Digestive System

• Nutrients


  Cannot be used in existing form, must be broken down into smaller components before body can make use of them
• Digestive system


  Acts as a disassembly line to break down nutrients into forms that can be used by the body, and to absorb them so they can be distributed to the tissues
• Gastroenterology


  The study of the digestive tract and the diagnosis and treatment of its disorders
• Five stages of digestion

  • Ingestion: selective intake of food
  • Digestion: mechanical and chemical breakdown of food into a form usable by the body
  • Absorption: uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph
  • Compaction: absorbing water and consolidating the indigestible residue into feces
  • Defecation: elimination of feces
• Mechanical digestion


  The physical breakdown of food into smaller particles through the cutting and grinding action of the teeth, and the churning action of the stomach and small intestines, which exposes more food surface to digestive enzymes
• Chemical digestion


  A series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues), carried out by digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine
• Products of chemical digestion

  • Polysaccharides into monosaccharides
  • Proteins into amino acids
  • Fats into monoglycerides and fatty acids
  • Nucleic acids into nucleotides
• Vitamins, amino acids, minerals, cholesterol, and water can be directly absorbed
• Subdivisions of the digestive system

  • Digestive tract (alimentary canal)
  • Accessory organs
• Digestive tract (alimentary canal)

  30 ft long muscular tube extending from mouth to anus, including mouth, pharynx, esophagus, stomach, small intestine, and large intestine
• Gastrointestinal (GI) tract


  The stomach and intestines
• Accessory organs


  • Teeth
  • Tongue
  • Salivary glands
  • Liver
  • Gallbladder
  • Pancreas
• Enteric nervous system


  Nervous network in esophagus, stomach, and intestines that regulates digestive tract motility, secretion, and blood flow, can function independently of central nervous system but CNS usually exerts influence on its action, often considered part of autonomic nervous system
• Mesenteries


  Connective tissue sheets that suspend stomach and intestines from abdominal wall, allowing freedom of movement and preventing twisting and tangling, provide passage of blood vessels and nerves, contain lymph nodes and lymphatic vessels
• Serous membranes line the abdominal cavity and cover the digestive organs (Figure 25.3a, Figure 25.3b)
• Mechanisms controlling motility and secretion of the digestive tract

  • Neural control: short (myenteric) reflexes, long (vasovagal) reflexes
  • Hormones: gastrin, secretin
  • Paracrine secretions: chemical messengers that diffuse through tissue fluids
• Mouth (oral or buccal cavity)

  Functions: ingestion, taste and sensory responses to food, chewing and chemical digestion, swallowing, speech, and respiration
• The mouth is enclosed by cheeks, lips, palate, and tongue
• Cheeks and lips


  Retain food and push it between the teeth, essential for speech, sucking and blowing actions, including suckling by infants
• Tongue


  Muscular, manipulates food between teeth, senses taste and texture of food
• Dentition


  • 20 deciduous (milk) teeth
  • 32 adult teeth (16 in mandible, 16 in maxilla)
• Mastication


  Chewing food into smaller pieces, makes food easier to swallow and exposes more surface area for action of digestive enzymes, speeding chemical digestion
• Functions of saliva


  • Moistens mouth
  • Begins starch and fat digestion (salivary amylase, lingual lipase)
  • Cleanses teeth
  • Inhibits bacterial growth
  • Dissolves molecules to stimulate taste buds
  • Moistens and binds food into a bolus to aid swallowing
• Salivation


  Extrinsic salivary glands secrete about 1 to 1.5 L of saliva per day, in response to signals generated by the presence of food, resulting in a bolus - a mass swallowed as a result of saliva binding food particles into a soft, slippery, easily swallowed mass
• Pharynx


  Muscular funnel connecting oral cavity to esophagus and nasal cavity to larynx, where digestive and respiratory tracts intersect, has deep layer of longitudinal skeletal muscle and superficial layer of circular skeletal muscles that form pharyngeal constrictors to force food downward during swallowing
• Stages of swallowing (deglutition)

  • Oral phase: under voluntary control, tongue collects food, presses it against palate forming bolus, and pushes it posteriorly
  • Pharyngeal phase: involuntary, prevents food and drink from reentering mouth or entering nasal cavity (breathing is suspended)
  • Esophageal phase: involuntary peristalsis - wave of muscular contraction that pushes the bolus ahead of it
• Swallowing involves over 22 muscles in the mouth, pharynx, and esophagus, coordinated by a pair of nuclei in the medulla oblongata
• Stomach


  A muscular sac in the upper left abdominal cavity immediately inferior to the diaphragm, primarily functions as a food storage organ with a volume of about 50 mL when empty, 1.0 to 1.5 L after a typical meal, up to 4 L when extremely full, mechanically breaks up food, liquefies it, and begins chemical digestion of protein and fat, chyme is the soupy or pasty mixture of semidigested food in the stomach, most digestion occurs after the chyme passes on to the small intestine
• Regions of the stomach

  • Cardia (cardial part) - small area within about 3 cm of the cardial orifice
  • Fundus - dome-shaped portion superior to esophageal attachment
  • Body - makes up the greatest part of stomach
  • Pyloric part - narrower pouch at the inferior end, with pyloric sphincter that regulates passage of chyme into the duodenum
• The stomach receives parasympathetic fibers from the vagus and sympathetic fibers from the celiac ganglia
• Gastric pits


  Depressions in the gastric mucosa, lined with simple columnar epithelium, with two or three tubular glands opening into the bottom of each pit
• Cell types in the gastric glands

  • Mucous cells - secrete mucus
  • Regenerative (stem) cells - found in base of pit and neck of gland, divide rapidly to replace cells that die
  • Parietal cells - found mostly in upper half of gland, secrete hydrochloric acid (HCl), intrinsic factor, and ghrelin
  • Chief cells - most numerous, secrete gastric lipase and pepsinogen
  • Enteroendocrine cells - concentrated in lower end of gland, secrete hormones and paracrine messengers that regulate digestion
• The mucosa of the stomach wall is shown in Figure 25.13a
• Gastric juice


  2 to 3 L per day produced by the gastric glands, mainly a mixture of water, hydrochloric acid, and pepsin
• Parietal cells


  Produce hydrochloric acid (HCl) and contain carbonic anhydrase
• Pepsin


  Aids in the digestion of proteins
• Intrinsic factor


  A glycoprotein secreted by parietal cells, essential for the absorption of vitamin B12 by the small intestine
• Gastric motility


  Swallowing center signals stomach to relax, vagus nerve activates a receptive-relaxation response, then stomach shows a rhythm of peristaltic contractions that churn the food, mix it with gastric juice, and promote its physical breakup and chemical digestion
• Only about 3 mL of chyme is squirted into the duodenum at a time, allowing the duodenum to neutralize the stomach acid, digest nutrients little by little
• A typical meal is emptied from the stomach in 4 hours, less time if the meal is more liquid, and as long as 6 hours for a high-fat meal