The Digestive System

• Food contains substances and energy required for cell component construction
• Digestion breaks down food into absorbable molecular sizes for use by cells.
• Organs performing digestion collectively form the digestive system.
• Gastroenterology studies upper digestive tract structures, functions, and disorders.
• Proctology studies the lower digestive tract.

Overview of the Digestive System

• The gastrointestinal (GI) tract is a tube open at both ends for food transit during processing.
• Functional segments of the GI tract include the mouth, esophagus, stomach, small intestine, and large intestine.
• Accessory structures aiding food processing include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
• Digestion involves six basic processes:
  • Ingestion of food through the mouth
  • Secretion of water, acid, buffers, and enzymes into the GI lumen by cells
  • Mixing and propulsion due to alternating contractions of GI tract smooth muscles
  • Mechanical digestion through GI tract movements, aiding chemical digestion
  • Chemical digestion via catabolic (hydrolysis) reactions breaking down large molecules
  • Absorption of digestion end-products into blood or lymph for cell distribution
  • Defecation: rectum emptying, eliminating indigestible substances

Layers of the GI Tract

• The gastrointestinal tract layers, from inside outward, include the mucosa, submucosa, muscularis, and serosa (visceral peritoneum).
• Mucosa components include epithelium, lamina propria, and muscularis mucosa.
• Epithelium: protective non-keratinized stratified squamous cells in the mouth, pharynx, and esophagus alongside simple columnar cells for secretion in the stomach and intestines.
• Lamina propria contains loose connective tissue, blood, lymph vessels, nerves, and sensors.
  • The lymph system is part of mucosa-associated lymph tissues (MALT) for immune response
  • It's estimated that the GI tract houses as many immune cells as the rest of the body combined.
• The muscularis mucosa increases surface area for digestion and absorption via local folding.
• The submucosa is composed of areolar connective tissue, and is highly vascular with submucosal plexus, glands, and lymphatic tissue.
• Muscularis composition varies:
  • Skeletal muscle produces voluntary swallowing and forms the external anal sphincter in the mouth, pharynx, and upper esophagus.
  • Smooth muscle consists of inner circular and outer longitudinal layers throughout the remainder of the tract.
• The serosa is the superficial layer of GI tract portions suspended in the abdominopelvic cavity.
  • Inferior to the diaphragm, it's known as the visceral peritoneum.

Neural Innervation of the GI Tract

• The enteric nervous system includes the submucosal plexus (Meissner's plexus), regulating mucosa movements, blood vessel constriction, and innervating mucosal gland secretory cells
• The myenteric plexus (Auerbach's plexus) contains fibers from ANS divisions and mainly controls GI tract motility.
• Parasympathetic nerve stimulation increases GI secretion and motility via ENS neuron activity
• Sympathetic nerves decrease GI secretion and motility by inhibiting ENS neurons.
• Plexuses form the gastrointestinal reflexes, regulating GI secretion and motility in response to stimuli in the GI tract.

Peritoneum

• The peritoneum stands as the body's largest serous membrane.
• The Parietal peritoneum lines the abdominal cavity walls.
• The Visceral peritoneum. covers some organs, constituting their serosa.
• The peritoneal cavity is a potential space between parietal and visceral portions containing serous fluid.
• Retroperitoneal organs lie behind the peritoneum on the posterior abdominal wall.
• The peritoneum contains large folds to support organs and contain blood vessels, lymphatic vessels, and nerves.
• Extensions of the peritoneum: mesentery, mesocolon, falciform ligament, lesser omentum, and greater omentum.
• Peritonitis denotes acute inflammation of the peritoneum.

