaphragm • 4 lobes • Connected to the gallbladder via the common hepatic duct • Functions: • makes bile to break down fat • makes cholesterol to transport fats • makes proteins for blood plasma • detoxifies blood from drugs and alcohol • Stores glycogen, vitamins, and minerals
Liver Figure 14.1
Bile • Yellowish-green fluid produced in the liver • Made of: • Pigments, mostly bilirubin • Made when the liver breaks down old RBCs • Causes yellow coloring of jaundice when there is liver disease • Salts • Cholesterol • Phospholipids • Electrolytes • Function: breaks large fat globules into smaller ones
Gallbladder • Sac found in hollow space of liver • When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder • When digestion of fatty food is occurring, bile is released into the duodenum from the gallbladder • Gallstones are crystallized cholesterol which can cause blockages
Figure 14.1
