The digestive system is a continuous tube (the alimentary canal) plus several accessory organs that together break down food, absorb nutrients, and eliminate waste. Understanding its overall structure and regulation gives you the framework for every topic that follows in this unit.
Digestive System Overview
Organs of the alimentary canal (gastrointestinal tract)
The alimentary canal (also called the GI tract) runs from the mouth to the anus. Each organ along the way has a specific job in moving, breaking down, or absorbing food.
- Mouth and oral cavity
- Performs mechanical digestion through chewing (mastication), breaking food into smaller pieces
- Initiates chemical digestion with salivary amylase, an enzyme that starts breaking down starches into simpler sugars
- Pharynx
- A shared passageway for food and air, connecting the oral and nasal cavities to the esophagus and larynx
- Esophagus
- A muscular tube that moves food from the pharynx to the stomach via peristalsis, which is a wave of rhythmic smooth muscle contractions that pushes the food bolus downward
- Stomach
- An expandable, J-shaped sac that stores food and mixes it with gastric secretions (hydrochloric acid, pepsinogen, and mucus)
- Protein digestion begins here: HCl activates pepsinogen into pepsin, which cleaves proteins into smaller peptides
- The resulting semi-liquid mixture is called chyme
- Small intestine
- Divided into three sections: duodenum (receives chyme, bile, and pancreatic juice), jejunum (primary site of nutrient absorption), and ileum (absorbs bile salts and vitamin B12)
- Completes chemical digestion using pancreatic enzymes (lipase, amylase, trypsin) and bile
- Absorbs nutrients through villi and microvilli, finger-like projections that dramatically increase surface area
- Large intestine
- Includes the cecum, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal
- Absorbs water and electrolytes (sodium, potassium) from remaining undigested material
- Houses bacteria that ferment some indigestible carbohydrates and synthesize certain vitamins (like vitamin K)
- Forms and stores feces, the solid waste product of digestion
- Anus
- The terminal opening of the anal canal; controlled by an internal smooth muscle sphincter (involuntary) and an external skeletal muscle sphincter (voluntary) during defecation
Accessory digestive organs
These organs are not part of the alimentary canal itself, but they produce secretions that are delivered into the GI tract to aid digestion.
- Salivary glands (parotid, submandibular, sublingual)
- Produce saliva containing salivary amylase (starch digestion), mucus (lubrication), and lysozyme (antibacterial)
- Liver
- Produces bile, a fluid that emulsifies fats, meaning it breaks large fat globules into smaller droplets so lipase can work more efficiently
- Also metabolizes absorbed nutrients (carbohydrates, proteins, fats) and detoxifies harmful substances like alcohol and drugs
- Gallbladder
- Stores and concentrates bile produced by the liver
- Releases bile into the duodenum when stimulated by the hormone cholecystokinin (CCK), which is released in response to fats and proteins entering the duodenum
- Pancreas
- Produces pancreatic juice containing digestive enzymes: lipase (fats), amylase (starches), and trypsin (proteins)
- Secretes bicarbonate to neutralize the acidic chyme arriving from the stomach, protecting the duodenal lining and creating the proper pH for enzyme activity
Tissue layers of the digestive tract
The wall of the alimentary canal has four basic layers, arranged from innermost to outermost. Knowing these layers helps you understand how the tract secretes, absorbs, and moves food.
- Mucosa (innermost)
- Lines the lumen (the hollow interior) of the tract
- Made of three sublayers: epithelium (surface cells), lamina propria (loose connective tissue with capillaries and lymphoid tissue), and muscularis mucosae (a thin smooth muscle layer that creates small folds)
- Functions include secreting mucus, digestive enzymes, and hormones, as well as absorbing nutrients (via specialized enterocytes in the small intestine)
- Submucosa
- Dense connective tissue layer containing blood vessels, lymphatics, and the submucosal plexus (Meissner's plexus), a nerve network that regulates glandular secretion and blood flow
- Muscularis externa
- Two layers of smooth muscle: an inner circular layer and an outer longitudinal layer
- Contractions of these layers produce peristalsis (propulsive waves that move food along) and segmentation (back-and-forth mixing contractions, especially in the small intestine)
- Contains the myenteric plexus (Auerbach's plexus) between the two muscle layers, which primarily controls motility
- Serosa (outermost)
- A thin layer of connective tissue covered by mesothelium (simple squamous epithelium) that secretes serous fluid to reduce friction
- In the esophagus and retroperitoneal portions of the tract, this layer is called the adventitia instead, because it lacks the mesothelial covering and instead blends into surrounding connective tissue
Nervous system regulation of digestion
Digestive activity is regulated by two interacting nervous systems: one built into the gut wall itself, and one that connects the gut to the brain and spinal cord.
- Enteric nervous system (ENS)
- Often called the "brain of the gut," this is an intrinsic nerve network embedded within the walls of the GI tract
- Contains two major plexuses:
- Submucosal plexus (Meissner's plexus): controls glandular secretion and local blood flow
- Myenteric plexus (Auerbach's plexus): controls motility (peristalsis and segmentation)
- Can operate independently of the CNS, though it normally communicates with it
- Autonomic nervous system (ANS)
- Provides extrinsic innervation from the brain and spinal cord
- Parasympathetic division (mainly via the vagus nerve, cranial nerve X): stimulates digestion by increasing secretion, motility, and blood flow to the GI tract ("rest and digest")
- Sympathetic division (via splanchnic nerves): inhibits digestion by decreasing secretion, motility, and blood flow, redirecting blood to skeletal muscles during stress or physical activity ("fight or flight")
The key takeaway: parasympathetic = promotes digestion; sympathetic = suppresses digestion. On exams, if you're asked what happens to digestion during a stressful event, the answer is that it slows down because sympathetic activity dominates.
Peritoneum's role in digestion
- Peritoneum
- A serous membrane with two layers: the parietal peritoneum (lines the abdominal wall) and the visceral peritoneum (covers the surface of abdominal organs)
- The space between these layers is the peritoneal cavity, which contains a thin film of serous fluid that reduces friction as organs move
- Mesentery
- A double-layered fold of peritoneum that anchors most of the small intestine (and other organs) to the posterior abdominal wall
- Carries blood vessels, lymphatics, and nerves to and from the organs
- Provides enough slack for the intestines to move and shift position while keeping them organized
- Omenta
- Greater omentum: a large, fatty, apron-like fold that hangs from the greater curvature of the stomach and drapes over the intestines. It stores fat, provides cushioning, and plays an immune role by helping wall off areas of infection or inflammation.
- Lesser omentum: a smaller fold connecting the lesser curvature of the stomach to the liver. It contains the hepatic artery, portal vein, and bile duct within its free edge.
Digestive processes
The digestive system carries out six major processes. Three of the most fundamental are:
- Digestion: breaking food into molecules small enough to absorb
- Mechanical digestion: physical breakdown through chewing (mouth), churning (stomach), and segmentation (small intestine)
- Chemical digestion: enzymatic breakdown of macromolecules. Amylase splits starches, pepsin and trypsin split proteins, lipase splits fats, and bile assists by emulsifying fat droplets.
- Absorption: the uptake of digested nutrients (amino acids, monosaccharides, fatty acids, vitamins, water) across the intestinal epithelium and into the blood or lymph. Most absorption occurs in the small intestine.
- Motility: movement of food through the GI tract, driven by smooth muscle contractions. Peristalsis propels food forward; segmentation mixes food with digestive secretions without significant forward movement.
Other processes you should be aware of: ingestion (taking food into the mouth), secretion (release of water, acid, enzymes, and bile into the lumen), and defecation (elimination of feces).