Mouth

• The mouth, otherwise known as the oral or buccal cavity, is composed by the cheeks, hard and soft palate, lips, and tongue
• The vestibule of the oral cavity is an area bounded externally by the cheeks and lips and internally by the gums and teeth.
• The oral cavity is a space that expands from the gums to the fauces - the opening between the oral cavity and pharynx
• Salivary glands secrete saliva; most saliva originates from salivary glands external to the mouth emptying into the oral cavity via ducts, with the rest lining the mucous membrane
• Three salivary gland pairs: parotid, submandibular (submaxillary), and sublingual.
• Saliva lubricates, dissolves food, starts carbohydrate chemical digestion, and keeps mouth/throat mucous membranes moist.
• Chemically, saliva consists of 99.5% water and 0.5% solutes (salts, gases, organic substances, and enzymes).
• Nervous control entirely regulates salivation.
• Mumps are an inflammation and enlargement of the parotid salivary glands caused by the mumps virus (myxovirus).
• Fever, malaise, pain, and swelling of the gland(s) are symptoms
• Male adults may experience inflammation of the testes and sterility.
• The tongue with muscle forms the oral cavity floor composed of skeletal msucle with mucous membrane
• To perform food manipulation for chewing/swallowing and speech, extrinsic/intrinsic muscles allow movement
• The lingual frenulum is a mucous membrane fold attaching to the tongue's undersurface midline.
• The upper surface and sides of the tongue contains the papillae, some of which have taste buds
• Glands on the tongue's dorsum release lingual lipase, initiating triglyceride digestion.

Teeth

• Teeth project into the mouth, enabling mechanical digestion.
• A typical tooth has three components: crown, root, and neck.
• Dentin constitutes most of the tooth, covered by enamel in the crown for wear protection.
• Another bonelike substance, cementum covers dentin in the root which then attaches the root to the periodontal ligament,
• Dentine encloses the pulp cavity in the crown, plus root canals in the root.
• The branch of dentistry concerned with the prevention, diagnosis, and treatment of diseases that affect the pulp, root, periodontal ligament, and alveolar bone is known as endodontics
• Orthodontics is a dental branch concerned with the prevention and correction of abnormally aligned teeth.
• Periodontics involves treatment of tissues around teeth.
• Throughout life, individuals possess two sets of teeth, deciduous and permanent
• Four shape-based teeth types: incisors, cuspids (canines), premolars (bicuspids), and molars; premolars are absent in deciduous teeth.
• Root canal therapy removes pulp tissue from a diseased tooth's cavity/canal.

Mechanical and Chemical Digestion in the Mouth

• Through mastication (chewing), food mixes with saliva and forms into a bolus.
• Only chemical digestion in the mouth: salivary amylase that converts starches/polysaccharides to disaccharides (maltose).

Pharynx

• The pharynx is a funnel-shaped tube extending from internal nares to the esophagus and larynx.
• This is composted of skeletal msucle with internal mucous membrane
• The nasopharynx functions in respiration while the oropharynx and laryngopharynx have digestive and respiratory roles.

Esophagus

• The esophagus is a collapsible, muscular tube, connecting the pharynx to the stomach located behind the trachea
• The walls of the esophagus contain mucosa, submucosa, and muscularis layers
• Adventitia composes the outer layer
• The esophagus contains upper and lower sphincters.
• During the esophageal stage of swallowing, peristalsis contractions push the bolus onwards
• Failure of a lower esophageal sphincter results in gastroesophageal reflux disease causes heartburn
• Deglutition, otherwise called swallowing, moves a bolus from the mouth to the stomach; it involves the mouth, pharynx and esophagus

Stomach

• The stomach is a J-shaped enlargement of the GI tract, beginning at the bottom of the esophagus, ending at the pyloric sphincter
• The stomach serves as a mixing and holding area, beginning protein digestion, continuing triglyceride digestion, converting a bolus to chyme, and absorbing substances.
• Stomach's subdivisions: cardia, fundus, body, and pylorus.
• When empty, mucosa lies in folds called rugae.
• Pylorospasm and pyloric stenosis are newborns abnormalities that can functionally or partially block the exit of food requiring drugs or surgery.
• The surface of the mucosa is a layer of simple columnar epithelial cells called mucous surface cells
• Epithelial cells extend down into the lamina propria forming gastric pits/glands.
• Gastic glands contain exocrine glands: mucous neck cells, chief/zymogenic cells, and parietal (oxyntic) cells.
• Gastric glands contain hormone-producing enteroendocrine cells; G cells secrete gastrin.
• The submucosa consists of areolar connective tissue.
• The muscularis consists of three smooth muscle layers: longitudinal, circular, and inner oblique.
• The serosa originates from the visceral peritoneum.
• At curves of the stomach, the visceral peritoneum transforms into the lesser (lesser curvature) or greater omentum (greater curvature).

