The gastrointestinal tract is a complex system that processes food and absorbs nutrients. From mouth to anus, it includes various organs working together to break down food, extract nutrients, and eliminate waste.

Understanding the anatomy of the digestive system is crucial for grasping how our bodies turn food into energy. Each part plays a unique role, from the stomach's acid production to the small intestine's nutrient absorption.

Gastrointestinal Tract Anatomy

Structure and Organization

• The gastrointestinal tract, also known as the alimentary canal or digestive tract, is a continuous tube extending from the mouth to the anus
• It consists of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine
• The gastrointestinal tract is approximately 9 meters long in an adult human
• It is divided into the upper gastrointestinal tract (oral cavity, pharynx, esophagus, and stomach) and the lower gastrointestinal tract (small intestine and large intestine)

Accessory Organs and Mucosa

• Accessory organs of the gastrointestinal tract include the salivary glands, liver, gallbladder, and pancreas
• These organs secrete enzymes and other substances that aid in digestion
• The gastrointestinal tract is lined with a mucous membrane called the mucosa
• The mucosa consists of an epithelial layer, a supportive connective tissue layer called the lamina propria, and a thin smooth muscle layer called the muscularis mucosae

Esophagus, Stomach, Small Intestine, Large Intestine

Esophagus

• The esophagus is a muscular tube that connects the pharynx to the stomach
• It measures about 25 cm in length
• The esophagus is lined with stratified squamous epithelium and mucus-secreting glands for protection against abrasion and stomach acid

Stomach

• The stomach is a J-shaped, muscular organ located in the upper left quadrant of the abdominal cavity
• It has four main regions: the cardia, fundus, body, and pylorus
• The stomach functions to store, mix, and digest food through the secretion of gastric juices (hydrochloric acid and digestive enzymes)

Small Intestine

• The small intestine is the longest part of the gastrointestinal tract, measuring about 6 meters in length
• It is divided into three regions: the duodenum, jejunum, and ileum
• The primary functions of the small intestine are digestion and absorption of nutrients
• The small intestine has a highly folded mucosa with finger-like projections called villi and microvilli, which greatly increase the surface area for absorption
• The wall of the small intestine contains circular folds called plicae circulares, which further increase the surface area and slow the passage of food for better absorption

Large Intestine

• The large intestine, also called the colon, is about 1.5 meters long
• It is divided into the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal
• The primary functions of the large intestine are absorbing water and electrolytes, and forming and storing feces until elimination
• The large intestine has a larger diameter than the small intestine and has pouches called haustra that give it a segmented appearance
• The mucosa of the large intestine lacks villi but contains numerous goblet cells that secrete mucus for lubrication and protection

Sphincters in the Gastrointestinal Tract

Function and Structure

• Sphincters are circular muscles that control the passage of contents through the gastrointestinal tract by alternately contracting and relaxing
• They allow for compartmentalization and unidirectional flow of digestive contents
• Sphincters are located at various junctions throughout the gastrointestinal tract

Major Sphincters

• The upper esophageal sphincter (UES) is located at the junction of the pharynx and esophagus and prevents air from entering the esophagus during breathing and swallowing
• The lower esophageal sphincter (LES) is located at the junction of the esophagus and stomach and prevents the reflux of stomach contents back into the esophagus, while allowing the passage of food and liquids into the stomach
• The pyloric sphincter is located at the junction of the stomach and duodenum and regulates the passage of partially digested food (chyme) from the stomach into the small intestine
• The ileocecal valve, located at the junction of the ileum and cecum, prevents the backflow of contents from the large intestine into the small intestine and regulates the passage of chyme into the large intestine
• The internal and external anal sphincters, located at the distal end of the anal canal, control the elimination of feces and maintain continence

Small Intestine Regions and Functions

Duodenum

• The duodenum is the shortest and most proximal segment of the small intestine, measuring about 25 cm in length
• It is the site where most chemical digestion occurs due to the entry of bile from the liver and gallbladder and pancreatic enzymes from the pancreas through the hepatopancreatic ampulla
• Bile aids in the emulsification and digestion of fats, while pancreatic enzymes break down proteins, carbohydrates, and lipids

Jejunum

• The jejunum is the middle segment of the small intestine, measuring about 2.5 meters in length
• It is the primary site of nutrient absorption due to its numerous plicae circulares, villi, and microvilli, which greatly increase the surface area for absorption
• The jejunum absorbs most of the carbohydrates, proteins, lipids, vitamins, and minerals from the digested food

Ileum

• The ileum is the longest and most distal segment of the small intestine, measuring about 3.5 meters in length
• It is responsible for the absorption of vitamin B12, bile salts, and any remaining nutrients not absorbed in the jejunum
• The ileum also contains numerous lymphoid follicles called Peyer's patches, which are part of the gut-associated lymphoid tissue (GALT) and play a role in immune surveillance and defense

pH Gradient

• The pH of the small intestine gradually increases from the duodenum to the ileum
• This pH gradient allows for optimal activity of different digestive enzymes and absorption processes in each region
• The duodenum has a slightly acidic pH due to the entry of acidic chyme from the stomach, while the jejunum and ileum have a more neutral to slightly alkaline pH