9.1 Structure and function of the gastrointestinal tract

The gastrointestinal tract is a complex system that breaks down food and absorbs nutrients. From the mouth to the anus, each organ plays a crucial role in digestion, with specialized structures and functions.

The tract's wall has four layers, including the mucosa for absorption and the muscularis externa for movement. The enteric nervous system regulates digestion, while the microbiome aids in immunity and nutrient production.

Gastrointestinal Tract Structures and Functions

Major Organs and Structures

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• The gastrointestinal tract is a continuous tube extending from the mouth to the anus consisting of the oral cavity, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine (cecum, colon, rectum, anal canal)
• The oral cavity breaks down food through mastication and salivary enzymes while the tongue mixes food with saliva and forms a bolus for swallowing
• The pharynx, a muscular tube, connects the oral cavity to the esophagus and aids in swallowing
• The esophagus transports food from the pharynx to the stomach through peristaltic contractions

Digestive Functions of Major Organs

• The stomach, a muscular sac, stores, mixes, and digests food using gastric secretions like hydrochloric acid and pepsin
• The small intestine, the primary site of digestion and nutrient absorption, receives digestive secretions from the pancreas, liver, and gallbladder in the duodenum to break down macronutrients
  • The jejunum and ileum contain villi and microvilli to increase surface area for nutrient absorption
• The large intestine absorbs water and electrolytes, forms feces, and eliminates waste

Layers of the Gastrointestinal Wall

Mucosa and Submucosa

• The gastrointestinal wall has four main layers: mucosa, submucosa, muscularis externa, and serosa
• The mucosa, the innermost layer, consists of the epithelium (secretion, absorption, protection), lamina propria (blood vessels, lymphatics, immune cells), and muscularis mucosae (smooth muscle for mucosal movement and folding)
• The submucosa, a connective tissue layer, contains blood vessels, lymphatics, and the submucosal plexus of the enteric nervous system

Muscularis Externa and Serosa

• The muscularis externa has an inner circular and outer longitudinal smooth muscle layer responsible for peristaltic contractions and segmentation
• The serosa, the outermost layer, consists of the mesothelium (simple squamous epithelium) and a thin connective tissue layer

Enteric Nervous System Regulation

Enteric Nervous System Structure

• The enteric nervous system (ENS), part of the autonomic nervous system, is embedded in the gastrointestinal wall and can function independently of the central nervous system
• The ENS has two main plexuses: the myenteric plexus (between muscularis externa layers, controls motility and peristalsis) and the submucosal plexus (in submucosa, regulates secretion, absorption, local blood flow)

Enteric Nervous System Function

• The ENS communicates with the central nervous system via the parasympathetic (vagus nerve) and sympathetic (splanchnic nerves) pathways to integrate gastrointestinal function with the body
• Sensory neurons, interneurons, and motor neurons in the ENS detect and respond to stimuli like mechanical distension, chemical changes, and nutrients
• Neurotransmitters in the ENS include acetylcholine, norepinephrine, serotonin, and peptides like substance P and vasoactive intestinal peptide (VIP)

Importance of the Gastrointestinal Microbiome

Microbiome Establishment and Composition

• The gastrointestinal microbiome is a diverse community of microorganisms, mainly bacteria, inhabiting the gastrointestinal tract
• The microbiome is established early in life and influenced by factors such as mode of delivery (vaginal or cesarean), diet, antibiotics, and environment

Microbiome Functions and Health Implications

• The microbiome maintains gut homeostasis by developing and regulating the immune system, protecting against pathogens, and producing short-chain fatty acids (SCFAs) like butyrate, propionate, and acetate through fermentation of undigested carbohydrates
  • SCFAs serve as energy sources for colonocytes, regulate intestinal pH, and have anti-inflammatory properties
• The microbiome synthesizes essential vitamins like vitamin K and some B vitamins
• Dysbiosis, an imbalance in the microbiome composition, is associated with gastrointestinal disorders such as inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and colorectal cancer
• Probiotics (live beneficial microorganisms) and prebiotics (non-digestible food components promoting beneficial bacteria growth) can potentially modulate the microbiome and improve gastrointestinal health