GI Tract Anatomy and Function

Learning Objectives

• Understand the functional anatomy of the GI tract through motility, absorptive, secretory, and digestive functions.
• Explain the anatomical and functional complexities of gut hormone regulation.
• Describe general mechanisms of hormonal cell-to-cell communication, including endocrine, neurocrine, paracrine, and autocrine signaling.
• Describe the gut as a hormone source, using examples such as gastrin, CCK, somatostatin, GIP, and ghrelin. Apply this to demonstrate function and regulation of gut hormonal processes.
• Understand how gut hormones are used in medical practice.
• Discuss the secretion mechanisms and functions of digestive juices.
• Understand how certain disease states impact digestive juice secretions.

Functions of the Digestive System

• The digestive system breaks down food, releases nutrients, and absorbs them into the body.
• This is achieved through GI functions: secretion, motility, digestion, and absorption.

The Digestive System

• The gastrointestinal (GI) tract consists of the alimentary canal organs and accessory glandular tissues.
• The alimentary canal is a muscular tube composed of different organs that coordinate to move food unidirectionally, from the mouth to the anus.

Alimentary Canal Components

• Mouth
• Pharynx
• Esophagus
• Stomach
• Small Intestine
• Large Intestine
• Anus

Accessory Organs of the Digestive System

• Exocrine organs lie outside the digestive tract and empty their secretions through ducts into the digestive tract lumen.
• Salivary glands
• Exocrine pancreas
• Biliary system
• Liver
• Gallbladder

Secretion Function

• Digestive juices are secreted into the digestive tract lumen by exocrine glands.
• Gastrointestinal hormones control digestive motility and exocrine gland secretion.
• Digestive juices are secreted into the digestive tract lumen by exocrine glands
• Gastrointestinal hormones are secreted by endocrine cells to modulate digestive motility and exocrine gland secretion.

Secretion in the GI Tract

• Exocrine gland cells secrete substances into the GI tract lumen via their duct system.
• Endocrine gland cells secrete products directly into the bloodstream.

Tissue Organization of the Alimentary Canal

• The alimentary canal is primarily lined by four basic tissues.
• The mucosa lines the alimentary canal, directly contacting food/chyme and containing secretory cells.

GI Tract Barrier to Environment

• The alimentary canal must create a barrier to protect the body from external assaults, like acidic stomach pH, digestive enzymes, and gut flora,.
• The stomach lumen pH can be as low as 2.

Peptic Ulcers

• H. pylori infection and overuse of NSAIDs can compromise the protective mucus of the GI tract.
• Compromising results in pepsin and HCl causing damage to the stomach wall, which in turn creates ulcers.

Digestive Juices

• Digestive juices are secreted into the digestive tract lumen by exocrine gland cells.
• There are five digestive juices secreted into the alimentary canal
• Saliva: Salivary gland
• Gastric juice: Gastric mucosa
• Pancreatic juice: Exocrine pancreas
• Intestinal juice: Intestinal mucosa
• Bile: Hepatic gland

Salivary Secretion

• Saliva is a mixture of about 99.5% water, 0.5% electrolytes, and proteins.
• Saliva's functions are digestion, lubricating swallowing assistance, rinsing teeth, neutralizing acids etc
• Salivary amylase aids with digestion
• Saliva helps moisten food and aid in speech

Autonomic Influence on Salivary Secretion

• Salivary secretion is unique as the only digestive secretion entirely under neural control. Other secretions are regulated by nervous reflexes and hormones.
• Parasympathetic stimulation produces a lot of watery saliva rich in enzymes.
• Sympathetic stimulation, however, produces less volume of thick saliva-rich mucus.

Autonomic Control of Salivary Reflexes

• Pressure and chemoreceptors in the mouth trigger salivary centre reflexes in medulla to communicate with the cerebral cortex as a conditioned refex.

Gastric Juice Secretion

• Chemical digestion starts in the stomach via gastric juice.
• Gastric juice is a mix of water, hydrochloric acid (HCl), electrolytes, mucus, pepsin, and protein.
• This gastric juice is highly acidic and contains digestive enzymes.

Gastric Juice HCL

• The stomach lumen pH can be as low as 2
• Activates pepsinogen to become the active pepsin
• Aids in the breakdown of connective tissue and muscle fibres
• Denatures protein
• Kills microorganisms ingested with food

Control of Gastric Secretion

• Gastric secretion has three phases: cephalic, gastric, and intestinal.

Cephalic Phase

• Thinking about, tasting, smelling, chewing, and swallowing food increases gastric secretion through vagal nerve activity.

Gastric Phase

• Begins when food reaches the stomach.
• The presence of protein, distension, caffeine, and alcohol increases gastric secretion.