Mechanical and Chemical Digestion in the Stomach

• Peristaltic mixing-waves causes mechanical digestion
• Chemical digestion: proteins becomes peptides by pepsin which requires an acidic stomach, pH 2, via HCl from parietal cells
• Gastric lipase has limited activity on butterfat in adult stomach, splitting molecules for fatty acids and monoglycerides
• A few substances can be absorbed like: some water, electrolytes, drugs especially aspirin, and alcohol

Pancreas

• Gastric emptying is the duodenum's periodic release of chyme from the stomach
• Post-ingestion, most food takes 2-6 hours to leave the stomach, first carbs, then proteins, lastly fat
• Vomiting expels GI tract contents from the stomach and duodenum through the mouth; prolonged vomiting disrupts fluid and acid-base balance.
• The pancreas's head, body, and tail connects to the duodenum via the pancreatic duct while the accessory duct connects to the duodenum
• Pancreatic islets secrete hormones while acini secretes pancreatic juice
• Pancreatic juice digests: starch (pancreatic amylase), proteins (trypsin, chymotrypsin, carboxypeptidase), fats (pancreatic lipase), and nucleic acids (ribonuclease, deoxyribonuclease)
• Juice includes sodium bicarbonate for converting the stomach's acidity to a slightly basic pH of 7.1-8.2 which halts pepsin while promoting pancreatic enzymes.
• Pancreatic cancer/pancreatitis greatly affects pancreas function.

Liver and Gallbladder

• The liver is the heaviest gland and second largest bodily organ
• The liver divides into left/right lobes while also containing caudate and quadrate lobes.
• The gallbladder resides in a posterior liver depression.
• Liver lobes contain hepatic cells/hepatocytes, sinusoids, Kupffer's cells, a central vein
• The gallbladder has simple columnar epithelium arranged in rugae, there is no submucosa; outer layer is the visceral peritoneum
• The gallbladder ejects bile which stores and concentrates until required, into the small intestine
• The liver receives a double blood supply; the hepatic artery/portal vein; the hepatic vein transports blood away from the liver
• Hepatic cells produces and transports bile through the duct system to the gallbladder for concentration
• The liver excretes wastes/produces digestive components; it also emulsifies triglycerides
• Liver metabolic functions: carbohydrate/lipid/protein metabolism; drug/hormone removal; bilirubin excretion; bile salts synthesis; vitamin/mineral storage; phagocytosis; vitamin D activation.
• Blockage of bile by gallstones halts outflow of bile in any portion of the duct; treatments involve drugs, lithotripsy, or surgery.
• The sclera, skin, and mucous membrane is jaundice, a yellowish coloration derived from bilirubin
• Jaundice categories: prehepatic, hepatic, and enterohepatic.

Small Intestine

• The small intestine is an area that digests and absorbs, and goes from pyloric to ileocecal sphincters
• Duodenum, jejunum, and ileum are the small intestine's subdivisions.
• The Circular folds are projections that remain as permanent ridges to promote absorption via inceased surface area which then promotes spiral movement for the chyme
• Mucosa contains fingerlike villi to provide more surface area for epithelium.
• Within the villus is the lacteal for fat absorption.
• The mucosal epithelium contains absorptive, goblet, enteroendocrine, and Paneth cells.
• Microvilli increases the surface area available on the free absorptive cells and also generates multiple enzymes that form the brush border
• Intestinal glands lie within the mucosa which consists of cavities with glandular epithelium
• The submucosa: Brunner's glands secreting alkaline mucus to neutralize acid in chyme in the duodenum, and Peyer's patches (aggregated lymphatic nodules) in the ileum's submucosa.
• Intestinal juice carries substances from chyme in contact with the villi
• Intestinal "brush border" enzymes break down foods inside epithelial cells of the mucosa

Mechanical and Chemical Digestion in the Small Intestine

• Segmentation movement is the localized contraction in areas with food.
• Peristalsis is the propulsion of chyme onwards.
• Carbohydrates are broken into monosaccharides prior to absorption
• Small intestine's enzymes break down starches into maltose, maltotriose, alpha-dextrins assisted through amylase
• Maltose, alpha-dextrins, sucrose, and lactose is then converted to glucose/fructose/galactose through a series of enzymes

Lipid & Protein Digestion

• Proteins becomes peptides via trypsin, chymotrypsin, with enzymes which attach terminal amino acids to the carboxyl(carboxypeptidases) and amino ends (aminopeptidases)
• Finally, dipeptidase splits the dipeptides into the amino acids
• Most lipid digestion is completed through and by bile which forms and emotes a droplet emulsion and pancreatic lipase hydrolyzes triglycerides into fatty acids and monoglycerides/bile to droplet
• Nucleic acids convert into nucleotides for absorption.

