The Digestive System

The Digestive System

• The digestive system is over 20 feet long
• It is specialized in mechanical and chemical digestion, absorption, and elimination
• Accessory structures are mainly involved in secreting digestive enzymes and fluids
• The GI tract has four tissue layers
  • Mucosa: Innermost, made of mucous membrane, secretes digestive fluid and enzymes, forming specialized structures (e.g., rugae in the stomach, villi and microvilli in the small intestine)
  • Submucosa: Second layer of fibrous connective tissue, supplies blood to the GI tract, has blood and lymphatic vessels to absorb nutrients
  • Muscularis: Third and thickest layer, made of smooth muscles to produce movements like peristalsis, mixing, or segmentation (e.g., 3 layers in the stomach)
  • Serosa: Outermost layer, made of serous membrane, binds the GI tract to body cavities and accessory structures
• Muscular movements:
  • Peristalsis: Wavelike movement that pushes food particles from the oropharynx to the rectum
  • Mixing: Mixing motion in the oral cavity and stomach to break down food into smaller particles
  • Segmentation: Regions of the small intestine contract and relax independently to digest and absorb more efficiently

Digestive Organs

• Mouth and oral cavity:
  • Food enters through ingestion
  • Food is broken down mechanically through mastication
  • The tongue manipulates food during mastication and has taste buds
  • Hard palate at the front of the roof of the oral cavity, formed by the palatine bone and palatine process of maxillae
  • Soft palate at the back of the roof of the oral cavity, made of smooth muscle and adipose tissue, forms a uvula
• Teeth: Adapted for mechanical digestion (mastication). A person has deciduous (milk) teeth and permanent teeth. Incisors, canines, premolars, and molars have different roles in breaking down food.
• Salivary glands: 3 pairs; parotid, submandibular, and sublingual. Saliva moistens food, disolves food to form bolus, cleanses teeth, contains amylase for carbohydrate digestion, bicarbonate ion (HCO3−) to maintain pH of saliva at 6.5-7.5
• Pharynx: Passageway for air and food. The nasopharynx connects to the nasal cavity to lungs, while the oropharynx connects to the oral cavity and esophagus, and the laryngopharynx directs to the larynx and trachea to receive food from the oropharynx
• Epiglottis: Cartilage that prevents food from entering the trachea
• Esophagus: A straight, collapsible tube connecting the pharynx to the stomach
  • Utilizes peristalsis to move food to the stomach
  • Has upper and lower esophageal sphincters to regulate food movement and prevent backflow

Stomach

• Pouch-like organ for food storage (2-4 hours)
• Some mechanical and chemical digestion
• 4 regions: cardia, fundus, body, and pylorus
• Rugae (folds) increase surface area for expansion, secretion, and some absorption
• Contains three layers of smooth muscle (peristalsis and mixing)
• Produces chyme = a yellowish substance mixing digestive juice with food bolus and passed to the small intestine

Pancreas

• Has an endocrine and exocrine function
  • Endocrine: Pancreatic islets secrete hormones
  • Exocrine: Secrete pancreatic fluid and enzymes for chemical digestion in the small intestine
• Pancreatic fluid includes sodium bicarbonate to raise pH of chime to 7.1-8.2 to stop pepsin activity and promote the activity of pancreatic enzymes such as pancreatic amylase (carbohydrate), pancreatic lipases (fatty acid), pancreatic proteases (amino acids), and pancreatic nucleases (nucleotides).

Liver

• Largest gland in the human body, divided into 4 lobes (right, left, quadrate, and caudate)
• Held together by the falciform ligament
• Coronary ligament connects the liver to the diaphragm
• Hepatic lobules are functional units of the liver
• Liver receives nutrients from the small intestine via the hepatic portal vein for detoxification and metabolism
• The hepatic vein transports nutrients back to the cardiovascular system

Gallbladder

• Small sac on the inferior surface of the liver
• Stores and concentrates bile produced by the liver
• Bile is ejected into the cystic duct, which joins the hepatic duct to form the common bile duct
• Common bile duct joins the pancreatic duct from the pancreas and attaches to the duodenum
• Bile functions to emulsify lipids so lipases can break them down easily

Small Intestine

• Long tube, small diameter (about 1 inch)
• Extends from the pyloric sphincter to the ileocecal valve
• Divided into duodenum, jejunum, and ileum
• Highly adapted for chemical digestion and absorption (90% of all absorption)
• Contains intestinal glands that secrete digestive enzymes and mucus to aid in chemical digestion
• Has villi and microvilli for a large surface area for nutrient absorption
• Muscular movement includes segmentation and peristalsis

Large Intestine

• Last segment of the GI tract, with large diameter (2-3 inches)
• Extends from the ileocecal valve to the anus
• Divided into cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal, and anus
• The mucosa has no villi or microvilli, but it does contain goblet cells to produce mucus for fecal formation and maintain an alkaline condition
• Absorbs only water, electrolytes, and some vitamins

Appendix

• Part of the large intestine (cecum). Not involved in absorption

Regulation of GI Tract Activities

• Autonomic Nervous System:
  • Parasympathetic nerves stimulate GI tract activity
  • Sympathetic nerves inhibit GI tract activity
• Hormonal Control: Hormones from endocrine glands and the GI tract itself help regulate GI tract activities
• Reflex Mechanisms: Regions of the GI tract (especially the stomach and small intestine) use reflexes to stimulate or inhibit each other

Clinical Terms

• Dysphagia: Difficulty swallowing
• Cholecystitis: Gallbladder inflammation
• Cholelithiasis: Gallstones
• Peptic ulcer: Ulcers in the stomach or duodenum
• Endoscopy: Visual examination of the interior of organs
• Colonoscopy: Endoscopy of the colon
• Sigmoidoscopy: Endoscopy of the sigmoid colon
• Pyloric stenosis: Congenital narrowing of the pyloric sphincter

Accessory Structures

• Teeth and tongue
• Salivary glands
• Liver
• Gallbladder
• Pancreas

Test your knowledge on the intricate structure and function of the digestive system. This quiz covers the various layers of the GI tract, accessory structures, and the processes involved in digestion and absorption. Challenge yourself and see how well you understand the amazing processes that aid in nutrition.