Intestinal Phase

• Fat, acid, hypertonicity, or distension in the duodenum can inhibit gastric secretions.
• This phase shuts off the flow of gastric juices when chyme is released into the small intestine.

Gastric Juice Secretion and Mucosa

• Gastric juice is secreted into the stomach lumen by exocrine cells within the mucosa.
• The gastric mucosa is arranged into invaginated gastric pits

Pancreatic Secretions

• Chyme in the small intestine mixes with secretions from the exocrine pancreas and the liver when released into the duodenal lumen.
• Exocrine secretions consist of trypsinogen, pancreatic amylase, pancreatic lipase, and aqueous alkaline solution secreted by duct cells.
• Endocrine tissue consists of the islets of Langerhans, which secrete insulin and glucagon.

Biliary System

• Bile is secreted into the duodenal lumen to aid in digestion of lipids
• The biliary system includes the liver, gallbladder, and associated ducts.
• After fat digestion and absorption, bile salts go through enterohepatic circulation and are reabsorbed into the blood and returned to the liver through the hepatic portal system.

Small and Large Intestine Secretions

• Small intestine mucosa secretes the succus entericus, secretion increases in response to Chyme prescence in the small intestine
• Large intestine secretion contains colonic secretion from alkaline mucous to protect from injury.

Cystic Fibrosis Impairs Digestive Secretions

• CFTR mutation causes impaired chloride transport
• CFTR is involved in exocrine gland secretions such as sweat, mucus, and digestive fluids.

Secretion of Gastrointestinal Hormones

• Gastrointestinal hormones are secreted into the blood by endocrine gland cells in the digestive tract's mucosa and control digestion and exocrine glands.

Cell-Cell Communication Controls GI Functions

• Cell signaling can occur through endocrine, paracrine, or autocrine mechanisms

Endocrine Cells in the Stomach Lining

• G cells: Found only in PGA gastric pits. Secrete gastrin.
• Enterochromaffin-like (ECL) cells: Secrete histamine.
• D cells: Secrete somatostatin in response to acid.

Hormones in the GI Tract

Gastrin

• Stimulated by protein or amino acids in the stomach.
• Increases HCl and pepsinogen secretion.
• Enhances gastric and ileal motility, relaxes the ileocecal sphincter, and induces mass movements in the colon.
• Gastrin secretion is inhibited by an accumulation of acid in the stomach and duodenal lumen.

Ghrelin

• Produced by endocrine cells of the stomach.
• Is called the "hunger hormone" by increasing food intake.
• Might stimulate food intake by increasing gastric motility and gastric acid secretion.

Glucagon-like peptide 1 (GLP-1)

• Secreted by intestinal L cells by nutrients in the lumen
• Promotes insulin secretion and sensitivity in the pancreas, decreases glucagon secretion
• Inhibits gastric secretion and gastric emptying
• Increases satiety, reducing food intake

Glucose-Dependent Insulinotropic Peptide (GIP)

• Released by duodenal K cells when food is present
• It is able to stimulate the release of the Pancreatic insulin, and Inhibit the small intestine motility
• GIP inhibits gastric secretion of acids and pepsin.

GLP-1 and Ghrelin

• Glucagon-like-peptide 1 and Ghrelin functions are opposite to each other.

Secretin Hormone

• Stimulated by acid in the duodenum
• Inhibits gastric emptying to prevent further acid from entering the duodenum.
• In response to being stimulated by acid in the duodenum, will inhibt gastric secretion to minimize the acid.
• Stimulates pancreatic exocrine NaHCO3 secretion, neutralizing duodenum acid.

CCK Hormone

• Stimulated by fat in the duodenum
• Inhibits gastric motility and secretion
• Stimulates acinar cells in the pancreas to secrete pancreatic enzymes and contractions of the gallbladder.
• An improtant regulator of food intake.

Motilin Hormone

• Secrethed by M cells in crypts in small intestine
• Released in the general circulation
• Inhibited by feeding
• Increases mirgrating molitiy complexes.

Somatostatin Hormone

• Secrete by various cells in the Gl tract
• Has various function in the body
• Inhibition of Gastric Secretion
• Parietal Cell - Reduces acid secretion
• G Cell & ECL - Reduces Gastrin Secretion
• Therefore, turining off HCL secreting Cells

Gut Secretion Review

• Digestive juices are secreted into the digestive tract lumen by exocrine gland cells.
• Mucus serves as a protective barrier against injury to the gastric mucosa.
• Gastric secretions are regulated by the cephalic, gastric and intestinal phases.
• Exocrine organs include Salivary glands, Exocrine pancreas, Biliary system, Liver, and Gallbladder
• Gastrointestinal hormones help control digestive motility and exocrine gland secretion.
• Hormones can signal change in effector cells through endocrine, paracrine and autocrine mechanisms