The Digestive System: Processes and Organs

• The digestive system's function is encompassed by four fundamental processes: digestion, secretion, motility, and absorption.

Organs and Glands

• Saliva is produced by the salivary glands.
• Bile comes from the liver and enters the digestive system.
• Pancreatic juice, containing enzymes and hormones, is produced by the pancreas.
• Gastric juice, containing acid, is produced by the stomach to kill bacteria and help decompose food.

Secretion vs. Excretion

• Secretion refers to the release of substances that aid in the digestion of food.
• Excretion is the elimination of waste products remaining after digestion.

Absorption

• Absorption occurs when food molecules pass through the cells of the gastrointestinal wall into the bloodstream.

Motility

• Motility involves the smooth muscles of the gastrointestinal tract using coordinated contractions and relaxations to mix nutrients with digestive secretions.
• This facilitates nutrient contact with the mucosa for absorption.

Gastrointestinal Wall

• The gastrointestinal wall contains a mucosa, submucosa, and muscularis externa.
• The mucosa is the innermost layer in contact with the gastrointestinal lumen, protecting and producing digestive juices.
• The submucosa is a layer following the mucosa, made of connective tissue, containing blood and lymphatic vessels and sometimes glands.
• The muscularis externa has two layers of muscle cells, mixing contents in the lumen, and functions in gastrointestinal motility and aboral propulsion.

Mucosa

• The epithelium helps absorb nutrients and secrete enzymes and hormones.
• The lamina propria, formed by loose connective tissue, contains collagen, elastin fibers, glands, capillaries, and lymphoid nodules.
• The muscularis mucosae is a thin layer of muscle fibers that, when contracted, cause folds and ridges in the mucosa, aiding its mobility.

Serosa or Adventitia

• The serosa or adventitia is the outermost layer in contact with the abdominal cavity; it is formed by connective tissue with mesothelial cells.
• It continues as the mesentery, which connects the gastrointestinal tract to the abdominal wall.
• Blood vessels, lymphatic vessels, and nerves travel through it to reach the digestive tube.
• This layer protects the digestive organs, keeps them in place, and produces fluid to reduce friction.

Serosa and Adventitia Specifics

• Serosa covers the stomach, small intestine, and large intestine.
• Adventitia covers the esophagus and rectum.

Gastrointestinal Lumen

• The gastrointestinal lumen is the space within the digestive tract.
• It is defined as the space inside a tubular structure.
• It contains everything within the lumen of an organ.
• The walls of the gastrointestinal tract are responsible for moving food.
• Aboral propulsion moves contents from the mouth to the anus through the gastrointestinal tract.

Innervation

• Innervation involves control split into intrinsic and extrinsic regulatory systems, for digestive activity based on external and emotional stimuli.

Intrinsic Nervous System

• The intrinsic nervous system is also known as the enteric nervous system.
• It is made up of two intramural nerve plexuses: submucosal or Meissner's plexus and myenteric or Auerbach's plexus.
• The intramural nerve plexuses are located within the submucosa and between the muscle layers.
• It is a large cluster of neurons that relates to the extrinsic innervation.

Extrinsic Nervous System

• The extrinsic nervous system comes from the autonomic nervous system's sympathetic and parasympathetic divisions.
• Sympathetic innervation involves main ganglia sending postganglionic sympathetic fibers to the celiac, mesenteric (superior and inferior), and hypogastric regions.
• It targets the intramural, myenteric, and submucosal plexuses.

Sympathetic and Parasympathetic

• Normally, when sympathetic fibers are activated, they inhibit the neurons of the enteric plexuses, reducing motor activity in the muscularis externa.
• Sympathetic activity generally inhibits digestive system structures.
• The parasympathetic innervation mainly involves the vagus nerve, starting from the digestive system's beginning to the transverse colon's first half.
• The remaining large intestine receives parasympathetic fibers from the pelvic nerves via the hypogastric plexus.
• Vagal fibers often terminate at the intramural plexuses, stimulating motor and secretory functions.

Glossary terms

• Intramural nerve plexus: neuron networks within digestive organ walls regulating movements, secretions, and blood flow in the gastrointestinal tract.
• Postganglionic fibers: carry nerve signals from ganglia to organs and body tissues.
• Ganglia: local centers that receive, transmit signal to control digestive functions.
• Parasympathetic innervation: the vagus nerve controls most of the digestive system up to the first half of the large intestine; it relies mostly on nerves and stimulates motor and secretory functions of the digestive tract.
• Sympathetic innervation: inhibits digestive functions.

Motility - Digestion

• Motility is mechanical digestion.

Gastrointestinal Motility

• The mouth, pharynx, and esophagus are the gastrointestinal structures involved.
• Mastication reduces food size and mixes it with saliva for lubrication.
• Deglutition involves voluntary and reflex mechanisms to move the bolus from the mouth to the stomach.
• Sphincters prevent air passage into the digestive tube and gastric reflux.

Gastric Motility

• Gastric motility allows the stomach to store food, reduce particle size, and mix with gastric secretions for chemical digestion.
• It empties gastric contents into the small intestine gradually.
• Gastric emptying involves draining small fractions of acidic chyme to the duodenum.

Gastric Emptying

• Gastric emptying is regulated by nervous and humoral mechanisms to control the flow of gastric content into the duodenum.
• An acidic chyme slows gastric emptying.
• Duodenal acidity triggers secretin release to reduce gastric motility and antral and pyloric contractions.
• Hyperosmotic luminal content decreases emptying.
• Lipids, particularly triglycerides, affect pyloric contractility.
• CCK is released by lipids.
• GIP is a gastric inhibitory peptide.
• The presence of long-chain fatty acids and amino acids also decrease tone in GI tract.