Human Digestive System Quiz

Digestive System: Introduction, Mouth, Esophagus and Deglutition

• The digestive system is composed of two groups of organs: the gastrointestinal tract and accessory digestive organs.
• The gastrointestinal tract extends from the mouth to the anus, passing through the thoracic and abdominopelvic cavities.
• The gastrointestinal tract includes the mouth, pharynx, esophagus, stomach, small intestine and large intestine.
• The length of the gastrointestinal tract is approximately 5-7 meters in a living person.
• Accessory digestive organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Digestive System Functions

• Ingestion: Taking food and liquids into the mouth (eating).
• Secretion: Cells in the GI tract and accessory organs secrete water, acid, buffers, and enzymes into the lumen of the tract (approximately 7 liters per day).
• Mixing and Propulsion: Motility, the capability of the tract to mix and move material along its length. Smooth muscle contractions and relaxations mix food with secretions and propel the mixture toward the anus.
• Digestion:
  • Mechanical: Teeth break down food; stomach and small intestine churn food.
  • Chemical: Large carbohydrate, lipid, protein, and nucleic acid molecules are broken down into smaller molecules by hydrolysis. Digestive enzymes catalyze these catabolic reactions. Some substances can be absorbed without chemical digestion (vitamins, ions, cholesterol, and water).
• Absorption: Products of digestion, ingested and secreted fluids and ions enter the epithelial cells lining the lumen of the GI tract. Absorbed substances enter blood or lymph and circulate to cells throughout the body.
• Defecation: Elimination of wastes (indigestible substances, bacteria, cells sloughed from the lining of the GI tract, and unabsorbed digestive materials). The eliminated material is called feces.

Layers of the GI Tract

• The wall of the GI tract (from the lower esophagus to the anal canal) has the same four-layered arrangement. From deep to superficial: mucosa, submucosa, muscularis, and serosa.

• Mucosa: epithelium, lamina propria, and muscularis mucosae.

• Epithelium in the mouth, pharynx, esophagus, and anal canal has a protective function. In the stomach and intestines, the epithelium has a protective, secretory, and absorptive function. Epithelial cells are renewed every 5-7 days. Exocrine cells (mucus cells, fluid cells), and endocrine cells (enteroendocrine cells – hormones) are embedded throughout the epithelium.

• Lamina propria contains many blood and lymphatic vessels for nutrient absorption and many immune system cells (MALT) to combat disease. MALT is present throughout the GI tract, but especially prominent in tonsils, small and large intestines.

• Muscularis mucosae is a smooth muscle layer that enables movement.

• Submucosa: contains blood and lymphatic vessels (to receive absorbed food molecules), and an extensive network of neurons (submucosal plexus).

• Muscularis: skeletal muscle in the mouth, pharynx, superior and middle esophagus (for voluntary swallowing), external anal sphincter (for voluntary control of defecation), smooth muscle in the rest of the GI tract (two layers: inner circular and outer longitudinal fibers). Involuntary contractions propel food, mix it with secretions, and help break it down.

• Serosa is the outer layer; in the esophagus, it's called adventitia. In the portions of the GI tract that are suspended in the abdominopelvic cavity, it is called visceral peritoneum. Peritonal cavity contains lubricating serous fluid between visceral and parietal peritoneum for movement.

• Salivary glands: secrete saliva into the oral cavity. Lubricates, dissolves, and chemically breaks down food. Labial, buccal, and palatal glands in the mouth, and lingual glands in the tongue.

• Teeth: accessory digestive organs in the alveolar processes of the mandible and maxilla, covered by gingivae (gums). Sockets are lined by periodontal ligaments; major parts: crown, root, and neck

  • Dentin, a calcified connective tissue, is the basic framework of the tooth.
  • Enamel, the hardest substance in the body, covers the dentin.
  • Pulp cavity: contains pulp (blood vessels, nerves, lymphatic vessels).
  • Root canals: open at the apical foramen.

• Deciduous (primary) teeth: 20 teeth that erupt from about 6 months of age, with approximately two teeth appearing per month until there are 20. They start being replaced by permanent teeth between ages 6 and 12.

• Permanent teeth: 32 teeth that continue erupting from age 6 until adulthood.

  • Central & lateral incisors: cutting
  • Cuspids (canines): tearing
  • Premolars: grinding
  • Molars: grinding

• Salivary Composition: Primarily water, various ions (sodium, potassium, chloride, bicarbonate, phosphate), urea, uric acid, mucin, immunoglobulin A (IgA), lysozyme, and salivary amylase.

• Digestion in the mouth: Mechanical (chewing) and chemical (salivary amylase to begin start digesting starch and lingual lipase begins digesting triglycerides).

• Deglutition (Swallowing): Three stages: voluntary, pharyngeal, and esophageal.

  • Movement of food from the mouth into the stomach.
  • Facilitated by saliva and mucus secretion, and involves mouth, pharynx, and esophagus.

• Pharynx: three parts; nasopharynx (respiration only), oropharynx, and laryngopharynx (respiration and digestion).

• Esophagus: a collapsible muscular tube (approximately 25 cm long) posterior to the trachea. Begins at the inferior end of the laryngopharynx and passes through the esophageal hiatus in the diaphragm to end in the superior portion of the stomach, with a skeletal muscle, a mixed skeletal and smooth muscle, and a smooth muscle layer in the upper, intermediate, and lower portion of the muscularis. Two sphincters: UES (upper esophageal) and LES (lower esophageal).

• Regulation of the GI Tract: Autonomic regulation; enteric nervous system, local regulation, hormonal regulation, paracrine regulation.

• Autonomic Nervous System: sympathetic and parasympathetic.

  • Parasympathetic (vagus nerve): stimulates motility and secretions.
  • Sympathetic: inhibits salivation and peristalsis.