Small Intestine Absorption

• In the GI tract, end product digestion happens by diffusing blood and the lymph, especially through the assisted epithelial cells
• Monosaccharides are essentially completely absorbed in the blood capillaries
• Proteins mostly become absorbed as amino acids by "active transport"
• Lipids: Dietary lipids are fully absorbed by simple diffusion.
• Micelles are long-chain fatty acids and monoglycerides, which are resynthesized introgen triglycerides.
• Lipids are then combined with lipoproteins for transportation in blood
• Electrolytes like potassium and water can be absorbed in specialized areas of the intestine via the venous system

Large Intestine

• The large intestine goes from ileocaecal sphincter to anus
• The main subdivisions include the: cecum, rectum, colon and the anal canal
• Inferior to the cecum is the vermiform appendix.
• Inflamed appendix causes appendicitis, and threatens a rupturing if conditions worsen
• Large Intestine structure consists of a colon which is separated into transverse, descending, and ascending sections
• Large intestine mucosa lacks villi/folds has goblet cells and columnar epithelium
• The muscularis's longitudinal portions causes pouches/haustra via contraction (taeniae coli).
• Haustral churning, peristalsis, and mass peristalsis are movements of the large intestine
• Bacterial in the large intestine digest matter.
• water, some vitamins/electrolytes goes into the large intestine.
• Waste contains: inorganic salts, bacteria, sloughed-off matter.

Defecation

• Defecation eliminates fecal matter - aided by contracting diaphragm and abdominal muscles,
• External anal sphincters can be controlled to stop and delay
• Increased motility will cause diarrhea
• Decreased motility leads to constipation and decreased stool frequency
• Diets will affect the speed of food - fiber and soluble.

Phases of Digestion

• Digestion functions in cephalic, gastric, and intestinal phases
• Cephalic triggers stimulation of gastric secretion and motility
• Gastric is regulated via neural/hormonal mechanisms triggered by stomach wall distension and high pH.
• Increased protein then prompts chemoreceptors and stretch receptors which prompts peristalsis/gastric flow.
• Chemo/receptors stimulates ANS for gastrin release, gastric growth, and sphincter contraction in the stomach.
• Distension then reduces gastric emptying - enterogatric reflex.
• Secretin improves HCO3 into bile and pancreatic juice while also improving growth/maintenance and CCK impacts
• Gastrin triggers juice with enzymes and contracts bile while slowing gastric emptying.
• There are other hormones secreted by GI tract like motilin and VIP.

Development of the Digestive System

• Endoderm creates epithelium for the glands - digestive system
• Mesoderm creates smooth muscle for connection tissue and function.

Aging and the Digestive Tract

• Elderly: secretory/tone decreases while sensation and feedback loop lowers as well
• Other potential occurrences involve sensitive mouth, swallowing and digestive dysfunction.
• Ulcers, hernias and digestive problems are expected with old aged digestion.

Homeostasis and the Digestive System

• Digestive provides for homeostasis to function at a high standard.

Disorders: Homeostatic Imbalances

• Bacteria creates caries that then act on demineralized enamel.
• Gum/alveolar degenerates - periodontal disease
• Ulcers are basically mucous injuries and creates a gastric exposure - bleeding is the most common occurance
• Diverticula and saccular outpouches becomes inflamed resulting in diverticulitis and causes pain for digestion
• Malignancy can be deadly since it creates tumors and can show in distal exams
• Hepatitis inflames the liver
• Hep A is spread by contamination of the liver
• B is caused sexually and potentially causes cancer
• C is blood and likely to cause liver issue - cirrhosis
• D happens if you already have hep B
• E is deadly for pregnant women
• Anorexia is a loss of weight and image.