The digestive system is a complex network of organs working together to break down food and absorb nutrients. From the mouth to the anus, each part plays a crucial role in processing what we eat and drink.

Accessory organs like the liver and pancreas aid digestion by producing essential enzymes and bile. The nervous system regulates digestive processes, while the peritoneum provides support and protection for abdominal organs.

Digestive System Overview

Organs of the alimentary canal (gastrointestinal tract)

• Mouth and oral cavity
  • Performs mechanical digestion through chewing (mastication) breaking down food into smaller pieces
  • Initiates chemical digestion with salivary amylase an enzyme that begins carbohydrate breakdown
• Pharynx
  • Serves as a passageway for both food and air connecting the oral and nasal cavities to the esophagus and larynx
• Esophagus
  • Muscular tube that transports food from the pharynx to the stomach via peristalsis rhythmic muscle contractions
• Stomach
  • Expandable sac that stores and mixes food with gastric secretions (hydrochloric acid and enzymes)
  • Begins protein digestion with the enzyme pepsin breaking down proteins into smaller peptides
• Small intestine
  • Divided into three sections: duodenum, jejunum, and ileum
  • Completes digestion with pancreatic enzymes (lipase, amylase, trypsin) and bile from the liver and gallbladder
  • Absorbs nutrients through finger-like projections called intestinal villi increasing surface area for absorption
• Large intestine
  • Includes the cecum, colon, rectum, and anal canal
  • Absorbs water and electrolytes (sodium, potassium) from remaining undigested material
  • Forms and stores feces the solid waste product of digestion
• Anus
  • Opening at the end of the anal canal for the expulsion of feces during defecation

Accessory digestive organs

• Salivary glands (parotid, submandibular, sublingual)
  • Produce saliva containing salivary amylase to initiate carbohydrate digestion and mucus to lubricate food
• Liver
  • Produces bile a fluid that emulsifies fats into smaller droplets for easier digestion
  • Metabolizes nutrients (carbohydrates, proteins, fats) and detoxifies harmful substances (alcohol, drugs)
• Gallbladder
  • Stores and concentrates bile produced by the liver
  • Releases bile into the duodenum when stimulated by the hormone cholecystokinin
• Pancreas
  • Produces pancreatic juice containing digestive enzymes (lipase, amylase, trypsin) for fat, carbohydrate, and protein digestion
  • Secretes bicarbonate to neutralize acidic chyme (partially digested food) entering the duodenum from the stomach

Tissue layers in digestive tract

• Mucosa
  • Innermost layer lining the lumen (hollow space) of the digestive tract
  • Consists of epithelium, lamina propria (connective tissue), and muscularis mucosae (thin muscle layer)
  • Secretes mucus for lubrication, enzymes for digestion, and hormones for regulation
  • Absorbs nutrients through specialized cells (enterocytes) in the small intestine
• Submucosa
  • Layer beneath the mucosa containing blood vessels, lymphatics, and submucosal plexus (nerve network)
  • Provides support and nourishment to the mucosa
• Muscularis externa
  • Consists of an inner circular and outer longitudinal layer of smooth muscle
  • Responsible for peristalsis (propulsive contractions) and segmentation (mixing contractions)
• Serosa (adventitia in esophagus and upper duodenum)
  • Outermost layer composed of connective tissue
  • Provides support and reduces friction with surrounding structures

Nervous system regulation of digestion

• Enteric nervous system (ENS)
  • Intrinsic innervation embedded within the digestive tract wall
  • Consists of submucosal plexus (Meissner's plexus) and myenteric plexus (Auerbach's plexus)
  • Can function independently of the central nervous system
  • Regulates motility (peristalsis, segmentation), secretion (mucus, enzymes), and local blood flow
• Autonomic nervous system (ANS)
  • Extrinsic innervation originating from the brain and spinal cord
  • Parasympathetic division via the vagus nerve (cranial nerve X)
    • Stimulates digestion by increasing secretion, motility, and blood flow ("rest and digest")
  • Sympathetic division via splanchnic nerves
    • Inhibits digestion by decreasing secretion, motility, and blood flow ("fight or flight")
    • Diverts blood flow to skeletal muscles during stress or physical activity

Peritoneum's role in digestion

• Peritoneum
  • Serous membrane lining the abdominal cavity (parietal peritoneum) and covering the surface of digestive organs (visceral peritoneum)
  • Secretes serous fluid to reduce friction between organs
• Mesentery
  • Double layer of peritoneum that attaches digestive organs to the posterior abdominal wall
  • Contains blood vessels, lymphatics, and nerves supplying the organs
  • Allows for mobility and flexibility of organs while maintaining their position
• Omenta (greater omentum, lesser omentum)
  • Folds of peritoneum that connect the stomach to other organs
  • Greater omentum extends from the greater curvature of the stomach, draping over the intestines
  • Lesser omentum extends from the lesser curvature of the stomach to the liver
  • Store fat, provide insulation, and help contain the spread of infections

Digestive processes

• Digestion: The breakdown of food into smaller, absorbable components
  • Mechanical digestion: Physical breakdown of food through chewing and muscular contractions
  • Chemical digestion: Breakdown of food molecules by digestive enzymes
• Absorption: The process by which nutrients from digested food are taken up by the cells lining the digestive tract
• Motility: The movement of food through the digestive system, facilitated by muscle contractions and peristalsis