13.1 Gastrointestinal Tract Anatomy

4 min read•august 28, 2024

The gastrointestinal tract is a long tube that runs from mouth to anus. It's the body's food processing system, breaking down what we eat into nutrients we can use. Each part has a specific job in digestion.

From the mouth to the large intestine, food goes through different stages of breakdown. Accessory organs like the liver and pancreas help out too. The whole system works together to turn our meals into fuel for our bodies.

Gastrointestinal Tract Anatomy

Organs and Structures

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  • The gastrointestinal tract forms a continuous tube from the mouth to the anus consists of the mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum, colon, rectum), and anus
  • The mouth performs mechanical digestion through chewing (mastication) and chemical digestion through salivary enzymes while the tongue helps mix food with saliva and forms a bolus for swallowing
  • The pharynx and esophagus transport the food bolus from the mouth to the stomach through peristaltic contractions
  • The stomach stores and mechanically breaks down food through muscular contractions also initiates protein digestion through the action of pepsin and hydrochloric acid
  • The small intestine serves as the primary site of chemical digestion and nutrient absorption receives digestive enzymes from the pancreas and bile from the liver and gallbladder
  • The large intestine absorbs water and electrolytes from the remaining chyme and stores feces until elimination through the anus

Functions of Gastrointestinal Organs

  • Mouth: Mechanical digestion (chewing), chemical digestion (salivary enzymes), bolus formation
  • Pharynx and esophagus: Transport of food bolus to the stomach via peristalsis
  • Stomach: Storage, mechanical breakdown, initiation of protein digestion (pepsin, HCl)
  • Small intestine: Primary site of chemical digestion and nutrient absorption, receives pancreatic enzymes and bile
  • Large intestine: Absorption of water and electrolytes, storage of feces before elimination
  • Anus: Elimination of feces from the body

Digestive Tract Histology

Layers of the Digestive Tract Wall

  • The digestive tract wall consists of four main layers: mucosa (innermost), submucosa, muscularis externa, and serosa (outermost) or adventitia
  • The mucosa contains epithelium, lamina propria, and muscularis mucosae varies throughout the digestive tract and is involved in secretion, absorption, and protection
  • The submucosa contains blood vessels, lymphatic vessels, and nerves supports the mucosa and contains submucosal glands that secrete mucus and other substances
  • The muscularis externa has an inner circular layer and an outer longitudinal layer of smooth muscle responsible for peristaltic contractions that move food through the digestive tract
  • The serosa consists of a layer of simple squamous epithelium (visceral peritoneum) and a small amount of underlying connective tissue while areas not covered by serosa have the adventitia (a layer of connective tissue)

Roles of Histological Layers in Digestion

  • Mucosa: Secretion of digestive enzymes and mucus, absorption of nutrients, protection against pathogens
  • Submucosa: Support for the mucosa, contains blood vessels for nutrient transport and glands for secretion
  • Muscularis externa: Peristaltic contractions for food propulsion and mixing
  • Serosa/Adventitia: Reduction of friction with surrounding structures, protection of the digestive tract

Accessory Organs in Digestion

Salivary Glands and Pancreas

  • The salivary glands (parotid, submandibular, and sublingual) secrete saliva containing enzymes (salivary amylase) that initiate carbohydrate digestion in the mouth
  • The pancreas secretes bicarbonate to neutralize acidic chyme from the stomach and digestive enzymes (pancreatic amylase, lipase, trypsin) that break down carbohydrates, fats, and proteins in the small intestine

Liver and Gallbladder

  • The liver produces bile, which is stored and concentrated in the gallbladder bile emulsifies fats in the small intestine, increasing their surface area for enzymatic digestion by lipases
  • The liver, gallbladder, and pancreas secrete their products into the duodenum of the small intestine through ducts (hepatic, cystic, and pancreatic ducts, respectively)

Peritoneal vs Retroperitoneal Regions

Peritoneal Cavity and Organs

  • The abdominal cavity is lined by a serous membrane called the peritoneum, which consists of the parietal peritoneum lining the abdominal wall and the visceral peritoneum covering the abdominal organs
  • The peritoneal cavity is the potential space between the parietal and visceral peritoneum, containing a small amount of serous fluid for lubrication
  • Intraperitoneal organs are suspended within the peritoneal cavity and are covered by visceral peritoneum on all sides (stomach, small intestine, transverse colon)

Retroperitoneal Organs and Mesentery

  • Retroperitoneal organs are located behind the peritoneum and are not suspended within the peritoneal cavity (kidneys, adrenal glands, pancreas, ascending and descending colon)
  • The mesentery is a double layer of peritoneum that attaches intraperitoneal organs to the posterior abdominal wall, allowing for blood vessels, lymphatics, and nerves to supply these organs
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